832 



TWENTY-FIR8T llEPORT. 



when compared with similar records from other states, will permit the 

 development of generalizations as to the nature and origin of certain of our 

 plant disease outbreaks. And lastly, it serves as an intelligence or recon- 

 naissance bureau, which in part prettk-ts disease outbreaks as indicated by 

 weather conditions, and which seeks to discover these outbreaks in one part 

 of the state or another, and give proi)t»r warning so that farmers may take 

 necessary precautions. In these various ways the Plant Disease Survey 

 functions to serve the people of the state. This reix^t. which represents only 

 incidental observations, is therefore a summary of the reports received, and 

 an attempt at an interpretation of these reiwrts in light of the weather 

 conditions. 



THE WEATHER OF 1918. 



The weather conditions of 1918 were extremely significant in their effect 

 on the plant disease developments. In general, the spring was exceptionally 

 wet — rain every two or three days, delaying farm work and giving the year 

 a lower temperature record than normal. The rainy weather persisted until 

 about the middle of June, when di\v weather began and was foUoweil by 

 nearly a month of drought. The weather conditions of the month may be 

 deduced from the following table (Table 1) showing the summaries for May 

 to October Period. 



TABLE 1 

 Rainfall. Michigan, 1918 



The interpretation of the effects of the weather <'an Ite made in general 

 lines. Tlie rainy season by giving so many periods for initiation of various 

 plant diseases whose pathogens winter over on trash, fallen leaves, etc., lead 

 to heavy i)riuiary infestation. Tliis was quickly followed in June by severe, 

 secondary establishment of infection so that in many diseases all the condi- 

 tions for epidemics were at liand ixinding favorable July and August weather. 

 As it happened, however, the July weather and that of August checked many 

 of the di.«ease outbreaks which had started. 



Some, however, were so well established and so widely distributed that in 

 spite of severe drought the diseases were not checked. Apple scab, for example, 

 wa.s present as a disease of nearly every aitple leaf in uusprayed orchards. 



