. Fungous diseases of plants, with chapters on physiology, culture methods and technique . ion Blight. Cornell Agl. Exp. Sta. Built. 218 : I-17. 1904. The onion mildew, blight, or mold is a disease which hasbeen recognized for more than half a century. At various timessince 1884 it has been reported as of consequence in parts ofthe United States from New England to Wisconsin. It is proba-bly far more common and destructive than has been supposed, asshown by the observations of Whetzel in 1903. The disease, in the regions referred to, appears in late June orJuly, and in the early
. Fungous diseases of plants, with chapters on physiology, culture methods and technique . ion Blight. Cornell Agl. Exp. Sta. Built. 218 : I-17. 1904. The onion mildew, blight, or mold is a disease which hasbeen recognized for more than half a century. At various timessince 1884 it has been reported as of consequence in parts ofthe United States from New England to Wisconsin. It is proba-bly far more common and destructive than has been supposed, asshown by the observations of Whetzel in 1903. The disease, in the regions referred to, appears in late June orJuly, and in the early morning while covered with dew it is inyoung stages conspicuous by a furry violet appearance of theaffected leaves. Later the leaves become moldy in character, pale,collapsed, and often broken. Fig. 57 shows a diseased plant in PHYCOMYCETES 163 an acute stage. Older onions are apparently more susceptiblethan young, and recovery in the former case is seldom. The fungus. The mycelium is considerable, and it penetratespractically all parts of the leaf. The minute haustoria are numerous, \. Fig. 57. Onjon Mildew(Photograph by H. ) Fig. 58. Mature Coxidiophore, Germinat-ing CONIDIUM, AND MYCELIUM OF OnIONPeronospora. {c after Whetzel) thread-like, and often branched at the tip. The conidiophoresarise through the stomates. They are of the characteristic type,often 320/X in height, and bear large elliptical conidia (44-52 x22-26/z) which germinate promptly by a side tube and effectpenetration through the stomates. The time required for infec-tion and the production of conidiophores again is extremely 164 FUNGOUS DISEASES OF PLANTS short, so that the fungus spreads with great rapidity. Oosporesare commonly produced. Control. It would seem that this fungus has been controlledin New York by systematically spraying where it is likely to beabundant after June 15. In addition, however, it is important todestroy the tops of diseased plants, and by no means to returnthese to the land
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