The common Leaf Spot of blackberry affects also the raspberry and dewberry. It is found in almost every locality where these fruits are grown. While the disease is common, it is not often serious. Rather it is enphytotic, doing but little damage under ordinary conditions. In 1905, however, 20 per cent foliage - losses were reported from Ohio and Florida.
The appearance of this disease is said to vary to a marked extent on different varieties. On some varieties the spots on the leaves (Fig. 46) are light-brown, while on others they are dark-brown. The lesions are comparatively small, measuring about one - eighth of an inch in diameter. The diseased portion is at first purplish, although a brown color is assumed as the affected tissue dies. In old spots the center is whitish, and the border is brownish or reddish.
The causal fungus is Septoria Rubi. It belongs to a group of organisms the life - histories of which are fairly well understood.
But the successive activities of Septoria Rubi have not been carefully studied; this is probably due to the fact that the disease which it produces has never been of great economic importance.
The mycelium of the fungus growing in local areas within the leaf-tissues causes the color changes and finally their death. The fungus forms fruiting bodies - pycnidia - just beneath the upper surface of the cuticle. These, as they mature, break through the cuticle and stand exposed over the affected area. Within the pycnidia spores are developed which at maturity, under moist weather conditions, ooze out and are scattered to other leaves. Here they germinate in water and the result is a germtube which in some way gets into the leaf - tissues. A spot is the outcome. Little is known of the winter condition and of the manner in which the first infections in the spring are brought about.
In cases in which the Leaf Spot disease becomes a menace it may be kept in check fairly well by the use of bordeaux mixture, 4-5-50, or lime sulfur 1-50. The spray should be applied as follows:. (1) shortly after the leaf-buds unfold, (2) subsequent applications at intervals of two to three weeks until the fruit is about two - thirds grown.
Fig. 46. - Blackberry Leaf Spot.
As a rule the disease is not of sufficient importance to warrant spraying for its control.