CHAPTER 5: DECAY AND INSECT CONTROL - 1

The first line of defense against insects and disease is good management during production. Planting resistant varieties, the use of irrigation practices that do not wet the leaves or flowers of plants, avoiding over-fertilization with nitrogen, and pruning during production to reduce canopy overgrowth can all serve to reduce produce decay before and after harvest. A second important defense is careful harvesting and preparation for market in the field. Thirdly, sorting out damaged or decaying produce will limit contamination of the remaining, healthy produce. Yet, even when the greatest care is taken, sometimes produce must be treated to control insects or decay-causing organisms.

While high humidity in the storage environment is important for maintenance of high quality produce, any free water on the surface of commodities can enhance germination and penetration by pathogens. When cold commodities are removed from storage and left at higher ambient temperatures, moisture from the surrounding warm air condenses on the colder product's surfaces. A temporary increase in ventilation rate (using a fan) or increasing exposure of the commodity to drier air can help to evaporate the condensed moisture and to reduce the chances of infection.

Control of storage insects in nuts and dried fruits and vegetables can be achieved by freezing, cold storage (less than 5°C or 41°F), heat treatments, or the exclusion of oxygen (0.5% or lower) using nitrogen. Packaging in insect-proof containers is needed to prevent subsequent insect infestation.

Some plant materials are useful as natural pesticides. Cassava leaves are known to protect harvested cassava roots from pests when used as packing material in boxes or bags during transport and short-term storage. It is thought that the leaves release cyanogens, which are toxic to insects (Aiyer, 1978). The ashes of the leaves of Lantana spp. and Ochroma logopur have been found to be very effective when used as a dust against aphids attacking stored potatoes (CIP, 1982). The pesticidal properties of the seeds of the neem tree (as an oil or aqueous extract) are becoming more widely known and used throughout the world. Native to India , neem acts as a powerful pesticide on food crops but appears to be completely non-toxic to humans, mammals and beneficial insects (NRC, 1992). Any "natural pesticide" must be shown to be safe for humans before its approval by regulatory authorities.

Chemical controls

Washing produce with chlorinated water can prevent decay caused by bacteria, mold and yeasts on the surface of produce. Calcium hypochlorite (powder) and sodium hypochlorite (liquid) are inexpensive and widely available. The effectiveness of the treatment will be decreased if organic matter is allowed to build up in the wash water. The effectiveness of chlorine increases as pH is reduced from pH 11 to pH 8, but at lower pH chlorine becomes unstable.

Fruits and vegetables can be washed with hypochlorite solution (25 ppm available chlorine for two minutes) then rinsed to control bacterial decay. Alternatively, these commodities can be dipped in hypochlorite solution (50 to 70 ppm available chlorine) then rinsed with tap water for control of bacteria, yeasts and molds.

Sources: Ogawa, J.M. and Manji, B.T. 1984. in: Moline , H.E. (Ed). Postharvest Pathology of Fruits and Vegetables. University of California , Division of Agriculture and Natural Resources, UC Bulletin 1914.

Sources: Bachman, J. and Earles, R. 2000. Postharvest Handling of Fruits and Vegetables. Horticulture Technical Note. ATTRA.

For a list of registered pesticides in the USA , see the EPA Compendium of Registered Pesticides published by the U.S. Environmental Protection Agency (http://epa.gov )

Sulfur:

Sulfur is used on bananas as a paste (0.1 % active ingredient) to control crown rot fungi.

Sulfur dioxide:

SO₂ is used as a fumigant (with a 10 ppm residue tolerance) on grapes to control Botrytis, Rhizopus and Aspergillus fungi. Careful calculation of the amount of sulfur dioxide required to treat grapes can greatly reduce the need to vent or scrub the storage air after fumigation to remove excess SO₂. For information on the "total utilization" fumigation technique that has been developed for treating grapes with sulfur dioxide, see Luvisi (1992).

Sodium or potassium bisulfite:

Bisulfites are usually contained within a paper or plastic pad that can be placed inside a carton to release SO₂ for control of molds on grapes during transportation and storage.

Sources: Luvisi, D.A. et al. 1992. Sulfur Dioxide Fumigation of Table Grapes. University of California , Division of Agriculture and Natural Resources, Bulletin 1932.
Ogawa, J.M. and Manji, B.T. 1984. in: Moline , H.E. (Ed). Postharvest Pathology of Fruits and Vegetables. University of California , Division of Agriculture and Natural Resources, UC Bulletin 1914.

The use of bicarbonate salts for prevention of postharvest decay has been used on fresh peppers, melons, potatoes, carrots and citrus fruits. These salts are very inexpensive, safe to use, readily available and accepted as “certified organic” and “chemical free” for marketing purposes.

Bicarbonate salts include:

Bicarbonate of soda, or “baking soda” (NaHCO₃)

Potassium bicarbonate (KHCO₃ )

Application methods (with or without chlorination as desired):

Sources: Smilanick, J. 2002 (personal communication) Research Plant Pathologist, USDA ARS San Joaquin Valley Agricultural Sciences Center .

Bacterial soft rot (Erwinia) of cabbage can be controlled by using lime powder or a 15% solution of alum (15g aluminum potassium sulfate in 100ml of water). After treatment of the butt-end of the cabbage heads, the produce should be allowed to dry for 20 to 30 minutes before packing.

Applying alum solution (spray or brush on):

Applying lime powder (press butt-end into powder):

Sources: Borromeo, E.S. and I lag, L.L. 1984. Alum and Lime Applications: Potential Postharvest Control of Cabbage Soft Rot. Appropriate Postharvest Technology 1(1):10-12.

On occasions when fungicides must be applied to produce, a simple tray with holes punched in the bottom can be used hold the commodity while it is sprayed. In the illustration below, a hand-operated knapsack sprayer is used to spray fungicides on bananas to the stage of run-off. The bananas can then dry in the perforated tray before further handling.

Sources: FAO. 1989. Prevention of Postharvest Food Losses: Fruits. Vegetables and Root Crops. A Training Manual. Rome : UNFAO. 157 pp.

A back-pack sprayer is a useful tool for applying fungicides and other postharvest chemical treatments. A wide variety of sizes and types of sprayers can be purchased at farm supply centers or via companies on the internet.

When fruit is packed for export, fungicides are often applied to meet the requirements of international quality standards and to reduce deterioration during transport. The "cascade applicator" illustrated below was developed to apply fungicide uniformly and effectively by using a liquid curtain to drench the fruit.

Fruit in a perforated plastic tray is introduced on a roller conveyor belt (not shown) into the applicator. Inside a simple fan shaped deflector creates a curtain of liquid fungicide. The fruit passes under the curtain where it is drenched, then out of the applicator to drain on a tilted return tray. The tank holds up to 50 liters of fungicide solution, and a pump is mounted at the level of the tank outlet. A filter is fitted on the top of the tank to remove foreign matter from the return flow of fungicide from the applicator box and the return tray.

Fungicide Applicator:

Sources: Overseas Div., AFRC. National Institute of Agricultural Engineering. 1974. Bulletin No. 6. Silsoe, Bedfordshire , England .

Vase life of Cut Flowers

Some simple chemical treatments and handling practices can be used to prevent wilting and decay when handling and marketing fresh cut flowers, and can help cut flowers last much longer in the vase at home. If you are selling cut flowers, you could give a printout listing some of these tips to your customers.

• Start with a clean bucket or vase (washed with hot soapy water and rinsed thoroughly).
• Gently remove lower leaves so there will be no leaves in the bucket or vase water.
• Re-cut the stems, removing one or two inches at an angle under water.

• Use de-mineralized water if the water in your area is “hard” (creates white deposits on faucets) or has been “softened” (full of salts).
• Use warm to hot water (40 C or 100 F).
• Use one of the following solutions. They contain acid to improve water flow, sugar to help buds open and a preservative to reduce growth of fungi and bacteria.
  1. mix 1 part lemon-lime soda with 3 parts warm water. Do not use diet sodas. Add ¼ teaspoon of household bleach per quart of water.
  2. Use 2 tablespoons of lemon juice or lemon juice concentrate, 1 tablespoon of sugar and ¼ teaspoon of bleach in a quart of warm water. Add another ¼ teaspoon of bleach to the vase every 4 days.
  3. Use a commercial floral preservative. These are inexpensive and convenient but may not be as effective as the two recipes provided above.
• If you are using florist foam to arrange flowers, let it soak in the solution until it sinks on its own. Pushing it down into the vase will cause air bubbles to remain in the foam and can cause early flower death.

Sources: Hesketh, K.A. et al. No date. Extending the life of cut flowers in your home. University of California Cooperative Extension DANR No. Publication 7169.