Cucumber Diseases 139 



as yellowish spots, which have no definite outline. 

 In a warm, moist atmosphere numerous spots coal- 

 esce, and soon the affected leaves turn yellow and 

 die. With cool temperatures, the spots seem to 

 spread less rapidly. The disease appears to work on 

 the older leaves, beginning on those at the center of 

 the plant and working outwards. With infected 

 plants the center of the hill is clearly marked by a 

 cluster of yellow leaves. Diseased plants may 

 flower profusely, but set no fruit. The few cucum- 

 bers which are set are small, deformed, and unfit for 

 the market. 



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 

 arise in small clusters through the leaf stomata and 

 are branched and flexuous. The zoosporangia are 

 hyaline but slightly violet, tinted in mass. Germina- 

 tion of zoosporangia is by means of motile zoospores. 

 The zoospores or sexual fruiting stage was first 

 found on the host by Rostovtsev. 



Control. Downy mildew seems to be most prev- 

 alent on greenhouse cucumbers planted in August. 

 Those set in October seem to be free from it. Where 

 the disease makes its appearance, it is advisable not 

 to syringe the plants, but on the other hand to keep 

 the foliage dry. Diseased plants or parts of plants 

 should be destroyed by fire. Late planting in Sep- 

 tember or October wherever practicable is also ad- 

 vised. 



