PHYCOMYCETES 



159 



been devoted to the cultivation of melons or of cucumbers, 

 especially for the pickling trade. Nevertheless, it is now a very 

 constant disease in greenhouse-grown cucumbers. 



The fungus has been found on most of the cultivated species 

 of the Cucurbitaceae, or gourd family, such as cucumbers, musk- 

 melons or watermelons, squash, 

 pumpkins, gherkin, and also 

 upon the star cucumber, Sicyos 

 angtdatus, and a few other wild 

 species. 



Symptoms and effects of the 

 disease. The effect of this dis- 

 ease upon the host, that is, 

 upon the cucumber, have been 

 very clearly presented by Stew- 

 art as follows : 



The leaves show yellow spots 

 which have no definite outline. If 

 the weather is warm and favorable 

 for the disease, these spots enlarge 

 rapidly and run together so that the 

 whole leaf becomes yellow and soon 

 dies and shrivels like a leaf killed 

 by frost. If the weather is cool, the 

 yellow spots spread less rapidly. In 

 the latter case the central portion of 

 the yellow spots becomes dead and 

 brittle and of a light-brown color. 

 . . The disease invariably begins 

 with the oldest leaves and proceeds 

 toward the tips of the vines. Hence the disease appears to proceed from the 

 center of a hill outward. In a field recently attacked, the center of every 

 hill will be clearly marked by a cluster of yellow leaves, so that the rows may 

 be plainly seen clear across the field, even though the plants are large and 

 cover the ground. Affected plants continue to grow at the tips and put out 

 new leaves, and it is interesting to note how the disease follows at a distance 

 of about four or five leaves behind the growing tip. After the disease is 

 once thoroughly established, very few cucumbers are produced, although the 

 plants may continue to flower profusely. The few cucumbers which are 

 formed grow slowly and become misshapen so that they are unsaleable. . . 

 Of the total shortage of 75 per cent, in the Long Island cucumber crop of 1896, 

 it is safe to say that 55 per cent, was due to the downy mildew. 



Fig. 54. Plasmopara on Cucumber. 



Salient Phases of Conidial Cycle 



(After G. P. Clinton) 



