190 DISEASE IN PLANTS. 



by Ustilagineae, and are commonest in the grain, 

 the soot-like powdery spores (Smut) being very 

 characteristic. Ustilago longissima induces black 

 streaks on the leaves. Many of these fungi cause 

 distortions or pustules on leaves and other organs. 



Brown and black leaf spots are frequently fur- 

 nished with concentric contours arranged round a 

 paler or other coloured central point e.g. Cerco- 

 spora on Beans, Ascochyta on Peas. 



Brown spots with bright red margins are formed 

 in young Beans by Gloeosporiuni. 



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. 



Mai 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 in- 

 gredients 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 



