WHAT IS BLACK SIGATOKA
Mycosphaerella fijiensis is an ascomycete fungus under the order Capnodiales, of the class Dothidiomycetes. It is haploid and hemibiotrophic, with name M. fijiensis specifically referring to the sexual telemorph stage producing ascospores, and the synonym Pseudocercospora fijiensis applied to the asexual anamorph stage which produces conidia. Relatively fewer conidia are formed by the fungus, and ascospores are mainly responsible for dispersal. Sexual reproduction of the fungus on the banana involves development of erumpent, pear-like spermatogonium that releases large numbers of small, cylindrical spermatia on the abaxial surface of the leaf. The spermatia fertilizes neighboring female hyphae or trichogynes (Bennett & Ameson, 2003) to form globose pseudothecia that are 47μm-85μm in diameter. Dark brown ostioles of the pseudothecia are recognizable above the leaf surface. The pseudothecia contain bitunicate, obclavate asci with no sterile elements or paraphyses. The asci produce at least 12.5μm by 2.5μm by 3.8μm ascospores that are two-celled, colorless, and constricted at the septum. Ascospores can travel long distances and can only germinate under conditions of high humidity. Asexual reproduction, on the other hand, is through pale brown, at least 16.5μm by 4μm single- to six- celled conidiophores that are cylindrical, straight to geniculate, and occasionally branched. These conidiophores bear cylindrical, six- to eight- celled conidia (ed. Ploetz, 2003). Conidia are pale to light olive-brown and taper from base to tip (Bennett & Ameson, 2003). The presence of basal scars on the conidia also distinguishes M. fijiensis (ed. Henderson, 2006). Conidia are mainly responsible for local spreading and disease development during dry seasons. Both conidia and ascospores may travel by means of wind, rainwash, or animals (Marin, et.al, 2003).
Infection by the black Sigatoka fungus is through penetration of stomata by germinated ascospores or conidia, at high humidity and preferably at 27ºC. M. fijiensis first grows epiphytically for 2-3 days before entering the banana leaf by means of a hydrotropic response involving stomatopodia. The resulting infection hyphae form a substomatal vesicle upon entering the leaf. Fine hyphae grow through mesophyll tissue and airchambers for about a month, with no visible leaf pathogenesis. After development of a streak on the surface of the banana leaf, fungal hyphae re-emerge, grow epiphytically, and penetrate adjacent stomata (ed. Henderson, 2006).
Contact between hyphae and mesophyll cells after two months coincide with Stage 2 symptoms. After 41 days the burn stage appears (ed. Henderson, 2003). M. fijiensis conidia are visible upon formation of red-brown streaks, while perithicia and ascospores are observable two weeks after the appearance of conidia (Bennett & Ameson, 2003). The whole leaf dies in 3-4 weeks after necrosis (IMA, 2004).
Symptoms caused by BLS disease were hypothesized to involve pathogen phytotoxic compounds. Phytotoxic metabolites found in studies of Mycosphaerella fijiensis culture filtrates were 2,4,8-trihydroxytetralone, 5-hydroxy-1,4-naphthalenedione (juglone), 2-carboxy-3-hydroxycinnamic acid, dimethyl ester of 2-carboxy-3-methoxycinnamic acid, isoochranic acid, 4-hydroxycytalone, and fijiensin specific only to bananas (Busogoro, 2000). Busogoro and colleagues (2002) were able to demonstrate that juglone directly inhibited the electron transfer properties of purified banana chloroplasts, a mode of action responsible for toxicity. Other alternative mechanisms of action may include depletion of NADH and NADPH, and partial deprivation of antioxidants in banana leaf tissue by the auto-oxidative properties of the pathogen's naphthoquinone metabolites such as juglone.
It was found that optimum sporulation if cultured M. fijiensis occurred in SDA and V8 plates at 25ºC for 10 days in darkness and 5 days in continuous light (Hanada, et.al, 2002). Puch-Ceh, et.al (2005) evaluated the growth of M. fijiensis in eleven liquid media under two lighting and aeration conditions in order to determine the best culture conditions for high phytotoxicity and yield of organic crude extracts. They discovered that neither banana infusion nor any of the media containing banana infusion showed fungal growth, which suggested the presence of antifungal phytoalexins in the banana infusion. The strongest phytotoxic activities were observed in the filtrates of the V8 juice medium-shaken cultures and the Czapek-Dox-still cultures kept under light/dark conditions, but the shaken V8 cultures kept under light/dark conditions had a higher yield of organic extracts. Thus V8 broth that is shaken was chosen as the optimum medium and condition for culturing M. fijiensis.
Various methods of control are available for getting rid of the fungus that causes black Sigatoka. However, it is still a feared opponent particularly due its penchant for fungicide resistance.
CONTROL OF BLACK SIGATOKA
Farmers who are in charge of cultivating bananas know how persistent black Sigatoka can be in damaging acres of the herb. Black Sigatoka's versatility makes it a veritable foe of all plantation owners, particularly of small-holder farmers. The disease is still rampant in unfortunate areas.
Although its demise has been planned by researchers in current years, the fungus has not been completely wiped out due to its talents in resistance. Over the decades it has acquired some defense over popular commercial fungicides, a grave problem faced in banana-producing countries today.
Plantations control the spread of black Sigatoka by a variety of methods. Normally these are in conjunction with traditional ways of preventing diseases in plants. Fungicide is frequently applied in areas that have succumbed to the disease, marking no-tolerance zones in areas where there used to be some fungal activity. Affected leaves are carefully removed without contact with the rest of the plants in the hectare, to isolate the disease to a single region of a plant.
As a precaution, herbs are planted with adequate spacing in between "individuals" (although most bananas in a plantation are of one genotype). Pools of water where spores could rest during their journey of infection are minimized by building drains that sufficiently collect and channel water to a region far from the banana plantation.
The methods mentioned, while simple to do due to their mechanistic processes, require loose finances to produce intensive labor. Such practices are ideal for implementation in areas that cannot afford spraying.
Large banana plantations that participate in the export market respond differently to the threat of black Sigatoka. The managers of these areas for cultivation generally resort to application of fungicides by aircraft. Small farms on the other hand, make use of backpack sprayers or tractors that are loaded with the fungus-eliminating material.
These control measures are said to collect large percentages (up to 20%) of the final retail price of banana imports. Small-holder farmers are naturally at a disadvantage when diseases such as black Sigatoka come a-calling, naturally since the cost of plane maintenance and spray materials are prohibitive to those with low incomes.
Chemically, the dreaded fungus is attacked by a slew of different types of fungicides: triazole, benzimidazole, mancozeb, clorothalonil, etc. These are generally categorized as either systemic or protectant. The former two mentioned belong to the systemic grouping, which acts by inhibiting M. fijiensis while inside the leaf. The latter two are spread on leaf surfaces to prohibit fungal spore germination and penetration of the fungus into the leaf.
Black Sigatoka has been shown to snub certain systemic fungicides, particularly benomyl and propiconazole. Central American populations of the fungi were guilty of this resistance, contributing to the enforcement of around 30 applications of fungicide in a single year for some areas in the world.
To quell the growing frustration with the rebellious nature of the fungus, banana caretakers turned to back-and-forth applications of systemic and protectant fungicides for help. Morpholine and strobilurin fungicides represent the common substances fighting on the side of the systemic category, while clorothalonil is used in water emulsion as protectant. Modern methods of destroying black Sigatoka employ mixtures of systemics and protectants applied with petroleum as base.
Mancozeb, another protectant fungicide, took precedence in some regions of Asia as an effective deterrent to M. fijiensis. However, issues have been raised as to the safety of this substance on the environment as well as on the health of the workers.
While mancozeb is not in itself severely toxic, the primary metabolite that results from its degradation, ethylenethiourea, has been a cause of concern since it was shown to possess carcinogenic effects in test animals, and to have potential mobility in soil. Another metabolite, carbon disulfide, is a cholinesterase inhibitor that affects the nervous system to induce fatigue, headache, blurred vision, and nausea, all symptoms brought forth by complainants of aerial spraying.
Those exposed to high levels of carbon disulfide have been reported to experience convulsions, slurred speech, confusion, and slowing heartbeat. As a result, mancozeb is listed by the State of California as carcinogenic to humans.
The good news for banana plantations is that recently organic fungicides that leave no residue and toxic effects have been introduced in the market. The use of such fungicides is not yet common.
Another method of controlling M. fijiensis is through biological control measures. However, this has been largely ignored due to the disease's polycyclic nature and the availability of highly effective fungicides (Bennett & Ameson, 2003).
It has been said that the only practical method of control is the culture of resistant varieties. This is especially important for small-scale and subsistence growers who often cannot afford chemicals. Existing resistant varieties are: Yangambi km5; wild banana species M. acuminata burmannica, M. acuminata malaccensis, and M. acuminata siamea; diploid cultivars "Paka" and "Pisang lilin"; and the wild diploid Calcutta 4. Breeding for black sigatoka-resistant varieties are the agenda of various international research groups. Resistant breeds however, do not cater to consumption needs such as flavor or taste. Also, conventional breeding of export-quality bananas such as Cavendish (highly susceptible to black Sigatoka) for resistance is not possible because of the sterility of the females in all of the group's cultivars.
Thus the banana has a ruthless enemy in this dimorphic fungus, which is even worse in its effects compared to yellow Sigatoka.
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