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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te, such as in the mosaic,or calico disease of plants formerly described. Yellowing may be in-duced experimentally by an excess of carbon dioxide, in fact yellowingaccompanies wilting, the attack of wire worms, the presence of poisons,or acid gases. Variegation and albinism may be apparently normal conditions ofsome varieties of plants, for gardeners and horticulturists grow suchplants by preference for decorative uses. This variegation, or albinism. 344 GENERAL PLANT PATHOLOGY is induced in all probability by the presence of oxidizing enzymes inpatches of cells where the chlorophyll pigment is destroyed and not inother adjoining areas. The formation of spots on leaves (Fig. 134), stems, flowers, or fruits isdue to a variety of causes. The grayish or whitish spots on the undersurface of grape leaves are due to mildews, on the stems of cruciferousplants to white rusts and on the leaves of the parsnip are found whitespots due to a fungus, Cercosporella. Grayish spots on the prickly pear
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Fig. 134.—Apple leaves showing leaf spots produced by natural infection withSphaeropsis malorum. (After Scott, W. M., and Rorer, J. B.. Bull. 121, U. S. Bureauof Plant Industry, 1908.) and on the leaves of the box trees are occasioned by a disease known asanthracnose. Many leaf spots are yellow as in violets, oaks, cucumbersand melons. The red or orange spots on plants usually suggest thepresence of rusts as on wheat, rye, alfalfa and a host of other cultivatedand wild plants. The so-called tar spots of the maple leaves are blackin color and such discolorations of the leaf surface are traceable to theattack of a fungus, Rhylisma acerinum. Apples are frequently markedby fly specks which are usually clustered as small circular black spots.A fungus is the causal agent. SYMPTOMS OF DISEASE (SYMPTOMATOLOGY) 345 2. Shot-holes (Fig. 135).—The perforations of leaves and the forma-tion of what are called shot-holes illustrate another form of fungousattack, where circular patches of dead ti

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