. California plant diseases. Plant diseases. Bulletin 218] CALIFORNIA PLANT DISEASES. 1133 Leaf Spot (Cycloconium oleaginum) Causes spots of considerable size on the leaves. No treatment is usually necessary. Dry Rqt Figure 62. Characterized by the appearance of numerous large spots on the fruit which dry and sink in. During some seasons the olive is considerably affected with this trouble. Cause not known. Root Rot (Toadstool disease). See Almond. ONION. Downy Mildew (Peronospora schleideniana). Causes serious losses some years in onions grown for seed during the win- ter. The fungus affects
. California plant diseases. Plant diseases. Bulletin 218] CALIFORNIA PLANT DISEASES. 1133 Leaf Spot (Cycloconium oleaginum) Causes spots of considerable size on the leaves. No treatment is usually necessary. Dry Rqt Figure 62. Characterized by the appearance of numerous large spots on the fruit which dry and sink in. During some seasons the olive is considerably affected with this trouble. Cause not known. Root Rot (Toadstool disease). See Almond. ONION. Downy Mildew (Peronospora schleideniana). Causes serious losses some years in onions grown for seed during the win- ter. The fungus affects the seed stalks and leaves with the production of large dead areas upon which may be seen the dark, purplish-colored mildew. The seed stalks topple over and fail to develop seed. This disease has not been successfully controlled in California, but in Eastern states has yielded fairly well to spray- ing with Bordeaux mixture, to which a resin "sticker" has been added. This spraying must be done at frequent in- tervals, commencing before the mildew appears. The development of resistant strains by seed selection is Fig. 62.—Dry rot of olive fruit. ORANGE. In the consideration of diseases of the orange and other citrus trees, it is desirable to mention at the outset certain peculiar characteristics of such trees which distinguish them to a considerable extent from any other cultivated plant. These features make the citrus tree specially liable to peculiar, obscure effects of climate, soil or other non-parasitic conditions, and in making an investigation of citrus diseases one should not fail to keep these peculiarities of the citrus tree prominently in mind. First, is the very free circulation of water from the roots up into the tops, which occurs when plenty of moisture is available in the soil. This water carries with it all the food materials from the soil which the tree takes up and thus we see that the citrus tree must take up large 7_Bul. 218. Please n
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