Black spot treatment
A term which can be used to describe certain plant diseases which reveal themselves as black spots on the leaves. Some are quite startling in the color contrast with the green of the leaf, for example black or tar spot on sycamore leaves. But the best known of all ornamental plant black spot diseases is that which affects roses, called rose black spot. In this the spots are usually circular and well defined but sometimes they are very diffuse and roughly follow the veins in a branched fashion. In the disease of delphiniums called black spot or black blotch the black spots are of all sizes and very irregular shape. In black spot disease of elm leaves the spots are shiny, coal black, and slightly raised.
In general most black spot diseases may be controlled by picking off and burning the affected leaves where this is practicable, or by spraying with a proprietary copper fungicide or with a modern fungicide containing thiram.
Rose black spot is often more difficult to control and it may be necessary to spray at fairly frequent intervals with one of the fungicides mentioned above, or with Bordeaux mixture. Spraying the bare bushes and the soil beneath them with tar-oil emulsion in winter is sometimes resorted to with success. All prunings and affected leaves should be picked up and burned. An excess of nitrogen in the feed may predispose roses towards an attack of black spot. Where the disease is troublesome it may be advisable to reduce the nitrogen content of the feed. A spring dressing consisting of 2 parts of superphosphate, 1 part of magnesium sulphate, 1/2 part of iron sulphate applied at 168g (6oz) per sq m (sq yd), is a suitable low nitrogen feed. A rose leaf badly affected by black spot is illustrated on google images.