DISEASES OF CULTIVATED PLANTS 545 



The first indication of downy mildew in the field is a yellowing of 

 the older leaves in the center of the plant. Faintly defined angular 

 spots bordered by the veins will then be detected. These become 

 more distinct, and if the weather is moist an obscure violet coating 

 of the spores may be noticed on the under side of the spots. The dis- 

 ease progresses from the center of the hill outward, the young leaves 

 at the tips of the branches living longest. It spreads slowly in bright 

 weather, but under the more favorable conditions afforded by cloudy, 

 humid weather it often develops with the greatest rapidity, so that 

 the fields quickly become as if scorched by fire. 



Downy mildew is caused by a parasitic fungus closely related to 

 the destructive downy mildews of grape, onion, etc., and to the late 

 blight of potato. So far as known, it is spread entirely by its conidia, 

 or summer spores, produced on the lower surface of diseased leaves. 

 These are blown about by the wind, but are very thin-walled, delicate 

 bodies, which perish quickly when dried. No oospore or resting stage 

 is known, and there is consequently no evidence that the fungus lives 

 over in dead vines or in the soil, and no logical reason for destroying 

 the vines or for selecting fresh land for the crop, aside from ques- 

 tions of soil fertility. 



The disease lives through the winter in Florida and probably 

 spreads northward each summer. There is also good evidence that 

 it lives over in greenhouses, which may later become the centers of 

 local epidemics. 



The earliest appearance of the disease in South Carolina in 1905 

 was May 1. It usually appears in that latitude during June. It 

 does not reach Ohio until August 1, while New York and New Eng- 

 land are visited later in August. The date of appearance and the 

 severity of the outbreak seem to be governed mainly by the weather. 

 During unusually dry seasons the disease may be entirely absent, 

 while warmth and moisture, especially warm, cloudy nights, soon 

 lead to its development and result in the greatest loss. Any weak- 

 ness of the plant appears also to predispose it to attacks of downy 

 mildew. Cold weather during spring, when the plants are small, is 

 unfavorable for this reason. The plant also becomes more susceptible 

 when the bearing of fruit is imposing an additional strain upon it. 

 Liberal fertilizing and careful attention to cultivation should be given 

 to keep the vines strong and vigorous. (F. B. 231.) 



Leaf-Blight. Alternaria brassicse, var. nigreseens, is the usual 

 cause of leaf-blight of musk-melons or cantaloupes throughout the 

 country, though downy mildew and anthracnose also occur and some- 

 times are associated with the leaf-blight. 



Leaf-blight begins in small, round spots, which usually show 

 faint concentric rings. These spots enlarge and their effect is very 

 quickly visible on the leaf. The cantaloupe leaf in particular, owing 

 to its thin and delicate structure, will curl at the margins and shrivel 

 up in a few days. 



The disease is caused by a fungus closely related to tho species 

 causing early blight of potato, leaf-blight of cabbage, etc. Leaf- 

 blight is spread by spores borne on the upper side of the leaves and 



