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Pathogen Biology
Leptosphaeria maculans was first described by Tode in 1791, who reported blackleg on dead cabbage stems and named the pathogen Sphaeria lingam. In 1849, Desmazieres found the same fungus on living plants and transferred it to the genus Phoma (Phoma lingam). Smith, in 1956, first discovered the perfect (sexual) stage of P. lingam and named it Leptosphaeria napi. In 1964, Smith, Sutton, Boerema and van Kesteren changed the name to L. maculans.
Sexual reproduction
Leptosphaeria maculans produces pseudothecia on the decaying stems and leaves of infected plants (Figure 12). These black, globose fruiting bodies with protruding ostioles 300-500µm in diameter, are immersed in the host tissue, and become erumpent when mature (Figure 13). The pseudothecia contain cylindrical to clavate asci. Eight ascospores are encapsulated within each bitunicate ascus (Figure 14). The ascospores are biseriate (in two rows) and have a cylindrical to ellipsoidal shape and mostly rounded ends (Figure 15). They are a yellow- brown color, with 5 transverse septa, and guttulate (containing small oil droplets). The fungus is heterothallic, i.e. two different mating types must be present for sexual reproduction to occur.
 Figure 12 |
 Figure 13 |
 Figure 14 |
 Figure 15 |
Asexual reproduction
Two types of pycnidia are found in diseased tissues. Type I (sclerotioid form) are immersed in the host tissue and become exposed in groups. The pycnidia lack a definite shape, have narrow ostioles, and have walls composed of several layers of thick-walled sclerenchymatous cells. Type II pycnidia are globose and black with a wall composed of several layers of cells. Only the cells in the outer layer are thick-walled. Pycnidiospores (conidia) are unicellular, hyaline (colorless) spores, cylindrical and straight, although a few may be curved. There is one guttule (oil droplet) at each end of the spore. These asexual spores are produced in pink to amethyst ooze under moist conditions (Figure 16).
 Figure 16 |
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by The American Phytopathological Society