Pseudocercospora fijiensis

Black tsigatoka is a leaf-spot disease of banana plants caused by the ascomycete fungus Mycosphaerella fijiensis (Morelet). Also known as black leaf streak, it was discovered in 1963 and named for its similarities with the yellow sigatoka, which is caused by Mycosphaerella musicola (Mulder), which was itself named after the Sigatoka Valley in Fiji, where an outbreak of this disease reached epidemic proportions from 1912 to 1923. According to new terminology, the Tsigatoka disease complex is a cluster of three closely related fungi—yellow sigatoka (Pseudocercospora musae), eumusae leaf spot (Ps. eumusae), and black sigatoka (Ps. fijiensis). Plants with leaves damaged by the disease may have up to 50% lower yield of fruit, and control can take up to 50 sprays a year. M. fijiensis reproduces both sexually and asexually, and both conidia and ascospores are important in its dispersal. The conidia are mainly waterborne for short distances, while ascospores are carried by wind to more remote places (the distances being limited by their susceptibility to ultraviolet light). Over 60 distinct strains with different pathogenetic potentials have been isolated. To better understand the mechanisms of its variability, projects to understand the genetic diversity of M. fijiensis have been initiated. When spores of M. fijiensis are deposited on a susceptible banana leaf, they germinate within three hours if the humidity is high or a film of water is present. The optimal temperature for germination of the conidia is 27 °C (81 °F). The germ tube grows epiphytically over the epidermis for two to three days before penetrating the leaf by a stoma. Once inside the leaf, the invasive hypha forms a vesicle and fine hyphae grow through the mesophyll layers into an air chamber. More hyphae then grow into the palisade tissue and continue on into other air chambers, eventually emerging through stomata in the streak that has developed. Further epiphytic growth occurs before the re-entry of the hypha into the leaf through another stoma repeats the process. The optimal conditions for M. fijiensis as compared with M. musicola are a higher temperatures and higher relative humidity, and the whole disease cycle is much faster in M. fijiensis. Black Sigatoka is also known as Black Leaf Streak, and is found mostly on the banana crop. The pathogen Mycosphaerella fijiensis causes streaks that run parallel to the leaves. It is an ascomycete fungus that affects banana trees specifically in tropical climates; including Asia, West Africa, China, and South America. Tropical weather is the preferred climate to grow banana trees, but it is also the environment that the pathogen thrives in, hot, humid, with plenty of rainfall to aid in dispersal. The optimal environment of the pathogen would be similar to that of the banana tree. The fungus infects mature banana leaves and will continue to cause infection without proper control. In the early stages of the infection of the plant, the lesions start off looking rusty brown and they appear to be faint, paint-like specks on the leaves. They become more visible on the undersides of the banana leaf as the lesions and leaves grow. The spots on the undersides of leaf are the fungus itself. The sign of the pathogen consists of the ascocarp which holds the ascospores used for dissemination to infect healthy new plants when the environment is conducive. The pathogen then survives on dead plant tissue as mycelium. The dimensions of the lesions are characteristically 20 x 2mm with a well defined wall surrounding it. After further development, they become darker, sink into the leaf, and turn into depressions. The depressions themselves and the chlorosis surrounding them are the visible symptoms of the plant pathogen. They eventually will merge, and causing the rapid decline of plant morphological and physiological functions. Leaves with a large infectious lesions will start to degrade and collapse off the branches because the leaf spots interrupt the plant’s ability to perform photosynthesis, leading to the ultimate death of the plant. Yellow Leaf Streak is in the same genus as Black Leaf Streak that affects banana trees as well. Yellow Leaf Streak in contrast has smaller, yellow-green lesions, and appear on the top of the leaves.