16 MASS. EXPERIMENT STATION BULLETIN 355 



By June 25, virtually all seed sets were badly affected, many fields of seed 

 onions partially affected, and some fields of set onions beginning to die back from 

 the tips. A heavy rain on June 26, 27, and 28 checked the progress of the blast 

 on almost all areas. Ten days of cool weather, following these rains, permitted 

 the set onions to mature fully without appreciable injury and allowed the seed 

 sets in most instances to "size up" sufficiently to be pulled, even though they 

 had been badly affected by blast. This was particularly true where fertilizer 

 topdressing had been applied to seed sets before the June 26th rain. A good crop 

 of seed sets was harvested in almost all instances. 



During the period June 28 to July 8, the seed onions made good top growth 

 and had begun to form bulbs. Even in those areas where the blast had made its 

 appearance earlier, the bulbs made good growth, and at this time, prospects 

 were excellent for a good crop of seed onions. Of course, small areas on light 

 land, particularly land flooded in 1936, made little recovery and indicated that 

 they would be a total lo.ss. A period of Avarmer weather followed from July 8 

 to July 17 but the progress of blast on seed onions was not rapid. On July 17, 

 however, a maximum temperature of 87'' seemed to induce blast symptoms again 

 in practically- all seed onion fields. Between July 18 and July 25, some five inches 

 of rain fell, with only one rainless day. The relative humidity was high at all 

 times, and the few hours of sunshine which occurred during the week seemed to 

 greatly accelerate the spread of blast symptoms. Clear, hot, humid weather on 

 July 25 and 26 followed this rainy week and greatly increased the severity of 

 blast in all seed onion fields. Even fields of onions on the higher terraces in the 

 vicinity of Amherst, which seldom show the effects of blast until much later than 

 Valley fields, were seriously affected at this time. In no instance, as a result of 

 blast, was either the yield or the quality of a field of seed onions much more than 

 40 to 50 percent of what could be considei ed a good crop. Set onions had reached 

 maturity before this date and as a result there was little damage to them from 

 blast. 



In the opinion of the writer, what principally happened to the onion crop of 

 1938 is as follows. The heavy rains and high temperature during the first part 

 of June resulted in a rank vegetative top growth accompanied by a relatively 

 small and shallow root system. The hot, dry weather toward the end of June 

 resulted in excessive transpiration from the rank vegetative tops, so that the root 

 system was unable to maintain normal water relationships within the plants. 

 As a result of this abnormal physiological condition, blast symptoms began to 

 appear and as the hot, clear weather continued, the spread of blast was rapid and 

 its severity increased. The reason that blast spreads and does not appear simul- 

 taneously on all areas is because in some soil types, the onion root system is able 

 to maintain an adequate supply of water to the leaves for a longer time period 

 than in other soil types. The greater leaching of plant food nutrients, as a result 

 of the heavy, early-June rain, from some soils (particularly sandy soils) than from 

 others was also a factor influencing the "spread" of blast in 1938. It appears 

 that when excessive leaching losses are experienced in June, blast will appear 

 first on the "light" sandy soils. On the other hand, if June leaching losses are 

 not excessive, the proportionately greater top growth and also shallower root 

 system of onions on "heavy" soil will result in blast appearing there first. 



The rains of June 26-28 followed by ten days of cool weather greatly reduced 

 transpiration rates and permitted the reestablishment of normal water relation- 

 ships within the plant and "recovery" from blast resulted. The fact that seed 

 onions at this time were still in a relatively early stage of growth also facilitated 

 recovery. 



When the sequence of weather conditions of the latter part of June were dup- 

 licated during the latter part of July, the seed onions behaved then much the same 



