Family Cucurbitaceae 231 



attacks cantaloupes, gourds, squashes, pumpkins, 

 and watermelons. 



Symptoms. The disease appears as yellowish spots 

 on the leaves, which have no definite outline (fig. 

 40 d). With warm moist weather, numerous spots 

 coalesce, and soon the affected leaves turn yellow 

 and die. With cool weather the spots seem to spread 

 less rapidly. The disease appears to work on the 

 older leaves, beginning on those on the center of the 

 hill and working outward. With infected plants 

 the center of the hill is clearly marked by a cluster 

 of yellow leaves. Diseased plants may flower pro- 

 fusely, but no fruit is produced. The few cucumbers 

 which set are small, deformed, and unfit for the 

 market. 



Downy mildew is most prevalent in August with 

 moderate rainfall and hot weather. The disease 

 spreads very rapidly and a large cucumber field 

 may be a total loss in less than from eight to ten 

 days. 



The Organism. The fungus derives its food from 

 the host cells by means of suckers or haustoria. The 

 mycelium is hyaline, non-septate ; the conidiophores 

 (fig. 40 e) arise in small clusters through the leaf 

 stomata and are branched and flexuous. The 

 zoosporangia are hyaline but slightly violet, tinted in 

 mass. Germination of zoosporangia is by means of 

 motile zoospores (fig. 40 f-g). The oospore, or sexual 

 fruiting stage, was first found on the host by Ros- 

 tovtsev. Downy mildew may be kept in check by 

 spraying with Bordeaux mixture. 



