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This article uses the following two articles:
Black Leaf Streak Disease, Pests and Diseases of American Samoa Number 10, American Samoa Community College Community & Natural Resources Cooperative Research & Extension, 2004
Black Sigatoka An Increasing Thread to Banana Cultivation, Douglas H. Marín, Del Monte Fresh Produce; Ronald A. Romero, Chiquita Brands; Mauricio Guzmán, National Banana Corporation of Costa Rica; Turner B. Sutton, North Carolina State University, Raleigh
Plants affected: Definite symptoms of Black sigatoka have only been recorded on Musa (Bananas), Musa paradisiaca (Plantains), Musa acuminata (Wild banana) and Musa acuminata (subsp. bantesii and subsp. zebrina). Cultivars differ in their reaction to the pathogen. Immunity is not known and it is possible that other wild species and subspecies of Musa are infected, but the disease does not develop significantly. Mycosphaerella fijiensis may also attack the seedling stage of wild bananas.
Season of occurrence: Black sigatoka spores form readily during tropical and sub-tropical summers or under conditions of high humidity and rainfall, especially if there is a film of free water on the leaves. The principle means of spread is through rainwash or splash of spores but later in the development of the disease spores are also forcibly discharged into air currents. Infection occurs on the youngest leaves of the plant during and immediately after unfurling. Older leaves are not readily infected. Obvious visual symptoms appear about 15-20 days after initial infection.
Symptoms: The first apparent symptoms of leaf infection are short, reddish-brown lines about 2 mm long. The lines turn into brown streaks 20-30 mm long (Fig. 2a) which broaden into elliptical spots (Fig. 2b), brown on the underside of the leaf, black on the upper surface. The spots are slightly sunken and surrounded by a yellowish halo. Next, the center of the large spot becomes gray, bordered by a black line and yellow halo (Fig. 2c). Finally, the spots merge, killing the leaf (Fig. 2d).
Fig. 2 a) - d) Symptoms of M. fijiensis on Banana Leafs
Ascospores are produced in pseudothecia in mature lesions, which are common on the older leaves of the plant or in dead leaves lying on the ground. Meredith and Lawrence (14) reported that pseudothecia are produced on both sides of the leaf surface, but higher numbers are present on the adaxial surface. Conversely, Gauhl et al. (6) found that more pseudothecia and ascospores were produced on the abaxial leaf surface. Although Burt et al. (2) found that approximately 4.5 ascospores are released per pseudothecium, this number seems very low considering the high concentration of inoculum that is present in the air following rain (4).
Ascospore release requires the presence of a film of water from rain or dew that imbibes the pseudothecia and results in the forcible ejection of the ascospores through the leaf boundary layer, where they are disseminated by air currents (15,20). Maturation of pseudothecia requires saturation of the dead leaf tissues for approximately 48 h (3,4,7,10,11,15). Under Hawaiian conditions, ascospore concentrations increased during the night, were highest about 0600 h, and decreased significantly during the day. On rainy days, peak concentrations occurred shortly after rain began. Seasonal increases in daily mean concentrations of ascospores are associated with increased rainfall and relative humidity (15). Gauhl (4), working with bananas, reported similar results under Costa Rican conditions. Whereas Meredith et al. (15) did not find evidence that ascospore production or release was affected by minimum temperatures, Gauhl (4) determined that there is a reduction in the production of inoculum during the drier (or less rainy) months of the year in the Caribbean zone of Costa Rica, which is also the season with the lowest temperatures. Ascospores are dispersed by wind; however, long-distance dispersal is limited to a few hundred kilometers due to their susceptibility to ultraviolet radiation (17). A consistent relationship between ascospore discharge and disease development has not been shown, thus limiting the usefulness of spore trapping for disease forecasting (5,11). Ascospores are deposited mainly on the lower leaf surface during the unfurling of a new leaf (Fig. 8A and , producing a band pattern of infections on the side that is first exposed, which is a reflection of the increased spore deposition on the cylindrical candela leaf during its unfurling as opposed to the entire open lamina of the leaf. Consequently, most infections occur on the abaxial surface of the leaves (3,7).
Latent period: Although conidia can be produced in lesions exhibiting early symptoms of M. fijiensis and contribute to the epidemic, the latent period is defined by the time the fungus starts to produce lesions with mature pseudothecia and ascospores, which are the main source of inoculum. Like the incubation period, the latent period also varies according to weather conditions, susceptibility of the host, and intensity of infections. Differences in the latent period from December 1993 to May 1995 for the susceptible cultivar Grande Naine, which is widely used for the fresh banana market, are shown in Figure 10. The latent period ranged from 25 days during the rainy season (June to December) to 70 days during the dry season at Guapiles, Costa Rica. When the weather is highly conducive for ascospore discharge and infection, many infections occur on the leaves. When infections are dense, they rapidly coalesce at a very early stage of development, accelerating the appearance of mature spots that are characterized by the presence of pseudothecia and ascospores (22). Under these conditions, leaves are rapidly and severely damaged. The latent period also varies according to the level of resistance. For instance, the time from leaf emergence to first mature spot symptom under the same natural conditions for the cultivar Curraré, a cooking banana belonging to the subgroup plantain, was 44 days compared with 34 days for the cultivar Valery, a banana belonging to the subgroup Cavendish (4). The term “disease development time” is widely used in the banana literature in the tropics to refer to the latent period, and it is defined as the time between infection and the formation of mature spots. Another common term used in the banana literature is “symptom evolution time” or “the transition period,” which is the time from first symptoms to the appearance of maturespots (6). The symptom evolution time gives a good indication of how fast the disease is progressing on the leaves.
|Table of Content:|
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