Brown and black leaf spots are frequently furnished with concentric contours arranged round a paler or other coloured central point—e.g. Cercospora on Beans, Ascochyta on Peas.
Brown spots with bright red margins are formed in young Beans by Gloeosporium.
Species of Fumago, Herpotrichia, etc., may cover the entire surface of the leaf with sooty patches, or even weave the leaves together as if with black spider-webs.
Mal nero of the Vine is a particular case of black spotting and streaking of the leaves for which no satisfactory explanation is as yet to hand. As with Chestnuts, Walnuts, and other plants containing much tannin, the dark spots appear to be due to this substance, but whether the predisposing cause is a lack of some ingredients in the soil, or some temperature reaction, or fungi at the roots, is as yet unknown. The most recent explanation puts the disease down to the action of bacteria, but the results obtained by different workers lead to uncertainty.
The "dying back" of leaves, especially of grasses, from the tip, is usually accompanied by a succession of colours—yellow, red, brown, to black—and is a common symptom of parching from summer drought; and spots of similar colours, frequently commencing at the margins of leaves, are characteristic symptoms of the injurious action of acid gases in the air.
Brown and blackish spots on Pears are caused by a species of Thrips.
In many cases the minute spots of Rust-fungi on one and the same leaf are bright orange yellow (uredo), deep brown, or almost purple-black (teleutospores), foxy-red brown (older uredospores), or dead slaty black where the old teleutospores have died off—e.g. Uromyces Fabae on Beans, U. Pisi on Peas, etc.
Parti-coloured leaves.—The leaves sometimes start shrivelling with red edges, while yellow, red, and finally brown and black blotches appear on the lamina, from no known cause—e.g. Vines. In other cases similar mimicry of the autumnal colouring of leaves results from the action of acid gases.
Burning is a common name for all cases where the leaves turn red or red-brown in hot, dry weather, and many varieties are distinguished in different countries and on different plants, because species react dissimilarly. The primary cause is usually want of water—drought.
Foxy leaves are a common sign of drought on hot soils, and the disease may usually be recognised by the gradual extension of the drying and fox-red colour proceeding from the older to the younger leaves, and from base to apex—e.g. Hops.