Identifier: textbookofmycol00hars (find matches)
Title: A text-book of mycology and plant pathology
Year: 1917 (1910s)
Authors: Harshberger, John W. (John William), 1869-1929
Subjects: Plant diseases Fungi
Publisher: Philadelphia : P. Blakiston's Son & Co.
Contributing Library: University of British Columbia Library
Digitizing Sponsor: University of British Columbia Library

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every year for the past six years. In some years, theattack is more virulent than in other years. It is also common on vio-lets grown under glass, and in some districts, commercial violet growinghas been practically abandoned. The fungus attacks plants that aremaking a rapid and vigorous growth. The first spots are circular,greenish or yellowish white ones. They have a light colored centralportion surrounded by a narrow ring of discolored tissue, usually DETAILED ACCOUNT OF SPECIFIC PLANT DISEASES 559 black or very dark brown at firsts but changing to a lighter shade, as thespots grow older. The first diseased part of the leaf looks as if water-logged, and in a few days, the diseased part of the leaf peripheral to thecentral spot fades, or bleaches, to a yellow, or grayish-white. Herethe disease may stop and the plants recover, the diseased areas separatefrom the healthy tissue and fall out leaving holes in the leaves. Thedisease may spread, however, until the whole leaf is destroyed.
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Fig. 203.—Violet leaves affected with leaf-spot (AUernaria violce). (Photo, byHeald, F. D. and Wolf, F. A., Bull. 135 (Sci. Ser. 14), Univ. of Tex., Nov. 15,1909.) The majority of the spots are free from fungous spores except underconditions favorable to their development. Some spots produce sporesin abundance, especially upon the central, or older portions of the spots.The spores are borne in chains on dark brownish hyphae that arisefrom the diseased surface. The conidiospores are clavately flask-shaped, muriform, strongly constricted at the septa, which are variable 560 SPECIAL PLANT PATHOLOGY in number, olivaceous, 10 to ijfx by 40 to 6ofi, exclusive of the isthmus,which is 3 to 5/x by 3 to 25/x.^ To prevent the disease, only healthy vigorous stock of known par-entage should be grown. These plants should be propagated at theseason most favorable to the growth of the violet. The frames, glasshouses and conservatories should be kept scrupulously clean. Wheat (Triticum sativum Lam.)

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