36 



DowxY Mildew. 

 (Plasm opara Cubensis. i B. & C. i Humphrey.) 



This fungus, like the preceding, has never been observed by us on 

 greenhouse cucumbers in winter, neither has it been observed in 

 greenhouses in spring and early summer. It has recently occurred 

 quite extensively on greenhouse cucumbers planted in August 

 and is found on late spring crops running into mid-summer. 

 During the past summer it appeared about August first on green- 

 house and outdoor crops. A considerable amount of this Mildew 

 has appeared within the last two years. Its presence can be detected 

 by the yellowish angular spots on the leaves, and it is not difl5cult to 

 distinguish from other leaf fungi when once carefully observed (see 

 fig. 1 1). This Mildew was first observed "in this State* by Humphrey 

 in 189 1 and it was not noticed again until 1900. Since that time it has 

 made its appearance annually on outdoor melons and cucumbers. 

 It occurs very extensively in the South, causing much damage to 

 cucurbitaceous plants. Conditions, however, do not appear to be 

 favorable for its appearance on winter crops in this latitude. Crops 

 of cucumbers set in August under glass appear to be subject to this 

 Mildew, but those set in October and later have shown no tendency 

 to, become infected. Prof. A. D. Selby of the Ohio Agricultural 

 Experiment Station is of the opinion that this fungus is perennial 

 in the South and travels north ever)- season, the plants farther north 

 becoming infected later than those in the south. At least this has 

 been the condition in Ohio. Prof. Selby writes as follows : "This 

 disease has occurred regularly on field cucurbits in Ohio at least 

 since 1896, without missing a year. The disease also appears earlier 

 in the southern part of the state than it does in the northern part. 

 I am entirely convinced through repeated experiments in our pathol- 

 ogium that it is not propagated in the cucumber fields by any resting 

 forms contained in old leaves or vines. My cooperative studies, 

 made through inquiry from Florida to your own state and Xew Hamp- 

 shire, have thoroughly convinced me of what I apprehended pre- 

 viously; namely, the Plasmopara spreads northward each year 

 by its conidia." It has been shown that spraying! with Bor- 



*Eighth Annual Rept. Mass. State Expt. Sta. pp. 210-213. 

 tF. C. Stewart. X. V. Agr. Expt. Sta., Bull. 138, p. 636-639. 



