Fungous diseases of plants . isease there is a well-defined spot with slightly raised bor-der ; but when the spots becomenumerous on a leaf, the latter beginsto turn yellow, and subsequently thefungus develops abundantly its conid-iophores in indefinite areas, thus giv-ing the characteristic ashen or velvetyspots of indiscriminate form. Whena leaf becomes seriously injured itwilts and dries. The conidia are thenproduced in quantity over the wholesurface, particularly during muggydays; thus the dead leaves increasemany times the chances of further in-fection. This disease does not usuallyappear
Fungous diseases of plants . isease there is a well-defined spot with slightly raised bor-der ; but when the spots becomenumerous on a leaf, the latter beginsto turn yellow, and subsequently thefungus develops abundantly its conid-iophores in indefinite areas, thus giv-ing the characteristic ashen or velvetyspots of indiscriminate form. Whena leaf becomes seriously injured itwilts and dries. The conidia are thenproduced in quantity over the wholesurface, particularly during muggydays; thus the dead leaves increasemany times the chances of further in-fection. This disease does not usuallyappear late in the season, being fre-quently followed by the late blight(Scptoria Petroselini var. Apii) with which it has no genetic con-nection. This fungus also occurs on cultivated and wild parsnip{Pastinaca sativd) and other related plants. The fungus. The conidiophores and conidia of this Cercosporaare in no way particularly characteristic. The conidiophores andspores are variable in size, depending upon the conditions under. Fig. 145. Cercospora Apir: Ab-normal Fruiting in Culture FUNGI IMPERFECTI 313 which produced ; the former measure in extreme cases 50-150 x4-5 iJL, and the spores, 50-280 x 4-5/-. They attain the maximumsize with both high humidity and temperature. The spores retaintheir vitahty for many months at least. Pure cultures of this fun-gus may be readily secured by the poured plate method, and themycelium grows well upon bean stems and other media. In suchcultures the conidiophores are most peculiar. They may attain alength of a millimeter (Fig. 145). Conidia may be produced andabscised for a time, leaving the customary geniculation ; thenwhen the hyphae are longer, conidia-like branches arise, which re-main attached, and eventually serve as tme branches of perma-nent hyphae. The mycelium, like that of the other Cercosporae, isolivaceous ; but the colonies show minor peculiarities distinguish-ing them from other species which have been thus cultivated. Control.
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