ANNUAL REPORT, 1938 17 



as they had earlier in June. On the hot, clear days of July 25 and 26 following a 

 week of cloudy, rainy weather, the water relationships within the plant were so 

 violently disrupted that the onion tops in all fields of seed onions simply dried up. 

 In this case very little recovery resulted and seed onion bulbs grew little if any 

 after July 25. 



The weather conditions, then, which usually favor the extensive development 

 of blast include wet weather in June, followed in July by a period which includes 

 a few days of cloudy, humid weather succeeded by a few clear, hot, dry days. 

 In previous years, this period in July has usually begun about the third week when 

 seed onion bulbs have reached about half their potential size. 



In checking through the Experiment Station Meteorological Observations 

 over the past eleven years, it was found that whenever a sequence of weather 

 conditions occurred similar to that described above, serious damage from blast 

 resulted. Such weather conditions existed during the growing seasons of 1927, 

 1928, 1931, 1935, 1937, and 1938, and blast was prevalent on seed onions to a 

 serious extent each of these seasons. 



Besides these weather conditions as the primary cause of blast, there are other 

 contributing factors. Foremost of these is injury from onion thrips. These 

 insects not only may weaken the plant so that it is more susceptible to blast but 

 may so further weaken the plant after the effects of a mild attack of blast are 

 gone that recovery is impossible. For example, in 1938 very few thrips were 

 present when blast symptoms appeared in June, but in July when blast symptoms 

 reappeared, thrips were abundant. The thrips population at this time was so 

 great that even had the plants been able to withstand the effect of the blast, 

 the added injury from thrips would have made recovery impossible. Onion thrips 

 then may play an important role in preventing recovery after weather conditions 

 producing blast have gone. By the time in Jul}- when blast usually strikes, set 

 onions have about reached maturity and harvesting operations have begun. 

 The thrips tend to migrate over from the matured set onion tops to actively 

 growing seed onion tops and thereby build up a high thrips population. Severe 

 thrips injury to seed onions consequently develops. Other contributing factors 

 favorable to the prevalence of blast include root diseases (primarily pink root, 

 Phoma terrestris) and lack of available plant food as a result of heavy seeding or 

 excessive leaching losses. In short, any factor or set of factors which adversely 

 affects the vitality of the onion plant will favor the development and increase the 

 severity of onion blast. 



It can be seen that the occurrence of blast in six out of eleven years has created 

 a very serious situation with respect to the growth of seed onions in the Connec- 

 ticut Valley. These "blast" years have resulted in large financial losses to growers 

 and have greatly reduced acreages of seed onions. 



Permanent Pastures. (W. G. Colby.) Results from outlying fertilizer top- 

 dressing experiments in Worcester County, Massachusetts, together with num- 

 erous observations on other permanent pastures throughout the State, lead tothe 

 conclusion that at best fertilizer topdressings on old pasture sods will only bring 

 these sods up to a moderate level of productivity. Even at this productivity' level, 

 herbage production is seasonable. Herbage yields reach peaks in early summer and 

 early fall and drop to low levels in midsummer and late summer. 



The first limiting factor to the presence of white clover in old pasture sods is 

 moisture. Native white clover can be found widely distributed after wet seasons, 

 but after dry seasons it will be found only in areas where good moisture relation- 

 ships have been maintained throughout most of the growing season. Lack of 

 available mineral elements, also a limiting factor to the prevalence of white 

 clover, is as important as favorable moisture relationships from the standpoint 



