10 ILLINOIS NATURAL HISTORY SURVEY CIRCULAR 46 



Fig. 13. — Leaf spot diseases produce small, scattered, circular to oval, 

 brown dead areas in the leaves. Frequently two or more spots coalesce and 

 form large, irregular diseased areas. On elm, show n in this picture, the leaf 

 spot fungus produces numerous fruiting bodies on the brown diseased tis- 

 sues; these fruiting bodies make the spots appear black and scab-like. 



hawthorn. Spots on walnut leaves are large, dark brown, and 

 round to oval. On cherry leaves the brown dead areas of tissue 

 drop out in time, leaving a shot-hole appearance of the leaf. One 

 leaf spot disease of crab apple and hawthorn produces minute 

 purple specks soon after the leaves unfold ; it is caused by the 

 fungus Physalospora obtusa. The specks soon enlarge to form 

 reddish-brown, circular to oval spots. After a few weeks, some 

 of the spots may start to enlarge and to form concentric rings of 

 dark brown, which give rise to the term f rogeye. This fungus also 

 produces black rot of fruit and cankers on twigs and branches. 



Species of fungi that cause leaf spot diseases in Illinois be- 

 long to the following genera : Alternaria, Ascochyta, Cercos'pora, 

 Coccomyces, Coniothyrium, Cylindrosporium, Dothiorella, Gloeo- 

 sporiuTYi, Gnomonia, Leptostroma, Marssonina, Microsphaerella, 

 Monochaetia, Phyllosticta, Physalospora, Septoria, and Venturia. 

 The bacterium Xanthomonas pruni causes a leaf spot of choke- 

 cherry. 



Several successive years of defoliation resulting from leaf 

 spot disease may weaken affected trees and increase their sus- 

 ceptibility to other diseases. Such affected trees can be given 

 plant food to stim.ulate vigorous growth. Leaves may be pro- 

 tected against leaf spot diseases by fungicides. Fungicidal sprays 

 recommended for the control of some common leaf spot diseases 

 are given in Table 1 (end of circular). Two or three applications 



