Fungous diseases of plants . anada. It is not en-tirely absent from regions muchfarther south and west, but insuch districts it seldom assumesany importance. In Europe itis widely distributed and maybe disastrous throughout GreatBritain, as well as east and westfrom Russia to Erance, extend-ing even as far south as Italy. The distribution of this fungus andits importance as a disease organism are entirely dependent uponclimatic conditions. It has been shown that it becomes of seri-ous importance only when favored by warm, moist weather. Asa rule, the fungus does not appear in the northeastern
Fungous diseases of plants . anada. It is not en-tirely absent from regions muchfarther south and west, but insuch districts it seldom assumesany importance. In Europe itis widely distributed and maybe disastrous throughout GreatBritain, as well as east and westfrom Russia to Erance, extend-ing even as far south as Italy. The distribution of this fungus andits importance as a disease organism are entirely dependent uponclimatic conditions. It has been shown that it becomes of seri-ous importance only when favored by warm, moist weather. Asa rule, the fungus does not appear in the northeastern UnitedStiites prior to the last days of July, and it is most abundantduring August and early September. A few days of rainyweather suffice to give it a start and to bring to fruiting theconidial stage on the leaves. The distribution of the fungusis then accomplished with alarming rapidity, and whole fieldsmay be devastated within a period covering only a few daysof such weather. While it is generally stated that warm weather. Fig. Phytophthora on Potato(Photograph by F. C. Stewart) PHYCOMYCETES 167 is required, it has also been shown that the high temperature ofsummer quickly checks its spread. Symptoms. Upon the leaves of the potato this fungus de-velops characteristic spots which cannot be easily confused withother potato diseases. These spots frequently begin at the edgeor tip and spread until the whole leaf may be involved. Theypresent in moist weather a dark,somewhat water-soaked appear-ance with slightly purplish tint(Fig. 60). In drier weather theyare brown without the definitemarkings of the early moist appearance of thespots accompanied by the wilt-ing of the leaf, or of that por-tion affected, offers an easydiagnosis. Generally there isno accompanying stem injury,but in some cases the troublemay extend to the stem; or,again, it may be found uponthe leaves as an extension of astem affection. Upon the tubersthis fungus develops the well-known dry r
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