. Fungi and fungicides; a practical manual, concerning the fungous diseases of cultivated plants and the means of preventing their ravages . FIG. 14. SECTIOX SHOWIXG SCAB FUXGUS. ^, spores in position, as developed; B, skin of fruit turned up; C, parenchyma-like cells of the parasite; Z» £>, tissues of the fruit. Much magnified. several other trees, small round whitish blotches of mil-dew, which at first have more or less of a radiated ap-pearance, due to the spreading of the threads. In ashort time many of the spots enlarge so that they run THE POWDERY MILDEW 31 toaetlier: and the mildew o
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. Fungi and fungicides; a practical manual, concerning the fungous diseases of cultivated plants and the means of preventing their ravages . FIG. 14. SECTIOX SHOWIXG SCAB FUXGUS. ^, spores in position, as developed; B, skin of fruit turned up; C, parenchyma-like cells of the parasite; Z» £>, tissues of the fruit. Much magnified. several other trees, small round whitish blotches of mil-dew, which at first have more or less of a radiated ap-pearance, due to the spreading of the threads. In ashort time many of the spots enlarge so that they run THE POWDERY MILDEW 31 toaetlier: and the mildew often covers the whole surfaceof the leaf, either as a thin coating through which thegreen surface can be seen, or as a thick, felt-like cover-ing, which entirely conceals the leaf surface. This mil-dew is composed of a great number of minute whitethreads, branching in all directions, which form themycelium of the powdery mildew fungus. In the fungiof this group the mycelium is external, t. e., instead ofdeveloping on the inside of the leaf of the host-plant, itdevelops on the outside, and in order to get nourishment. c d FIG. 15. APPLE POWDERY MILDEW. c, Mycelium forming summer spores; </, winter spore case (peritliecium). Magnified. the mycelium threads send little suckers into the cellsof the leaf and absorb their contents, for the benefit ofthe fungus. Soon after the mycelium gets well devel-oped it assumes a powdery appearance, due to the pro-duction of great numbers of the minute white spores, called the summer spores, or conidia (Fig. 15 h). Theseare very light, and are scattered by every breath of air.When one of them falls upon an unaffected leaf, wheresufficient moisture is present, it germinates and startsthe disease anew. By means of these summer spores—a 3^ FUXGI AXD FUNGICIDES yast number of which may develop from the myceliumof a single leaf—the fungus is able to spread very rap-idly. Toward the end of summer the mycelium devel-ops small round black masses