The rapid development of intensive seafarming methods has led to an increase in the number of diseases that attack commercially cultivated Laminaria plants. Diseases have occasionally been widespread, at times threatening large reductions in yields in various regions of China.
Chinese scientists have done a great deal of research on the symptoms, causes and cures of Laminaria diseases. Many preventive measures and methods of disease control have been discovered, though much further research remains to be done.
In this chapter we will look at diseases that affect both sporelings during cultivation in seedling stations and maturing sporophytes in grow–out stages during raft culture. Table 9.1 shows some of the most common diseases affecting Laminaria together with a brief description of their causes.
|- green rot disease||poor illumination|
|- white rot disease||change in transparency + insufficient nutrients|
|- blister disease||freshwater mixing with seawater after heavy rainfalls|
|- twisted blade disease||excessive illumination|
|- malformation diseases||hydrogen sulfide + sulfate- reducing and saprophytic bacteria, e.g. Macrococcus|
|- sporeling detachment disease||decomposing Pseudomonas bacteria|
|- twisted frond disease||mycoplasm–like organisms|
Table 9.1. Some of the most common diseases affecting commercial Laminaria production and their causes.
Diseases affecting Laminaria can be divided into two broad categories: (a) environmental diseases and (b) pathogenic diseases. The former conditions are caused by adverse environmental factors, such as poor illumination, fluctuations in seawater transparency accompanied by a deficiency of nutrient salts, or mixing of seawater with freshwater after heavy rainfalls. The latter infections are caused by pathogenic agents, such as bacterial and fungal or mycoplasm–like organisms.
Symptoms: The apical part of the kelp frond turns greenish and becomes soft, these symptoms gradually spreading to the lower part of the frond. Serious advancement of the disease eventually results in decay and death of the entire plant.
Causes: The disease is caused by insufficient illumination. It generally occurs in kelp plants that are attached to the lower ends of hanging culture ropes.
Cures: The disease can be cured: (a) by raising culture ropes, and (b) by periodically reversing culture ropes.
Symptoms: The kelp blade exhibits fading, turning in colour from brown to yellowish and finally to white. The disease spreads from the apical part to the lower part of the blade. Eventually the whole frond decays and drops from the culture rope.
Causes: The disease usually occurs in April or May in regions where seawater transparency suddenly increases and where dissolved levels of nitrogen/nitrates in the seawater are insufficient for the needs of rapid kelp growth. Injury of intracellular structures blocks photosynthesizing capability. With loss of pigmentation, fronds turn white and die.
Cures: The disease can be cured: (a) by applying nitrogen-based fertilizers over the seafarming area, and (b) by lowering kelp culture ropes to decrease illumination.
Symptoms: Blisters appear on different parts of the frond. Decay spots also appear following eruption of the blisters.
Causes: The disease is caused by a sudden decrease in salinity due to mixing of rainwater with seawater. The disease often appears after heavy rainfalls and generally occurs in shallow bays which are vulnerable to salinity changes caused by freshwater run-off.
Control: The disease can be controlled by lowering culture ropes below the mass of freshwater run-off which, because of its lower specific gravity than seawater, tends to float to the surface.
Symptoms: Blades show extensive twisting. Blade edges develop convolutions or wrinkles and are crisp and hard rather than pliable. In serious cases, twisting fronds develop spiral-like turns, accompanied by extensive fading of colouration, rotting and disintegration along edges and tips. The disease typically affects young and robust sporophytes during mid-to-late stages of grow-out.
Cause: Excessive illumination causes this condition.
Prevention and control: Steps should be taken to lessen light intensity. The number of raft floats may be decreased to lower horizontal culture ropes in the seawater. Hanging culture ropes should also be lowered 1–2 m. All adjustments should take into consideration water transparency as a factor affecting light intensity.
Symptoms: This disease infects sporelings raised under artificial conditions in seedling stations. Not infrequently sporelings in a seedling station suddenly die within a few days. Sometimes virtually the entire crop of sporelings is destroyed. The symptoms of this disease vary considerably in the different stages of sporeling growth, causing what appear to be different diseases, such as “gametophyte metamorphosis disease” and “young sporeling rot and malformation disease”. These diseases are in fact manifestations of the same infection.
(i) Gametophyte metamorphosis disease: Typical morphological symptoms include very slow gametophyte growth, thickening of cell walls, degeneration of chloroplasts with pigment change to a yellowish colour, and vacuolar contraction with shrinking of the cell protoplasm. In serious cases, lipophanerosis occurs, where chloroplasts and protoplasm form transparent or dark droplets in the center of the cell.
(ii) Death of the egg: The infected egg shrinks inside the oogonium, unable to extrude and attach to the lip of the oogonium in preparation for fertilization. As the disease advances, the egg further contracts and gradually dies. Some eggs may extrude but, because of pronounced loss of vigour, fail to attach and drop off from the apical lip of the oogonium.
(iii) Rot and malformation of the young sporeling: One or a group of cells expands abnormally to between 3–6 times larger than normal cells. Division of these enlarged cells occurs abnormally, leading to very disordered cell arrangement and thus deforming the shape of the sporeling. Grape-like malformed sporelings may be composed of from several to a hundred cells. The deformation is irreversible. Infected sporelings are unable to recover to a normal shape and gradually die.
Causes: The symptoms appearing in the different stages of gametophyte and sporeling growth have two main causes: (a) tannic acid poisoning due to improper treatment of substrate materials, and (b) infection due to an anaerobic sulfate-reducing bacterium which produces toxic hydrogen sulfide gas as a metabolic byproduct.
Poor treatment of substrate ropes: If the palm rope used for making sporeling substrates is poorly treated before use, lethal concentrations of tannic acid may exude into the culture tanks and poison gametophytes. This problem can be prevented only by carefully treating palm ropes before use, through soaking, hammering and boiling procedures which remove toxic chemicals. (Substrate treatment procedures are described in Chapter III).
Presence of hydrogen sulfide (H2S): During ovulation, when the egg is being extruded from the oogonium, gametophytes are very sensitive to low concentrations of dissolved hydrogen sulfide gas. A concentration as low as .005 ml/L will cause shrinkage of eggs in oogonia. A concentration of .018 ml/L will terminate ovulation in 50% of gametophyte plants. Zygotes that are successfully fertilized exhibit symptoms that include concave plasmolysis, appearance of a large vacuole and abnormal cell divisions leading to sporeling malformation. Fig. 9.1 shows effects of varying concentrations of H2S on formation of zoospores in sporangial cells, on gametophyte development and on sporeling development.
Hydrogen sulfide is produced as one of the metabolic byproducts of sulfate–reducing bacteria and certain saprophytic bacteria. It can be inferred that these bacteria are the main cause of the malformation disease. The bacteria are known to thrive in two critically important locations: (a) in cast iron pipes used for making the indoor water supply system in seedling-rearing stations, and (b) in rotted spots on parent Laminaria blades used for spore collection.
Fig. 9.1. Effects of varying H2S concentration on zoospores, gametophytes and sporelings.
The sulfate–reducing bacteria multiply in old rusting cast iron pipes such as those used for making pipelines in seedling stations. If hydrochloric acid is added to the blackened puffy rust areas observed in these pipes, hydrogen sulfide gas is given off. The black rust is the byproduct of anaerobic corrosion of iron in the presence of sulfate–reducing bacteria, such as Macrococcus sp.
The bacteria are also found in the rotted spots of parent Laminaria blades used for spore collection. The collected zoospores immediately attach to the palm ropes and thus infect the substrate materials. Under favourable conditions (at 10–12° C), bacteria imbedded in the palm rope substrates multiply quickly, emitting hydrogen sulfide which causes the sporeling malformation disease.
Preventive measures: There are two main measures for preventing outbreak of this disease: (a) The indoor water circulation system should be sterilized with bleaching powder before spore collection proceeds. (b) The mature sporophyte cultivation system (tanks used to collect spores from parent Laminaria) should be separated from the sporeling cultivation system (tanks used to cultivate young sporelings). Additionally, great care should be taken to remove any rotted spots on parent Laminaria blades used for spore collection purposes.
Control measures: If abnormal growth of sporeling plants is observed in the seedling-rearing station, the following control measures should be taken: a) clean all sporeling substrates and culture tanks, b) change all seawater in the indoor water system, c) reverse flush the seawater filtration tanks, and d) lower the circulating water temperature below 10° C.
Mature parent Laminaria sporophytes used for collecting zoospores on substrates in seedling stations harbour numerous kinds of bacteria, including sulfate–reducing bacteria. During spore collection the seedling station may be contaminated with these bacteria which, if they multiply in the sporeling culture tanks, will cause the sporeling malformation disease.
Preventive measures can reduce the likelihood of this source of contamination. Preventive measures focus on health characteristics of the parent Laminaria stock. The bacteria grow most prolifically in rotting spots on the parent Laminaria blades. The extent of rotting spots on blades, in turn, depends on various biological and environmental factors, especially age of fronds and seawater temperature. Older fronds having reached full maturity tend to decompose. Higher seawater temperatures result in greater deterioration of kelp blades. Both factors contribute to the rotting of fronds which, in turn, increases contamination of parent Laminaria with sulfate–reducing bacteria.
Other injuries to parent Laminaria fronds will also increase the level of bacterial contamination, for example injuries sustained during transport from the raft culture site to the seedling–rearing station, or injuries sustained during the drying stimulation procedure. Reducing the probability of contaminating the seedling station with sulfate–reducing bacteria, therefore, consists in taking steps to reduce the above–mentioned origins and causes of bacterial contamination. Since parent Laminaria are the “carriers” of the bacterial infection, the following preventive measures may be taken to reduce the likelihood of contaminating the seedling–rearing station:
Parent Laminaria fronds selected for spore production should not be overly–mature.
If zoospores are collected from parent Laminaria stock growing naturally in seawater, zoospore collection should take place before the seawater temperature rises above 21° C.
The natural seawater temperature at the time that selected parent Laminaria stock are transferred to the seedling–rearing station, for spore collection, should not exceed 21° C.
Injury of parent Laminaria fronds should be avoided during transportation to the seedling–rearing station.
Parent Laminaria plants should be protected from high temperatures, direct exposure to sunlight, excessive water loss and physical injury that may be suffered during prolonged drying stimulation. The stimulation period should be as short as possible and plants should be kept well–shaded during the procedure.
Filtered seawater should be used to clean the parent Laminaria fronds over and over again before placing them in indoor culture tanks for spore collection. This reduces excessive exudation of mucus and thus reduces the level of bacterial contamination.
Symptoms: Symptoms vary somewhat for this disease, though common symptoms are detachment of sporelings from culture ropes, rotting of the stipe and the blade tip, and abnormal growth of holdfast rhizoids from the sides of the stipe.
In one set of symptoms, sporelings drop from culture ropes because of infected holdfasts. Holdfast development is abnormal with the appearance of numerous rhizoidal outgrowths on the stipe. These false roots intertwine, with holdfast growths on stipes of adjacent kelp plants criss–crossing and interlocking. Stipe ends grow very thin. Blade cells enlarge, are loosely compacted and gradually wither away. The apical blade tip becomes curved and growth in length is very slow. Holdfasts are hard, brittle and weakened. In advanced stages sporelings drop from the culture mats. The disease is most prevalent in the early development of sporelings.
In a similar set of symptoms, the kelp stipe rots away, resulting in falling–off of sporelings from the culture mats. First the stipe becomes soft and its colouration lightens and fades. Blades may continue to appear normal. Some sporelings develop rhizoidal holdfast outgrowths from the stipe, some do not. The disease often appears during the middle and later stages of sporeling growth in the seedling station, when seedlings are 0.3– 1.0 cm in length. Its occurrence is widespread, threatening seedling–rearing enterprises throughout China.
Causes: Experiments have shown that there are two causes of these symptoms: (a) the bacterium Pseudomonas, and (b) excessive illumination. The first is a pathogenic cause, the second an environmental cause.
Cause A: Pseudomonas is an alginic acid decomposing bacterium which causes decay of holdfasts by the enzymatic action of alginase, finally resulting in the detachment and loss of sporelings. Generally, alginic acid decomposing bacteria easily contaminate cultured sporelings in seedling stations. Like other bacterial infections, Pseudomonas can be introduced into culture tanks and sporeling substrates from infected rotting spots on parent Laminaria fronds used for spore collection. The highest density of Pseudomonas bacteria has been found in the foam near water inlets to culture tanks, where concentrations may reach 107 bacteria/ml.
Preventive measures: As with other potential infections carried by parent Laminaria, precautions can be taken to reduce the spread of infection to the seedling station. At end of season, culture tanks should be cleaned many times. Before spore collection begins in the new season, all culture tanks should be scrubbed and the water supply system should be disinfected with bleaching powder. All rope substrate materials should be carefully disinfected by repeated soaking, boiling and hammering to eliminate toxic substances and bacterial infections. Tanks used for sporeling cultivation should be kept separate from tanks used for sporeling culture. Any rotten spots of decaying tissue observed on parent Laminaria fronds should be excised and discarded. Parent fronds should be washed repeatedly in newly-filtered seawater before placing them in spore–collecting tanks. Density of spores attaching to substrate materials should not be allowed to exceed 30–50 spores per field area viewed under 100x magnification. Finally, the seawater temperature in the indoor water supply system should be lowered to reduce the spread of microbial infections. Towards the end of September, for example, water temperature can be lowered to between 4.0–4.5° C. Using these various management techniques, bacterial infections can be prevented, reduced, or controlled.
Control measures: If this bacterial infection reaches serious levels, its spread may be contained, somewhat, by leaving the substrate materials undisturbed. I.e. cleaning of substrate mats should be avoided. Instead, water in the culture tanks should be stirred with moderate force 6–8 times daily and 3–4 times nightly. In this way gaseous exchange is improved, conditions favourable to bacterial infection are disturbed and kelp plant metabolism may be reinvigorated, thereby increasing resistance to the disease.
Cause B: Excessive illumination is another cause of holdfast loosening and detachment of sporelings. The consequences may bear resemblance to a bacterial infection, when in fact this is a physiological–environmental problem. Photophobic holdfasts loosen from culture ropes in excessive illumination, resulting in heavy loss of sporelings. The problem is especially acute during late stages of sporeling development in the seedling station.
Preventive measures: Light intensity in the culture room should be monitored carefully and steps taken to reduce excessive illumination. Fluctuations in both light intensity and light periodicity should be recorded to establish regularity of changing patterns. This enables appropriate adjustments of the moveable curtains to control illumination in the culture room.
Symptoms: The disease affects maturing sporophyte plants during grow–out on culture rafts at sea. There are three characteristic symptoms: (a) coarsened, swollen and hollowed stipes, (b) twisted or spiral–shaped fronds, and (c) withered or shortened holdfasts. The frond surface feels rough or coarse. Plants develop thick holdfasts with little branching. Some stipes are divided into two parts joining the blade to the holdfast (Fig. 9.2b, c). Other stipes appear unaffected (Fig. 9.2a). The disease has been very destructive in the past, first causing serious damage to the kelp industry around Dalian in northern China in 1973, when 85% of plants under cultivation were infected, resulting in a production loss of 3,000 tons dry weight for the region. The disease has not recurred with such devastating effects over the past 15 years.
Causes: Twisted frond disease often occurs in shallow inner bay areas where tide and current action are relatively calm and where current flow measures less than 10 cm/sec. Nutrient levels in such regions are often insufficient for kelp growth. Light penetration may be impeded by high turbidity. Often seafarmers increase the length of culture ropes beyond the normal 2.5 m and raise stocking density far above the region's carrying capacity. All of these factors, which contribute to the degeneration of environmental conditions required for Laminaria production, seem to play a role in causing this pathogenic disease.
The pathogen, itself, has been observed. Tissue examination by electron microscopy has revealed the presence of numerous polymorphic mycoplasma–like organisms, mostly coccoid, some ovoid, dumbbell and amoeboid in shape. The disease is thus biotic in nature rather than being caused solely by environmental factors. It is contagious, taking hold after a long latent period of about 60–70 days. Experiments have demonstrated that the disease can be effectively controlled using tetracycline antibiotics, confirming its microbial nature.
Prevention and control measures: All infected kelp plants with swollen stipes and twisted fronds should be removed from culture ropes at the raft site. Appearance of the disease is closely correlated with deterioration of environmental factors. Therefore preventive measures consist in improving ecological conditions at seafarming sites. Water depth of culture ropes should be adjusted to about 2.5 m and culture density should not exceed the carrying capacity of the region. Cultivation should be undertaken in areas where water–flow is greater than 10 cm/sec, thereby ensuring adequate gaseous exchange and nutrient levels for kelp growth. Distances between rafts should be increased to lower intensity of cultivation. Fertilizer additives may need to be applied to the seafarming region.
Fig. 9.2. Characteristic symptoms of the swollen stipe / twisted frond disease.
a: thick holdfast b–c: divided stipe d–e: twisted fronds
This disease has been identified only recently. It is a serious problem which occasionally causes high mortality of sporelings in seedling–rearing stations. The disease progresses quickly so that within only a few days entire sporeling production runs may be suddenly destroyed.
Symptoms: Pigmentation in chloroplasts lightens and fades. Cell plasma shrinks and cellular arrangement in tissues is disordered. Sporelings turn greenish yellow and quickly die.
Causes: Experiments have shown that seawater pollution is the main cause of this disease. A strong correlation has been drawn between the occurrence of red tides and outbreaks of this infection (Table 9.2). The disease typically appears on the second day of a red tide. Seawater intake into the culture room of the seedling station is apparently contaminated by an infection closely associated with red tides.
Prevention and control: It is presently unknown whether this condition/disease is caused by biotic pathogens or by abiotic factors (water pollution). Possibly an as yet unidentified pathogen is responsible for the infection. Correlation with red tides is significant. When the disease is identified in the seedling station, all sporeling mats should be spray–cleaned immediately, the seawater supply should be completely changed and filtration tanks should be cleaned by reverse flushing.
Prevention mainly entails avoiding intake of polluted seawater, especially after red tides. Technically this means that settling tanks may have to be enlarged and that a 30 cm layer of activated carbon may have to be added to the filtration tanks to adsorb all incoming pollutants. Trace amounts of copper ions and other metals, both near the main seawater inlet and in the indoor water supply system, should be determined.