Studies on the Downy Mildew of Onions 13 



very slowly at that time and is not noticed. By July or August, how- 

 ever, more plants have become infected, and with favorable weather 

 conditions the disease develops and spreads very rapidly. 



Mildew usually is found about two weeks earlier on set and seed 

 plants than on market onions. In 1928 the disease was first observed 

 by the writer on set onions on July 10, and the grower on whose 

 farm it was found stated that it had already been present for two weeks. 

 It was not found, however, on market onions growing in the next row 

 until July 20. By that time it was present also on market onions in all 

 other fields about Elba, indicating that there were sources of inoculum 

 other than the fungus on set onions. 



Mildew was found on Egyptian, or top, onions as early as the first 

 week in July in some seasons, and was observed fruiting on them in 

 one planting at Ithaca as late as November. The fungus seems to be 

 perennial in these plants, and fruits whenever favorable conditions 

 exist. 



Experience has shown that a few diseased plants always can be found, 

 even in seasons when the disease is considered to be absent. Con- 

 sequently, when favorable weather conditions exist an epiphytotic may 

 occur. 



LOCATION OF DISEASED PLANTS 



Each season it was observed that the disease occurred at first on 

 individual plants which were scattered uniformly throughout the fields. 

 Furthermore, during this early period mildew was as prevalent in fields 

 where onions had never been grown as on old onion land. Nor did 

 there seem to be any difference in the occurrence of the disease in fields 

 near plantings of sets and seed onions and in fields that were isolated. 

 After a few days of weather favoring the development and spread of 

 the disease, the mildew became more severe on the plants next to the 

 hedges, in low spots, where the top growth was heavy, and in other 

 sheltered situations. It is observations on these latter stages of the 

 disease that have given rise to the frequent erroneous statements that the 

 disease starts in certain sheltered parts of the field and spreads to the 

 remainder. The mildew at first is distributed uniformly throughout 

 the field, but it spreads more rapidly in certain parts of the field than 

 in others. 



TEMPERATURE AND MOISTURE RELATIONS 



Moisture was observed to have a great influence on the development 

 and spread of the onion-mildew fungus. The formation of the fruiting 

 bodies appears to be dependent, for the most part, on an abundance 

 of moisture. The conidia are produced in greatest numbers during 

 rainy periods and when the Leaves are wet with dew. They are never 



