Feb., 1907.] Phytophthora and Plasmopora. 83 



The data at hand indicate that when we have cyclar periods 

 of low summer temperatures we may anticipate one or more 

 seasons of Phytophthora outbreaks. A single season, or a single 

 month scarcely controls; abundant moisture alone may not de- 

 termine, as is shown in contrasting the years 1905 and 1906 in 

 Ohio. In 1905 the July rainfall was 3.93 inches; that for August 

 4.46 inches, while in 1906 the July rainfall was greater, 5.14 

 inches, and that for August 4.72 inches; however, in 1906 the 

 August temperatures rose above the optimum, the mean being 

 74.6° F. 



It will be well at all times to bear in mind that hot and cold 

 are relative terms; our mean summer climate is above the op- 

 timum for Phytophthora injestans — so that in Ohio the seasons 

 that are below normal are the ones which favor the fungus. In 

 Maine, Nova Scotia, New Brunswick and Ireland, this may not 

 be relatively the case. The mean summer isotherm of 70° F. 

 crosses Northeastern Ohio, in an irregular line, entering the state 

 in southern Columbiana county and emerging at Vermillion 

 Erie County ; a second area in northwestern Ohio is crossed by 

 the same isotherm. The southern area of the state is excluded 

 from the Phytophthora areas by the higher temperatures, as well 

 as by the early potato crop grown there. It may be mentioned 

 in passing, that very early potatoes in Georgia and Florida are 

 also attacked by Phytophthora. 



From other known considerations, the foregoing suggestions 

 do not appear unreasonable. The potato plant is native in cool 

 regions and is most successfully cultivated in the cooler portions 

 of Ohio and in states of more northerly latitude. That the de- 

 velopment of the parasitic Phytophthora should be faA^ored by 

 analagous conditions even though limited by a much narrower 

 range of temperature and rainfall than the host itself, can scarcely 

 be regarded as strange. The irregularity of the outbreaks of 

 Phytophthora in Ohio, is an economic difficulty in its control, 

 since the public mind acts only after the fact. For Ohio it 

 seems not improbable to the writer, that a succession of favorable 

 or cool seasons leads to the gradual southward advance of Phy- 

 tophthora, until established within our area; we then have one 

 or more violent outbreaks of disease, followed in turn by the 

 gardual disappearance of the fungus during a cvcle of hot or 

 dry seasons, or both hot and dry seasons. It seems very evi- 

 dent that we do not alwaj'S have it with us. 



Concerning Plasmopora Cuhensis. 



This second fungus, Plasmopora Cuhensis (B. & C.) Humph., 

 parasitic upon the cucumber, Cucumis sativa, and upon other 

 cultivated and wild species of Cucurhitaceae, offers a contrasting 

 history of development. There is a brief history given in Bulle- 



