Oidium or powdery mildew of the vine . thebranching threads or mycelium, which constitute the vegetative bodyof the fungus. This mycelium grows only on the surface, unlike themycelia of Peronospora and other parasitic fungi which enter the tis-sues of the host plant. It obtains its nourishment from the epidermalcells of the plant attacked, by means of suckers—haustoria—whichpenetrate these outer cells. The outer cells thus attacked become darkcolored. The fact that the Oidium does not penetrate the tissues of thevine much facilitates its control and renders its destruction possible atany stage
Oidium or powdery mildew of the vine . thebranching threads or mycelium, which constitute the vegetative bodyof the fungus. This mycelium grows only on the surface, unlike themycelia of Peronospora and other parasitic fungi which enter the tis-sues of the host plant. It obtains its nourishment from the epidermalcells of the plant attacked, by means of suckers—haustoria—whichpenetrate these outer cells. The outer cells thus attacked become darkcolored. The fact that the Oidium does not penetrate the tissues of thevine much facilitates its control and renders its destruction possible atany stage without injury to the host plant from the remedies applied. Summer Form of the Fungus. The mycelial threads of the Unci-nula spiralis are extremely fine—not over two ten-thousandths of aninch in diameter. They elongate and grow over the surface to whichthey are attached by their haustoria and at intervals send out uprightbranches—hyphae—thirty to forty ten-thousands of an inch long. OIDIUM OR POWDERY MILDEW OF THE VINE. 321. Fig. 5. Various forms of Uncinula , II. Perithecia showing /, appendages, and a, Summer form showing m, mycelium; t, hyphae; c, conidia; and h, Group of asci removed from perithecium emitting s, ascospores. (Ill andIV, after Viala.)V. Summer form of Oidium as it appears on surface of Ditto showing effect of sulfur (d). (V and VI, after Mares.) 322 UNIVERSITY OF CALIFORNIA—EXPERIMENT STATION. These hyphae are somewhat thicker than the mycelial threads, grad-ually increasing in diameter from the bottom to the top until theybecome two or three times as thick as the threads from which theyarise. Cross partitions divide the hyphae into several cells, the upper-most of which finally becomes detached and forms a conidium orsummer spore. The hypha continues to grow and the second cell de-velops into another conidium. This process continues as long as theconditions are favorable throughout the summer, each hyph
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