Diseases of onion and garlic (
Purple blotch
Purple blotch is caused by the fungus Alternaria porri. The fungus causes small water-soaked lesions on leaves or seed stalks that rapidly develop white centers. As the lesions grow in size, they become zonate and brown to purple in color. The margin of the lesion is often a shade of red or purple and is surrounded by a yellow zone. In moist weather, the surface of the lesion may be covered with brown to dark gray masses of spores. Seed stalks, flowers of seed onions and, at harvest, bulbs may become infected. Purple blotch lesions may be invaded by another fungus, Stemphylium vesicarium, and turn black with spore masses of this fungus. Stemphylium also can cause lesions on onion by itself.
Disease progression requires plenty of moisture from rain or dew. Susceptibility to infection is higher on older leaves when compared to younger leaves and is higher on leaves infested with onion thrips compared to leaves free of this insect. Mycelium in plant debris can survive between seasons.
Cultural control strategies include reducing plant density and promoting good
field drainage to reduce hours of leaf wetness. Once disease occurs
in a field, long rotations are recommended. Preventive application of
fungicides such as chlorothalonil, iprodione, mancozeb, maneb, and vinclozolin
can reduce the severity of purple blotch. Alternate applications
of different fungicides is advised to prevent development of resistance.
February 10, 2003