Section 5: Problem description and solutions

Environmental factors

Temperature effects

The most important effect of temperature is that it affects the rate at which plants develop through their stages (p 8) and produce their leaves, tillers and other components. Everything goes progressively faster as temperature rises between a base and an optimum temperature (p 81), and similarly development slows at lower temperatures. Calculating the size of these effects is discussed later (p 81). Good farm management can do more to counter the negative effects of high temperature than it can with low temperature and particularly frosts.

Frost on a crop

AF van Herwaarden

Low temperature

Development slows at lower temperature. But when temperatures are low enough for frosts, severe damage is done to young tissues. Vegetative shoots can be killed below -5° C. The consequences to yield of two or more successive frosts from spike emergence (Z5.1) through anthesis into early grain filling (Z7.1) can be severe. Official Stephenson Screen temperatures (p 90) of 1.5° C measured at 1.5 metres above the ground are cold enough for effects. This temperature is equivalent to 0° C on the crop surface. A single night frost during this period may not be overwhelming because only the new tissue that has been exposed to the air during the last few days is killed. This is seen as banding of dead and live spikelets on the spikes. All tissues become more frost resistant after exposure to the air.

Frost bands

AF van Herwaarden

Is frost or low temperature a problem?

Spikes in Nepal blackened and sterile after low temperature - not frost

KD Subedi

Can you do anything about frost?

Winter temperatures well below freezing can kill vegetative growth

H Gomez Macpherson

High temperature

To keep pace with increasing temperature, the crop requires more of everything (nutrients, water, sunlight) each day. So to avoid significant yield losses as temperature rises, farm management must become increasingly precise. With water and supply of nutrients optimised, high yields can still be achieved at high temperatures. During grain filling, development accelerates faster than growth as temperature rises. Consequently, even if management is optimum yield can decline by up to 4% for every 1° C rise in mean temperature at higher temperatures (Stapper and Fischer 1990) because the grain filling period becomes very short.

High temperature damage is commonly associated with water stress so water management is critical. As long as plants can transpire freely they cope with high temperature. Field crops provided with sufficient water can withstand air temperatures to 40° C. But if water is limiting, 40° C will kill leaves. The reason for this is that water-stressed plants attempt to conserve water by closing their stomata (p 90). As a consequence evaporative cooling diminishes and, without that cooling, leaf temperatures might approach 50° C. At such temperatures plant processes break down. Seedlings in very hot dry soils can readily reach those critical temperatures.

Is high temperature a problem?

What you can do about high temperatures

Sunlight effects

Crop growth is primarily determined by the amount of sunlight (solar radiation) that the crop can intercept and use during its life. Too much radiation is rarely a problem as long as water and nutrients are in adequate supply. To get high yields, leaves should expand to cover the ground surface as soon as possible after planting. With delay, solar radiation is wasted just heating up bare soil and evaporating soil moisture. Solar radiation is particularly important to yield during the period from late stem elongation to one week after anthesis (Z3.3 to 7.05). Low solar radiation accompanying hot weather during this period reduces the numbers of grain sites (p 8) and severely limits potential yield.

Has the sun been hidden by clouds or fogs?

Sterile spikes in boron-deficient soil. The spikes appear to be translucent but other parts of the crop will seem normal

KD Subedi

Can you do anything about solar radiation?

Acid or alkaline soils

Acidity or alkalinity is measured in pH units on a scale of 1 to 14 though the extreme values do not occur in agricultural soils. A pH 7 is neutral. Values from 7 to 4 are increasingly more acid and from 7 to 10 increasingly alkaline. Wheat grows best between pH 5.5 and 7.5, shown as a green zone in the figure, but can grow well beyond this range with additives to the soil.

This figure shows how soil pH affects nutrient availabiity. The green zone from pH 5.5 to 7.5 is most suitable for wheat. Narrower red bars mean less availability of the nutrient.

Figure based on Pratt (1965)

A main effect of too high or too low pH is that certain nutrients become too available and toxic to the crop while others become less available and show up as crop deficiencies (symptoms on p 52). In the figure the deficiencies are where the red bars are narrow.

In acid soils aluminium and manganese can become very soluble and toxic, but additionally, they reduce the plant’s ability to take up calcium, phosphorus, magnesium and molybdenum. Phosphorus in particular is unavailable to the plant in acid soils. If boron, copper and zinc are present in the soil, they too can become toxic at low pH. In medium alkaline soils boron, cop-per and zinc become deficient and phosphorus again becomes unavailable. Soil pH has relatively little direct effect on nitrogen.

Substituting acid-tolerant species for wheat can boost productivity in acid soils. How-ever, gains may be relatively short-term as these species can further acidify the soil until it becomes limiting for them also. Soil amelioration is a preferred approach.

Is your soil too acid or too alkaline?

Causes of extreme soil pH

• The soil is geologically very old and heavily leached, with high levels of aluminium and iron oxides. These soils are acid.

• Acidifying fertilizers have been applied to the soil for many years. These include those with ammonium nitrogen and superphosphate.

• Large amounts of organic matter have been added to a very wet soil over many years with resulting acidification.

• The soil is inherently alkaline being derived from limestone parent materials.

What you can do about acid or alkaline soils

Saline soils

All soils contain salts, but salinity becomes a problem only when certain salts concentrate in the crop’s rooting zone. Sodium chloride or table salt is sometimes the problem but other salts can be. Salt can destroy soil structure causing swelling of clays and dispersion of fine particles that then clog the pores in the soil through which oxygen and water move. It also encourages crust formation.

Salt on the soil surface

The key to control of salt in agricultural soils is to hold or leach the salt below the root zone. Keeping a net downward movement of water through the soil does this. Even slightly saline water can be used for leaching purposes. Problems occur when the direction of water flow reverses to an upward movement as occurs with rising water tables. Salt can also migrate upwards to the surface by capillarity, climbing slowly through the fine pores. This is a particular problem if the water table is already high and is salty, as the salt does not have far to creep to destroy the whole rooting zone. White salt crystals can be seen at the surface when the soil dries.

When plants grow in salty soils their growth is reduced; leaves and tillers do not expand. As salt accumulates in tissues it kills the leaves and eventually whole plants. The higher the salinity level the more rapidly effects become obvious and the lower the yield. The accompanying photograph shows poor growth and early death of plants in a salty patch in a wheat crop. Salt crystals are on the soil surface.

There are considerable variations in tolerance to salt. Bread wheat is more tolerant than durum wheat and than species such as rice and maize. Some varieties of bread wheat are more tolerant than others and plants filling their grain are more tolerant than seedlings (Rawson et al 1988).

A salty patch in wheat in Australia

Rana Munns

Sodic soils deserve a mention here. They are not saline as such but do contain relatively high levels of sodium. This causes them to be physically unstable, crumbling and cracking when dry and collapsing when wet. They set hard and are relatively impermeable to water so that run off is a problem carrying with it suspended clay, organic matter and nutrients. High sodium irrigation water should not be used if these soils are highly impermeable and care should be taken not to over water otherwise the soil will rapidly worsen (Russell 1961). One way to improve these alkaline soils is to add gypsum and use deep-rooted crops such as legumes and canola in the rotation to move the gypsum down the soil profile and improve drainage. Sulphur is also used to acidify the soil.

Is your soil salty?

Causes of soil salinity

• Your soil is inherently saline.

• Your irrigation water is saline and has been applied in too little quantity to flush the soil.

• Drainage is inadequate so that a net downward movement of water through the soil is not achieved.

• Too much irrigation water is used and this accumulates as a water table over a shallow impermeable subsoil.

• There is a high and rising water table lifting salt from depth

• High-transpiring, deep-rooted plants have been cleared from the vicinity allowing water tables to rise, bringing salt with them.

Salt causing yellowing of a crop and death of leaves in Haryana

H Gomez Macpherson

What you can do about salty soil

Management factors

Wheat is grown from the equator through to 60° latitude. It will yield over a wide range of temperatures rising as high as 40° C, though yields decline outside the ranges of its base and optimum temperatures (p 81). As temperatures rise closer to the optimum, management has to become more stringent, more timely and more precise for yield to be maintained. This is because the crop grows faster so daily demands on resources are greater. Any minor delays in providing the resources when the crop needs them, or minor delays in removing competitors for those resources, such as weeds and disease, will have big negative impacts on yield.

Crop yield begins at Day One, the day the seed is sown (see yield components in figure on p 8), but good management starts before that.

A uniform crop

AF van Herwaarden

Poor crop stands. Why?

Wheat has the useful growth habit that it tillers. A few plants can therefore produce many leaves in a relatively short time to exploit the resources of a land surface. This means that seedlings do not have to be spaced perfectly as tillers will eventually fill most of the gaps in a crop canopy. However, it takes longer to fill gaps by tillering than if all seedlings emerge at the same time and at uniform spacing, and time can be important to yield. Poor crop canopies are the major constraint to yield, particularly in warm areas (Ageeb 1994, Olugbemi 1994). Poor canopies result from one or more of the following: poor seedbeds (inadequate land preparation); seedbed too dry; poor seed (viability) and poor planting technique. Planting at the ‘wrong time’ can make these problems worse.

Land preparation

Preparing your land prior to sowing should have the following aims.

• It should create a soil structure that encourages the seedlings to emerge rapidly and uniformly and allows the young plants ready access to the vital resources of nutrients, water and aeration. Both full tillage (also called conventional or clean) and minimal tillage systems have this aim, but minimal tillage (p 84) limits soil disturbance to the surface layers or just a shallow slot for each crop row. Because minimal tillage often uses light machinery that can be taken into the field when the soil is wet, as perhaps after a previous rice crop, it has the prime advantage that it commonly shortens the time between successive crops/rotations. This can be critical for maximising yield per year.

• It should aim to incorporate any additives such as lime, compost, farmyard manure and chemicals for plant nutrition and pest control and, depending on location, may incorporate residues remaining from previous crops.

• It should control weeds, pests and diseases.

• It should aim to shape the land so that irrigation water can be applied efficiently and drained effectively and so that waterlogging is avoided or minimised. This may involve levelling, preparation of furrows, beds and so on.

Using a laser plane to level land in the Punjab of Pakistan

H Gomez Macpherson

If you have little or no equipment or have limited access to equipment for timely and quality land preparation, perfect seedbeds may not be possible, particularly if the farming system uses flood or furrow irrigation.

Drilling seeds directly into the stubble of the previous crop (zero tillage) in Pakistan

Hafiz Mujeeb

That system depends on multiple tillage operations and levelling, with construction of borders to channel and contain the irrigation water. It may be beneficial to use minimal or zero (no) tillage (p 84) or to spend time developing a permanent raised bed system (p 84). If raised beds are used, the process of forming the soil is more complex. Here the field is formed or reformed into long levelled raised beds, 60-80 cm wide with two or three defined planting rows, the beds separated by irrigation/drainage channels. However, in later years when the bed structure is well established and undamaged from the previous crop and residues/mulches are thick, only the predefined planting rows need to be cultivated, similar to minimal tillage farming on flat land.

A raised bed system in Mexico with two rows of seedlings on each bed

H Gomez Macpherson

Problems of land preparation

Clods resulting in uneven seedlings

AF van Herwaarden

Causes of poor seedbeds

• Cultivation occurred when the soil was too wet, resulting in clods.

• Cultivation occurred when the soil was too dry and/or there was too much secondary cultivation, causing loss of structure. If this is followed by excess rain or irrigation a surface crust will form (p 87).

• Soil structure is poor because of salinity or sodicity (p 33).

• Residues were not incorporated sufficiently or excesses were not removed prior to land preparation.

• The gap between cultivation and sowing was too long and the seedbed dried.

How to improve your seedbed

Seed viability

Is the seed poor?

Causes of poor seed

• Seed was stored at high temperature, high humidity or near chemicals. Chemicals diffusing from some manufactured woods and soft plastics can be a problem in unventilated stores.

• Seed for planting was harvested when the crop was very dry, leading to a high proportion of cracked and broken seed that is dead or, if alive, has too little reserves to grow to the soil surface.

• The seed was harvested too early and still retained some dormancy. If this is the case, the later seedlings to emerge will appear strong and healthy.

• Seed was infested with insects or disease.

What you can do about poor seed

NOTE: Do a germination test by counting out several lots of preferably 100 seeds taken from well inside the seed sacks keeping each seed lot separate. Dampen squares of paper or towelling and spread each group of seeds on a towel so the seeds are not touching each other. Cover them with a second damp paper towel. Roll up each sandwich of seeds and put in a plastic bag to prevent the towels from drying out. Keep the bags at room temperature. After 4-5 days count how many seeds have germinated in each lot. Percentage germination is the number of germinated seeds divided by the number of seeds in the sample x 100.

Planting depth and method

The shallower the planting depth, the sooner the seedlings can emerge and commence photosynthesis (p 89) and the earlier tillering can start. Healthy seedlings can be produced from seed laid on the soil surface. The correct planting depth is one that places the seed where it can imbibe water for germination but not desiccate thereafter. It is often necessary to plant deeper to protect from birds. Though seedlings of some genotypes can emerge from below 5 cm, this is too deep for many modern genotypes that have short coleoptiles. The plants in the photograph were sown on the same day but at different depths. It shows that at the same age, seedlings planted shallowly are larger than those emerging from depth. They have many more leaves (15 vs 5) that are shorter and broader, more main shoot leaves (5 vs 3) and more tillers (4 vs 1). Later, these differences will be reflected in spike numbers and yield.

Shallower sowing speeds seedling and increases tillering

M Stapper

Are the seedlings uniform?

Causes of planting and emergence problems

• Many of the seeds were dead before planting or had low vigour. Check all the causes under poor viability (p 39) and poor seedbeds (p 38).

• Too few seeds were planted. If broadcasting, many more seeds are required to allow for irregular distribution.

• Seeds were planted too deep for the genotype. i.e. coleoptiles were too short to reach the surface. Measure the length of the shoot between the seed and soil surface. Check this against the recommended depth for the variety.

• Seeds were planted into a seedbed that had dried out leading to uneven germination.

• Seeds were not uniformly distributed, perhaps hand broadcast by an inexperienced worker prior to covering with soil.

• Seeds were planted too shallow and dug up by birds or moved by insects.

• Sowing occurred too long after cultivation allowing weeds (p 58) to establish and compete with crop seedlings for resources.

• Heavy rain fell after sowing and the soil surface crusted.

What you can do to fix emergence problems

Optimum sowing time

There is an optimum time for sowing for every location determined primarily by weather and availability of the field and irrigation water, but also by the variety that is being used and the likely timing of serious disease in the area. The best sowing time is that which gives the highest yield within the local limitations. It is usually decided on by working backwards from the date that is best for anthesis (see dot points below). Once the best sowing time has been determined, any delays in sowing at that date can reduce yield. Yield loss from delays will generally be greater in hotter regions.

The most suitable variety will be one that fits its growth stages into the available time or growing season. When deciding on variety and calculating your sowing date you should bear in mind the following risks:

• Frost from spike emergence (Z5.1) through early grain filling should be avoid-ed.

• High temperature during anthesis and early grain filling is best avoided.

• Avoid very overcast and misty conditions during 2 weeks before to 1 week after anthesis - solar radiation should be high for this period.

• Irrigation water should be available from stem elongation through anthesis to early grain filling.

• Avoid varieties that run to head too quickly before producing tillers unless the available season demands a very short duration crop.

Think not only of the current crop. Work out the best compromise planting times for all sequential crops in the rotation so that annual production is optimised.

Optimum seed rate

Numbers of plants in a crop depends on seed rate, seed viability, percentage seedling emergence and plant survival. However, plant numbers can often vary widely without appreciably affecting yield in irrigated crops. This is because wheat plants produce tillers (p 90), which themselves can produce leaves, spikes and grain. So seed rate is commonly less critical to final yield than many of the other factors dealt with in this section.

This figure shows seed rate may have a small effect on yield because as plant numbers decline, each plant yields more

For example, in the study from Sudan shown in the figure, yields did not change much over a 15-fold range of seed rate, because at low seed rates each plant produced more tillers and spikes. But note that final yields were also not very high.

Recommendations for seed rates are usually between 100-150 kg/ha, which is higher than necessary but allows for losses due to poor preparation of seed beds, poor seed, and poor distribution of seed as occurs with hand broadcasting (p 37). Very high plant populations can encourage disease (Du Daiwen 1994) but do reduce the effects of weeds by competing better, a more important consideration in some areas. Some consider that very high seed rates lead to a higher chance of lodging (p 49).

The crop canopy: control of tillers and spikes

A good crop of 4 t/ha has to produce more than 400 spikes/m2 as, on average, each spike produces 1 g of grain. With normal sowing rates of 100 kg/ha (10 g/m2), which is 200 viable seeds/m, these spikes could all be from plants with a main shoot and one tiller. But main shoots have few leaves, so do not capture enough solar radiation early enough to produce a good crop (p 28). Using its tillers a plant can produce many leaves quickly. These in turn capture the radiation needed for rapid crop growth.

Wheat plants can produce a tiller in the axil of every leaf (drawings on p 9) and a tiller bud is ready to grow into a tiller shortly after its surrounding leaf is fully expanded. The first tiller usually appears in the axil of leaf 1 when there are 2.5 leaves on the main shoot (check this out for a normal crop on the following figure).

Any tillers can be missing from the normal sequence. If water or nitrogen are limiting at the time the tiller bud is ready to grow, or if light is very low, that tiller bud will not grow and will not be used by the plant. Missing tillers, by their position, tell when the problem occurred.

Once the developing spike on the main shoot of the plant reaches Z3.0, few further tillers will start; maximum shoot number per m2 has been reached (figure on p 8). If conditions have been poor until then, fewer shoots will have been produced.

Growing conditions after Z3.0 determine how many of the shoots or tillers survive to produce spikes. Even if you do everything right, some tillers will die, but with poor management many tillers will die without bearing a spike. Greater death means less interception of solar radiation during the important period leading up to anthesis. As importantly, it means less spikes and grains.

Is the crop producing enough tillers and spikes?

Ground cover for three crops estimated at 25%, 45% and 90%

AF van Herwaarden

from data of AF van Herwaarden

Causes of low tiller and spike populations

• Seeds were planted very deep. If the first one or two tillers are absent and the sub crown stem section is very long, this is the likely cause (compare the photos of shallow and deep sown plants on p 40).

• Basal fertilizers were inadequate to stimulate tillering. Check how much nitrogen was used and when it was applied. Pull up some plants and assess whether some tillers in the tillering sequence are absent in spite of correct sowing depth. An absent position will indicate the problem occurred when that tiller was ready to grow.

• Death of tillers has resulted in low spike numbers. Was nitrogen provided at node 1 (Z3.0)? Count spike numbers (p 13) and compare with earlier tiller counts. Did many tillers abort?

• Irrigations were inadequate (p 64) during tillering? Did the wilting score (see p 15) fall to or below 1 at the time of irrigations? Again check the timing of the problem by noting which tillers are absent or weak.

• Waterlogging occurred in association with hot weather during early tillering. Two or more days waterlogging will kill tillers if temperatures exceed 30° C.

• Solar radiation was low and it was hot. Were there several days of very over-cast or foggy weather during tillering?

• Stem borers, root rots (p 76) that reduce green area have suppressed tillering.

• A variety with genetically suppressed tillering may have been used.

• The seed rate was too high.

• Planting was late during rapidly rising temperatures with long days. This caused the crop to rush through to heading quickly (see p 81 for explanations) leaving no time for tillering (see photo on p 58).

What you can do to increase tiller and spike number

The crop canopy: green leaves after heading

The crop fills its grains by using carbohydrates from two sources, either produced freshly from photosynthesis (p 89) in the green leaves or from storage in the stem. Stored material alone can only produce a small grain yield of less than 1 t/ha, so for high yields it is important to keep several leaves green and active on each shoot during the grain filling process.

Growing grains need nitrogen as well as carbohydrates. The plant extracts little nitrogen from the soil after anthesis so the developing grains, which grow entirely after anthesis, have to get virtually all their nitrogen from storage in the plant. An important store is the green leaves.

A main reason that leaves lose their green colour and die is because their nitrogen is removed and redistributed to the grains. The less nitrogen stored, the more quickly the leaves die. How much is stored depends on how much was available in the soil before anthesis, as determined by management.

The figure indicates how many green leaves each shoot should have at each stage of grain growth to achieve yields ranging from 4 to 10 t/ha. Any diseases such as rusts and leaf blights (p 75) that reduce the number of green leaves and the green leaf area at this time also reduce yield. The field sheets (p 17) have a column for recording green leaf number.

Does the crop have enough green leaves after heading?

Causes of leaf death after heading

• In nutrient-poor soils, nitrogen was not applied at first node (Z3.0) or later.

• The crop is short of water or waterlogged.

• Leaf diseases, root rots or insect attacks have caused leaf death.

Making sure there are enough green leaves

Lodging

Lodging is when the crop falls over. A normal vertical crop is finely balanced, so anything that upsets the balance will cause it to lodge: strong winds, heavy rain, very wet soil during late grain filling, tall thin stems that bend, root or stem rots that weaken the plant base. Winds associated with excess water are the worst combination.

Lodging destroys the canopy structure. Solar radiation is no longer intercepted efficiently with high light to young upper leaves and low light to old leaves. Heads are covered in the tangle and the collapsed crop becomes more susceptible to pests and diseases.

Lodging during early stem elongation has a relatively small effect on yield as the crop will right itself and reform the canopy. The stem nodes alter the angle of extension making new growth vertical (see the nodes in the centre of the adjacent photograph).

Lodging in wheat. The worst lodged stems here have fallen about 60° from the vertical

HM Rawson

From anthesis onwards the effects of lodging are large. For every day that the crop is lodged yield declines by more than 1% (Stapper and Fischer 1990). So for crops severely lodged shortly after anthesis and remaining so, final yield may be less than half that of the upright crop.

Any lodging also makes harvesting more difficult and increases the likelihood of losing grain during harvesting.

Has your crop fallen over?

Why has the crop fallen over?

• There were high winds and heavy rain after stem extension began or it was windy while irrigations were in progress during grain filling.

• Excess nitrogen was applied resulting in too much top growth and top-heavy plants. Are other farmers having similar problems with different varieties?

• A tall, unimproved variety was used, unsuited to high nitrogen nutrition.

• Eyespot, root or crown rots (p 76) weakened the stem or base of the plant.

• Low potassium availability.

• The seed rate was too high, preventing tillering and resulting in weak stems.

• Machinery or animals in the crop caused physical damage.

What you can do to stop it falling over

Mineral nutrition

For best growth, wheat needs many nutrients, in particular, the macronutrients oxygen (O; about 48% of dry matter), carbon (C; 42%), hydrogen (H; 6%), nitrogen (N 2%), potassium (K), phosphorus (P), calcium, magnesium and sulphur. The amounts of each steadily accumulate in the crop as it grows but their concentrations decline (see figure) as the crop accrues old tissues. Old tissues have lower concentrations of nutrients than young ones. Wheat also needs very small amounts of the microelements iron, manganese, boron, zinc, copper, sodium, molybdenum, chlorine, cobalt, and silicon. Apart from the first 3 elements (O, C and H), which come from the air and water, the remaining 16 can be managed to some degree by soil or crop treatments.

The concentration of nutrients declines in the crop as a whole as it develops and accumulates more old tissues

Recommendations for applications of N, P and K differ widely with soil type and fertility and expected use of the fertilizer by the crop. A very short duration crop may not have sufficient time to use as much fertilizer as a long duration crop. Fertilizer policy must be based on local practice, your target yield and crop rotation. Bigger crops need more fertilizer (see discussion and graph on p 45). Be cautious though, an excess of nitrogen may result in ground water pollution and lodging (p 50). If nitrogen is limiting, yield and probably grain protein will be reduced. Applying nitrogen after spikes emerge generally increases grain protein.

As a rough guide, a 7-t/ha crop removes from the field as grain 150-190 kg N, 25-35 kg phosphate and 45-60 kg potassium. These nutrients must be returned to the soil after every big crop to avoid depleting reserves. Work out the approximate amounts removed for your target yield. For a 4-t/ha yield for example, about 150 x 4/7 kg N would be removed (85 kg N).

When plants do not receive enough of a nutrient to satisfy their requirements, or receive too much, they grow poorly and, if the imbalance is large enough, they show symptoms of the problem. Symptoms for most deficiencies or toxicities are generally most obvious on the leaves.

If a soil nutrient is being progressively depleted by the growing crop, and the nutrient can not be moved from older to newer leaves, the symptoms will be more apparent on young leaves. If the nutrient is mobile, the plant will extract it from the old leaves for use in the young leaves. Then the old leaves will show the symptom. Be careful with boron however, which shows no significant symptom on the leaves. Only at anthesis, when the sterile florets of heads gape, does the deficiency become apparent.

Be careful also that you do not confuse these symptoms with similar ones due to disease. If the cause is nutritional, symptoms will occur in large areas of the field. If it is disease, the symptoms are likely to be on isolated plants or in patches in the crop.

First check whether the symptoms are on older or younger leaves and then use the appropriate flow chart and photographs (old leaves p 54, young leaves p 56) to make a first-order identification of the problem (checklists and photographs are based on those in Grundon 1987).

Is mineral nutrition a problem?

Causes of nutrient problems

• Soil has a long history of heavy cropping without sufficient replacement of nutrients (not enough fertilizer was applied). Check the cropping history and fertilizer applications of this and earlier years. Calculate whether nutrients are likely to be limiting from the difference between nutrients removed in yields and fertilizer applied.

• Soil is low in organic matter (or insufficient fertilizer was applied).

• Fertilizer was lost due to leaching from heavy rain or over irrigation, run off or volatilisation or lost to competing weeds or an intercrop.

• Fertilizer was applied when the crop could not use it optimally. What was the crop stage (p 6) when the fertilizer was applied, its type and amount and was it broadcast or banded?

• Soil pH is such that certain nutrients were unavailable (p 30). Test soil pH. If it is less than 5.5, magnesium deficiency is possible and phosphorus may be unavailable. If greater than 8, deficiencies of zinc, iron, copper and boron are possible. Compare with assessments already reached from the photo keys.

• Waterlogging occurred from heavy rainfall, over irrigation and/or poor drain-age on heavy soil. Check for symptoms of waterlogging (p 68), soil type (p 15) and check rainfall and irrigations.

• Wheat or maize straw was used as a mulch or large amounts of residues were incorporated. Some reports suggest nitrate may be lost with this practice under high temperature (p 62).

• Soil is saline (see p 33).

• Check soil depth (p 17) as a plough pan or other restriction may be limiting roots to the upper soil profile so nutrients in deeper layers are unavailable.

What to do about nutrient problems

Symptoms on fully expanded and older leaves

Adapted from NJ Grundon

Symptoms on young leaves, recently expanded and expanding

Adapted from NJ Grundon

Weeds

Weeds compete with the crop for light, nutrients, water and root space. Some weeds can damage the crop by producing toxic substances or acting as hosts for diseases. Annual weeds compete most effectively with wheat during the seedling stages and early tillering. So this is the critical period for weed control. Once the crop is covering 50-70% of the soil surface at jointing, it will dominate most newly germinating weeds.

Early wheat sown too late to tiller dominated by broad leaved weeds

H Gomez Macpherson

Many selective herbicides are highly effective against weeds in the crop but they may cause some damage to the crop, as may manual or mechanical weed control methods. The likely yield loss from chemical or mechanical damage should be assessed against the yield loss from the weeds. Pre-emergence herbicides cause little or no damage so often result in better yields than later sprays.

Weeds can be a problem during harvesting; weed seeds can contaminate the grain and the green matter from late maturing weeds can contaminate the straw. Herbicides and desiccants can be used at this late stage, but only in particularly weedy situations and then with great care.

Conventional tillage if shallow and done hurriedly during seedbed preparation can exacerbate a weed problem. It can break up and spread the propagating root systems of some perennial weeds and bring buried seeds of annuals to the soil surface where they can germinate. Consider whether the option of minimum or zero tillage (direct drilling, p 84) may better suit your requirements in some years. If there is a well-established problem of perennial weeds (p 89), deep tillage to undercut and expose the deep propagating roots may be the first step towards control.

Wheat in Nepal completely suppressed by the grass weed Phalaris minor

KD Subedi

Is the crop weedy?

Check the crop within 2-3 weeks of crop emergence.

Causes of weed infestation

• Your wheat seed was contaminated with weed seeds. Always examine your seed before sowing. Machines used for harvesting and tillage are a source of weed seeds and should be cleaned before leaving each field.

• Weed removal by hand was not effective. Check on its timing and frequency.

• Herbicide application was ineffective. Check what herbicide was used, its concentration and whether there was rain or heavy dew during application. Were the manufacturer’s recommendations on the label followed?

• Monoculture and repeated use of herbicides with the same mode of action led to herbicide resistant weeds as reported in Malik et al. (1998) in South Asia.

• Perennial weeds propagating from broken roots or underground stem sections are difficult to control manually. Was a herbicide used? Is it effective against perennial weeds?

• Planting was delayed too long after seedbed preparation. Check the field sheets for dates of land preparation and planting. Did rain occur at this time encouraging weeds to germinate? Use minimum tillage (p 84) if a fast turn around between crops is essential.

• Each crop species has its own set of cultural practices that create niches for certain weeds. If the land has been used continuously for wheat, a bank of weed seeds may have built up in the soil. As some weed seeds can lie dormant for many years, the problem will continue annually for some time despite control methods.

What you can do about weeds

Late removal of weeds by hand is possible but hard

H. Gomez Macpherson

Crop Residues

Many farmers in Asia remove crop residues for livestock fodder and for cooking fuel. Others incorporate the residues during initial tillage operations or later, after they have been passed through animals to become farmyard manure. Burning is becoming more popular as it shortens turn-around time between crops. However, burning leads to reduced soil organic matter and degradation of soil physical properties, in turn increasing the likelihood of waterlogging, crusting and disease.

 

 

 

 

Felton et al 1987

Residues up to 9 t/ha can come from a preceding rice or maize crop and this amount completely covers the soil with a layer several centimetres in thickness. As shown in the photos, 3.5 t/ha of residues will just cover the soil surface when spread uniformly. Zero and minimum till equipment is available to sow through deeper layers of residues and still achieve uniform seedling emergence.

The positive attributes of crop residues should not be overlooked. Used as mulch they conserve water, keep the soil cool and free from crusting. They also increase soil microbial activity and maintain or increase populations of earthworms, which in turn increase soil aeration. Retained stubble reduces wind and water erosion and can be grazed by animals, though not when the soil is wet as this can lead to the formation of a soil pan. The negative attribute of crop residues is that if managed incorrectly in wet soils they increase the incidence of disease, which may reduce yield. Root rots such as Rhizoctonia (p 76) and Pythium species (p 80) can be a problem. Furthermore, if trash from the previous crop was full of weeds the incidence of weeds in the current crop will also be significant.

Are crop residues a problem?

Causes of residue problems

• Too great a thickness of residue remained on parts of the soil surface during seeding which the planting machinery could not penetrate. What method was used for planting and for spreading the residues from the previous crop? How thick were the residues?

• The stubble was diseased. Were there disease problems in the previous crop? Has the soil been wet for a long period, conditions that encourage take-all disease and Pythium and Rhizoctonia root rots? This may have shown as stunting and uneven height of the crop, poor tillering for the leaf number (p 43) and yellow leaves.

• The planting machinery was not suitable. Did it block frequently?

Doing good things with residues

A crop in Pakistan drilled directly into the residues of the preceding rice crop

H Gomez Macpherson

Irrigation timing and moisture stress

If you are irrigating your crop at the recommended intervals for your region and making frequent assessments of the water status of the crop using the wilting score (p 15) and using that score to confirm when you should irrigate, moisture stress should not become a problem. Irrigation frequency and amount will differ depending on many factors, but there are four stages additional to sowing when water should not be limiting. These are crown root initiation when tillering is starting (Z2.1 to Z2.2), the jointing stage (Z3.0), anthesis (Z5.0) and the grain milk stage (Z7.0). Of the four stages, tillering and anthesis are most sensitive to water stress.

Is moisture stress a problem?

A stressed crop rapidly loses potential yield. When a young crop has too little water its first response is to conserve what water there is by closing its stomata (p 90). These are small pores all over green surfaces that let water out and carbon dioxide in. Without carbon dioxide photosynthesis (p 89) stops and so the sugars it normally makes from the carbon are no longer available for growth. So growth stops. The first growth response to water shortage is that leaves stop expanding. The tiller buds that are ready to grow into tillers remain as dormant buds. Usually the main shoot struggles on with its development. If moisture remains limiting, the crop eventually runs out of time to produce leaves, tillers and spikes and the consequence is a thin canopy with few small spikes, few grains and low yield.

If moisture stress occurs after anthesis the grains are affected, as they are now the growing part of the crop. Again the stomata close, the leaves roll and die from oldest to youngest and the plant rushes to move as much material from storage into the grains to fill them as full as possible before everything dies. The consequence of post-anthesis stress (milk stage) is that grains are pinched, shrivelled and small.

Estimating how much soil moisture is available to the crop

You can work out how many days it will take before your crop is short of water if you do not irrigate or if it does not rain. If around 50% of the water that the soil can hold has already been used, the crop is probably already stressed and potential yield is declining. To make the calculation you need to use soil moisture content (p 16) for your soil textural class (p 15) and weather data. The following three step method is adapted from Lafitte (1994).

·Step 1. How much water does the non-stressed crop use each day?

The number you calculate in Step 1 approximates the amount of water you would need to apply as irrigation to replace each day’s loss.

Evaporative demand in mm/day (evapotranspiration) can be taken from Table 1 or weather data, or from your local pan evaporimeter (p 89).

Table 1. Evapotranspiration values for different environments (mm/day)

From FAO Irrigation and Drainage Paper 24
Use the larger value of each pair if the area around the crop has no transpiring vegetation

Table 2. Evapotranspiration Crop Coefficient for crops reaching 80-90% full ground cover by heading. Reduce the coefficients for crops with less ground cover*

* See p.6 for information on ground cover
Coefficients from Wright 1981

·Step 2. How much water in your soil is available for crop use before stress begins? (Calculate for the current rooting depth of the crop-see p 17.)

·Step 3. How many days can the crop go without irrigation or significant rainfall before stress begins?

This calculation may seem fairly difficult so try an example to give yourself confidence. Pretend your crop is at heading and growing on a clay-loam soil in the semiarid tropics where mean temperature is 20° C.

According to Table 1 the evapotranspiration for that environment is 7 mm water per day. Because the crop is at heading (Z3.0-Z6.8), it has a crop coefficient of 1.0 (Table 2, column 3) so is losing 7 mm water/day (i.e. 7 mm x 1.0, from Step 1).

Your soil is a clay loam (p 15) that forms a ball when you squeeze it, but the ball breaks easily. According to that description in the table on p 16 your soil contains 110 mm water. And assuming the crop roots reach down to 1 m all that 110 mm water is available for crop use (p 16).

The table on p 16 also shows that at 50% moisture, when stress becomes important, your soil holds 80 mm of available water, so the crop can use 110 - 80 = 30 mm before stress. At 7 mm per day water loss, you do not need to irrigate for 4 days (i.e. 30/7 days).

Is the crop short of water?

Causes of water shortage

• Irrigations were too far apart for the water holding capacity of the soil (p 16) and the demands of the crop.

• Rain may have fallen that was overestimated in its useful quantity because of incorrect measurement or run off.

• The roots of the crop may be restricted to a part of the soil profile by a plough pan, or shallow or compacted soil (p 17).

• The roots may be diseased with rots limiting water uptake (p 76).

• The soil or the irrigation water may be saline resulting in the crop limiting its water uptake (p 33).

• The soil may be poorly aerated/waterlogged (p 68) preventing water uptake.

• Irrigation water was not available when it was needed.

Solutions to crop water shortage

Waterlogging

Waterlogging occurs when the soil is saturated with water. Heavy soils are most likely waterlog. They have limited pore space through which water and air can move only slowly. If the soil is saturated for too long oxygen is used up. Then roots stop growing and absorbing nutrients, stomata close preventing photosynthesis (p 89) and soil denitrification commences. Because plants cannot absorb their nitrogen from the soil, they have to extract it from older leaves to support the growth of new leaves. During this extraction, old leaves become ‘nitrogen deficient’ and yellow during a period of waterlogging. Generally there is not enough nitrogen available in old leaves support new tiller growth, so tillering does not occur.

Nitrogen deficient leaves

HM Rawson

Wheat deteriorates rapidly in waterlogged soils if temperatures are high; seedlings within as little as 2 days. Later stages are more tolerant but can still lose a high proportion of their leaf area and yield. Waterlogging is avoided by ensuring that any water drains through the soil before it has time to stagnate. Wheat growing acceptably in mildly saline soils will not survive if waterlogging occurs.

Do you have too much water?

Causes of waterlogging

• The soil is inadequately drained. Is the seedbed above the level of the drain-age channels and are any tied ridges allowing water to drain away?

• The field is not level. Can you see that there are low spots? Are these areas of poor growth?

• Too much irrigation water has been applied which cannot drain sufficiently quickly. Check records for amounts and timing.

• The soil is naturally heavy with poor structure and inadequate pore space.

• Rainfall has been heavy. Was the irrigation regime altered to allow for this?

• After heavy rainfall waterlogging can occur even in light soils because crusting seals the soil surface and prevents air from entering.

What you can do about waterlogging

H Gomez Macpherson

Biotic factors

Above-ground insects

Worldwide hundreds of insect species feed on wheat. Some are adapted specifically to wheat and the conditions in which it has been grown historically. They are generally in balance with the system. But with the spread of wheat into new regions in recent years, particularly regions with hotter, wetter and more humid conditions than traditional areas, populations of many new and old pests have erupted into significant out-breaks. It will take time for predators to build up to control pest levels in these regions, and to develop varieties resistant to all pests. Until then, crop rotations, careful selection of planting time to put the crop out of step with the pest, the use of resistant varieties when they are available and chemical approaches to control are required. These break the cycles of the pests. This discussion will focus on a few of the problem insects in the new wheat zones. The pests described do not occur in all wheat areas.

Adult sawfly. Sawflies are a widespread and serious pest of wheat and barley

RM Miller

Sawfly damage

ICARDA collection

Are above ground insects a problem?

Mosquito-like hessian fly

RH Miller

Adult suni bug on a spike

RH Miller

Suni bug nymphs on a leaf

RH Miller

Flax-seed puparia of the hessian fly

RH Miller

Causes of insect problems

• Continuous wheat cropping or rotating wheat with alternative host crops allows insect populations to build up in the locality. What rotations are used in the area? Is it feasible to alter them to include a non-host crop?

• Are fallows used in the area? Were isolated self-sown wheat and barley plants rogued from these fallows? If not, these could be infested with Russian wheat aphid and be the source of the pest re-infecting cropped areas.

• Insects have been blown in or migrated from outside the region such as might occur with locusts. When were the symptoms first noticed and were they a problem last year?

What to do about insect problems

NOTE. Litsinger & Barrion, 1988 and Miller & Pike 2000 have more detail on the insects discussed and extensive information on other insects.

Oat bird-cherry aphid (Rhopalosiphum padi) and greenbug

RH Miller

Greenbug (Schizaphis gramium) proliferating on a leaf

RH Miller

Soil pests

Soil pests such as termites, ants, mole and field crickets, ground beetles, white grubs, wireworms, root aphids and root bugs usually pose a major problem only in dryland agriculture. Under irrigation crop growth is faster and more regular and so the crop has greater capacity to grow through any damage by pests. Tillage also protects the crop from soil pests. It exposes soil pests to dessication and disrupts their food sup-plies while regular flooding disturbs their habitat. Removal or incorporation of crop residues further limits habitat and food resources. Under minimum tillage or direct drill practices some of the soil pests may build up over the years.

What to do about soil pests

If problems do arise, try the following after the pest has been identified

Diseases

There are many diseases that attack wheat some causing seedlings to die even before emergence (bare patch) or shortly thereafter (damping-off). Infections can continue throughout crop growth and impacts be seen right through to harvest.

Fungal diseases are the most common and widespread diseases. They have distinctive symptoms that can appear on spikes as well as on leaves and stems when the infection is severe. Symptoms on leaves range from small, infected patches (necrotic lesions) to discolouration and premature drying of whole leaves. Flat spots and blotches that progressively increase in size and spread from lower to upper leaves are generally due to fungal diseases that are transmitted in stubble or splashed up by water. These include septoria and tan spot. Rust spores cause protruding dusty spots or streaks on the surface of leaves, stems and spikes. The colour of the spots or pustules indicates the type of rust present. Rust spores leave a coloured smear on fingers when touched.

White heads in wheat

HM Rawson

Appearance of whitened spikes in the crop commonly indicates the presence of root rot pathogens such as take all, a Fusarium disease or Bipolaris sorokiniana. These white heads are completely dead having had their vascular links to water and nutrients completely severed by the root or crown rot disease. The plants have probably been diseased for some time prior to heading. Symptoms associated with seed and soil-borne diseases also usually become apparent on the crop after heading. Smut diseases for example are seed borne and very common in areas where farmers recycle their own seed.

Considering the diversity of wheat diseases, it is critical to keep an eye open for problems right from the beginning. Loss of plants and loss of green leaf area reduce the potential yield of a crop (p 44). Many of the problems are seed borne, while others are already in the soil at planting, carried over from the previous crop or in residues, or even associated with perennial weeds (p 89). Conditions that are less than optimal for the crop are often better for disease infections. Poor aeration, poor nutrition and water logging are examples. However, dense crop canopies necessary for highest yields are also perfect for rapid spread of diseases (and pests). Consequently, good management with regard to disease is a continuous balancing act. It starts before the crop is sown with a clean field and choice of the most disease resistant wheat variety. It demands vigilance right through until harvest as the crop is continually exposed to new disease threats as it develops. Finally, it requires thought and action about the crop rotations best able to prevent build up of diseases across seasons.

Are leaf diseases a problem?

Symptoms associated with leaf diseases are sometimes similar to those resulting from nutritional imbalance. However, many of the following symptoms cannot be confused.

Yellow rust on leaf and crop but not on a row of a resistant variety

A Yahyaoui

Stem rust

A Yahyaoui

Leaf rust

A Yahyaoui

Septoria blotch

A Yahyaoui

Are root diseases a problem?

Before Z1.3 (seedlings)

Are there sections of crop where plants are missing or seedling leaves look discoloured or weak? Carefully dig up seedlings and wash out their roots. Are the roots stunted, are they discoloured, either brown or grey?

Around Z3.3 (jointing)

Check for poor plant vigour in the crop looking particularly for defined patches of low vigour. Are any leaves yellow? Dig up plants and wash out their roots. Are roots stunted with discoloured, sharp tips?

Cereal cyst nematode causes roots to appear

Poor roots due to take all

Crown rot causes crown stems with internal pink colour

A Yahyaoui

Around Z5.5 to Z6.5 (heading to anthesis)

Are some spikes completely white and dead? Take all or crown rot causes this problem. The disease will by this stage have completely destroyed the conducting system of the stems, thus killing and whitening the spikes. See above for earlier symptoms. Severe frost can produce similar symptoms, but then large continuous sections of the crop will be affected.

Other root rots are prevalent in South Asia, particularly when conditions are warm. Bipolaris sorokiniana causes brown necrotic roots and brown subcrown internodes. The root rot Sclerotium rolfsii is also prevalent.

Seedlings made brown by Bipolaris

A. Yahyaoui

Head scab - pink spikelets are arrowed

A. Yahyaoui

Ear with bunt - arrows point to the bunt balls

A. Yahyaoui

Causes of plant diseases

• Resistant varieties were not used. Some resistance is available to the rusts, to septoria, to cereal cyst (Heterodera spp) and stem nematodes (Pratylenchus spp) and to a lesser extent to crown and root rots. Check what variety was planted. Were other varieties planted in the area less affected? If so, were the farming methods also different?

• Infected seeds were planted. Was black point apparent in the seed sample? Where did the seed come from? Was it from last season’s crop on that field or purchased from a seed merchant? Absence of black point is unfortunately no insurance that the seed is clean of Bipolaris sorokiniana.

• Diseased standing crop residues were not removed prior to planting in a disease prone area, or an alternate host is present close by (grass species harbour yellow rust while non-grass species harbour leaf and stem rust). Look for remains of crop residues and other plants that could host the disease.

• No seed treatment or an inappropriate treatment was applied. Get details of the seed treatment and check whether it is an acceptable treatment for the disease identified.

• The soil was waterlogged during the seedling stages slowing emergence and early growth. Check the soil type and look for evidence of poor drainage. Was there heavy rain during seedling growth?

• Was wheat planted after wheat or after barley? Is the wheat crop next to a field of corn?

Solutions to disease

Rusts & powdery mildew

Smuts

Blotch diseases (Septoriaspp, tan spot)

Root rot disease-complex