8 MASS. EXPERIMENT STATION BULLETIN No. 283 



stages as the host plants are grown farther north; the absence of the disease in 

 the North when it is already present in the South being due as much to the ab- 

 sence of the host as to the absence of inoculum. The evidence being, in the nature 

 of the case, necessarily largely circumstantial, it is easier to prove that the inocu- 

 lum can be carried northward by winds than that it is so carried. It is the belief 

 of the writer, however, on the basis of the earlier discussion and the evidence 

 presented below, that the conidia of P. ciibensis, the primary inoculum, reach 

 Massachusetts from Connecticut, Long Island, and the lower Hudson Valley 

 after the fungus has reached these regions by the aid of wind from more southerly 

 points. Because of downward air currents and rain, it seems unlikely, though 

 possible, that the spores move the whole or nearly the whole length of the Atlan- 

 tic Coast of the United States on any one wind or in any one fight. Even so, 

 the total distance could be covered in a relatively short time, and with only 

 short interruptions, for, as is discussed below, infection by, and incubation period 

 and sporulation of P. cubensis may all occur in a few days. 



In each of the seven years from 1924 to 1930 inclusive, downy mildew of cu- 

 cumber has been found in the field in Massachusetts, and has always appeared 

 within the first 18 days of August, whether the summer was rainy or dry. But 

 the disease was not found in this State in 1931 although many cucumber fields 

 were examined at weekly intervals, or oftener, up to the time of killing frosts. 

 The weather of the season was not significantly different from that of previous 

 seasons in which downy mildew had been severe, and the absence of the disease 

 in 1931 is attributable only to lack of primary infection rather than to failure 

 of the disease to spread within a field. 



In 1931, downy mildew of cucurbits seems to have been decidedly less com- 

 mon and less severe south of here also.'' In Connecticut, the disease was not found 

 until the very end of the season and then only in one field, although the weather 

 of the season was, in the opinion of Clinton, not unfavorable to it and it was pres- 

 ent as usual the previous year. It was not reported as present in either New York 

 or Pennsylvania. Only a trace of infection was found in Delaware. It was present 

 in West Virginia, but to a lesser extent than the previous year. It was either 

 absent or very unimportant in South Carolina. There was very little of the 

 disease in Georgia, although the weather (rainfall) was considered favorable 

 to it in June and July. In Florida, and this is of particular interest, fields were 

 remarkably free from downy mildew, and the disease was not of economic im- 

 portance. 



There was, therefore, a miuinmm chance of the causal fungus reaching Mass- 

 achusetts from states farther south and under these conditions, downy mildew 

 of cucurbits did not appear in this State. 



Most of the primary infection in Massachusetts occurs in August. It is at 

 least interesting to note that in August, 1931, there were 30 per cent fewer 

 days with southerly winds and about 39 per cent less total southerly wind move- 

 ment in miles here than the averages of the preceding five years. AMth very 

 little inoculum in states to the south in 1931 and with much less southerly wind 

 movement here than usual, the fungus did not appear in Massachusetts that 

 year. 



^The .statements in this paragraph are based on information received from G. P. Chnton, C. 

 Chupp, G. L. Zundel, J. F. Adams, C. R. Orton, G. M. Armstrong, F. Van Haltern, and G. F. 

 Weber, representing respectively the Agricultural Experiment Stations of Connecticut, New 

 York, Pennsylvania, Delaware, West Virginia, South Carolina, Georgia, and Florida. 

 'This information w-as furnished by C. I. Gunness, Meteorologist of Mass. Agr. Expt. Sta. in 

 letter of March 8, 1932. 



