Fungous diseases of plants . andbrittle and of a light-brown . The disease invariably beginswith the oldest leaves and proceedstoward the tips of the vines. Hence the disease appears to proceed from thecenter of a hill outward. In a field recently attacked, the center of everyhill will be clearly marked by a cluster of yellow leaves, so that the rows maybe plainly seen clear across the field, even though the plants are large andcover the ground. Affected plants continue to grow at the tips and put outnew leaves, and it is interesting to note how the disease follows at a distanceof abou
Fungous diseases of plants . andbrittle and of a light-brown . The disease invariably beginswith the oldest leaves and proceedstoward the tips of the vines. Hence the disease appears to proceed from thecenter of a hill outward. In a field recently attacked, the center of everyhill will be clearly marked by a cluster of yellow leaves, so that the rows maybe plainly seen clear across the field, even though the plants are large andcover the ground. Affected plants continue to grow at the tips and put outnew leaves, and it is interesting to note how the disease follows at a distanceof about four or five leaves behind the growing tip. After the disease isonce thoroughly established, very few cucumbers are produced, although theplants may continue to flower profusely. The few cucumbers which areformed 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 Fig. 54. Plasmopara on Cucumber. Salient Phases of Conidial Cycle (After G. P. Clinton) i6o FUNGOUS DISEASES OF PLANTS The fungus. The mycelium of this fungus is typical of thefamily. The haustoria have very much the form of those pre-viously described for the grape mycelium. The sporophores arise through the stomates,singly or in small clus-ters, and they areconsiderably branched,somewhat more flexu-ous than those of thegrape mildew, and thefruiting tips are lessrigid and more widelyseparated one fromanother, correspondingmore nearly to separatebranches. The sporesor zoosporangia areslightly violet tinted inmass and generallyovate in outline. Thegermination of the sporehas been followed care-fully and is known totake place by means ofthe characteristic motilezoospores. Clintonalone has illustrated thevarious stages of germi-nation (Fig, 54, r and d).Fig. 55. Peronospora on Young Cabbage Leaf Occasionally the normal sporophores are accompanied by short, swollen
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