SPRAYING AND DTTSTINC ONIONS 177 



Blast 



In 1927, 1928, and 1931, seed^ onions died prematurely in the Connecticut 

 N'alley, although downy niildew was practically or entirely absent, and it was 

 apparent that thrips {Thrips tabaci L.), a common cause of the early death of 

 onion leaves, was not responsible. In each of these years, this disease or abnor- 

 mal condition here designated as blast, first appeared in or about the third week 

 of July when periods of rain were interrupted and followed by high temperatures, 

 bright sunlight, and drying winds. 



Macrosporium porri Ell., the pathogenicity of which to onions has been 

 established by Angell (1), Macrosporium parasiticum Thum., and Botrytis sp. 

 were often, but by no means always, present on blasting onions. 



Onions dying from blast were often found to have pink and decaying roots, 

 and Plioma terreslris, which according to Hansen (7) is the true causal agent of 

 the pink-root disease of onions, was isolated by him from onions grown in Massa- 

 chusetts, which died prematurely in a year of blast. The pink-root disease was, 

 however, no less common in the Connecticut Valley on onions which did not 

 blast, but which died prematurely from other causes, notably thrips and drouth. 



These and other considerations indicate, in our opinion, a physiological 

 rather than a parasitic cause of onion blast. The disease did not spread in the 

 sense that plant diseases caused by fungi or bacteria usually spread, as from 

 one part of a field to another; nor was there any apparent relation to differences 

 in soil or topograj)hy, soil reaction or previous cropping systems. Whole fields 

 of onions and all fields over a large area, having little if anything in conunon 

 except climatic environment, began to blast on or about the same day. 



While the fungi named, and others, may be contributing factors which hasten 

 the death of blasting onions, our observations and those of other investigators 

 lead us to believe that blast of onion, like tip-burn of potato, has its first cause 

 in the atmospheric conditions indicated above. Lutman (12) has demonstrated 

 that tip-burn of potato is due to too bright sunshine and too high temperatures, 

 and under these same conditions, he observed tip-burn (incipient blast) of 

 onions. Furthermore, onion plants subjected by Jones (8) to conilitions of high 

 relative humidity and of light of low intensity, followed by exposure to relatively 

 dry air and to bright sunshine, developed the fading, withering, and dying 

 leaves symptomatic of blast. 



In the experiments of Lutman (11) tip-burn of potato was found to be pre- 

 ventable by spraying with Bordeaux mixture, and these experiments with 

 onion were undertaken with the thought that spraying with Bordeaux mixture, 

 or with milk of lime, might be similarly efi^ective in protecting onions against 

 blast. Milk of lime was included, although it is without the fungicidal properties 

 of Bordeaux mixture, for in the experiments of Butler (4) the action of both 

 milk of lime and Bordeaux mixture (4:4:50) on sprayed plants was found to be 

 one of shade. 



Methods and Costs 



The sprayed plots and their checks were in triplicate and the dusted plots 

 and their checks were in duplicate each year. The treatments of all plots, in- 

 cluding materials used, numbers of applications, and intervals of time between 

 applications are recorded in Tables 1, 2, and 3. 



* Blast as observed by us was confined almost entirely to seed onions, i.e., onions grown from 

 seed, as distinguished from set onions, i.e., onions grown from seta and harvested earlier. 



