14 Harold Thurston Cook 



produced during dry periods when the leaves are entirely free from 

 moisture. An examination of the thermograph records which were 

 kept during the course of this investigation showed that the fungus 

 fruits at a time of the year when the night temperatures vary between 

 5° and 13° C. 



The effect of temperature and moisture on the viability of the conidia 

 was even more marked. In obtaining spores for germination tests and 

 inoculation purposes, it was found necessary to collect them early in the 

 morning, before the dew had dried or the spores had been exposed 

 to the direct rays of the sun. Desiccation and heat are fatal to the 

 conidia. 



With market onions it was observed that the mildew spreads most 

 rapidly where the foliage is heavy, near hedges, in low spots, and in 

 fields that are sheltered from the sun and wind. These conditions favor 

 the accumulation and retention of water on the foliage of the plants, 

 and thereby make it possible for the fungus to fruit and to infect other 

 plants. Two seed plots on one farm, which were observed in 1927, are 

 good illustrations of the effect of environmental conditions on the 

 occurrence of onion mildew. One plot was located in a field that was 

 bounded on the south by an orchard and on the west by a large barn. 

 The planting was poorly cultivated and consequently many weeds were 

 present. The protection thus afforded the plants prevented the rapid 

 drying of the leaves. In this plot 85 per cent of the plants became 

 diseased. The other plot was kept free of weeds, and, being situated on 

 the west side of the barn, was more exposed to the prevailing winds, 

 which favored drying of the leaves. Only 15 per cent of the plants in 

 this field were mildewed. Another seed plot which was observed in that 

 year was located on muck soil where the humidity was high. That also 

 was allowed to be overgrown with weeds, and practically every onion 

 plant became diseased. 



SOURCES OF PRIMARY INOCULUM 



It was deemed necessary to determine the principal sources of the 

 primary inoculum so that methods could be developed to eliminate them 

 and thus prevent the disease from gaining a foothold in the field. This 

 conclusion was reached when attempts to combat the disease after it 

 had appeared in the field resulted in failure. Even if it were possible 

 to control the disease after it had appeared, the injury suffered by the 

 plants from the initial attack would often be so great that they would 

 not recover sufficiently to produce a full crop. Therefore the evidence 

 in support of four theories as to sources of primary inoculum was con- 

 sidered. These possible sources were: (1) systemically infected plants; 

 (2) infested soil; (3) infested seed; and (4) infected seed. 



