350 Keport of the Mycologist op the 



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. For an illustration of this see Plate X. 

 The disease invariably begins with the oldest leaves and pro- 

 ceeds 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 diseasa 

 follows at a distance of about four or five leaves behind the grow- 

 ing 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 unsalable. 



Besides the downy mildew there are several other fungous 

 diseases which sometimes do damage to cucumbers, but the 

 downy mildew was the only one which did serious harm to late 

 cucumbers on Long Island in 1896. The anthracnose, Colleto- 

 trichum lagenarium (Pass.) Hals., has recently done much damage 

 to cucumbers in New Jersey, but it has not been destructive on 

 Long Island during the past season. 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, while the 

 remaining 20 per cent was due to all other diseases and insects. 

 In the vicinity of Hicksville and Central Park practically all of 

 the damage was done by the downy mildew. \ 



STRUCTURE OF THE CUCUMBER LEAF. 



In order that the nature of the downy mildew and its method 

 of killing the leaves may be better understood, it is perhaps best 

 1o first describe the structure of the cucumber leaf. 



