CARTER: ILLINOIS TREES: THEIR DISEASES 9 



blig-ht, rust, scorch, and chlorosis. Leaf diseases common to most 

 species of trees are powdery mildew, leaf spot, scorch, chlorosis, 

 and those resulting- from chemical injuries. 



Powdery Mildew. — This fungus disease, which usually ap- 

 pears in late July and August, produces an unsightly gray to 

 white powdery covering on the leaves of affected trees. In many 

 cases, small black dots, fruiting bodies of the fungus, are visible 

 on the powdery growth. Although most trees are susceptible to 

 this disease, it occurs infrequently and usually causes very little 

 noticeable leaf injury. It can be controlled by applications of a 

 sulfur fungicide at 10-day intervals after the first appearance 

 of the disease. 



The fungi that cause powdery mildew in Illinois, and the 

 trees they affect, are as follows : 



Microsphaera abii on catalpa, chestnut, elm, honey locust, 

 oak, sycamore, and walnut 



Microsphaera arineophila on beech 



Microsphaera elerata on catalpa 



Mic7'0sphaera diffusa on honey locust 



Phyllactinia conjiea on ash. birch, catalpa, elm. mulberry, 

 and oak 



Phyllactinia suffidta on ash, birch, and catalpa 



Podosphaera leiicotricha on crab apple 



Podosphaera oxycanthae on wild cherry and hawthorn 



Sphaerotheca lanestris (brown mildew) on white oak 



Uncinula circiyiata on maple 



Uncinida clintonii on linden 



Uncinula flexuosa on horsechestnut and elm 



Uncinula macrospora on elm 



Uncinula salicis on poplar and willow 



Leaf Spot. — Leaf spot diseases become conspicuous usually 

 during July and August. Most of them are caused by fungi or 

 by bacteria. Occasionally some of them deform or kill flowers, 

 fruits, twigs, or young branches. Most leaf spot diseases develop 

 as small, scattered, circular to oval, light to dark brown dead 

 areas on the leaves (Fig. 13). Later, these spots may enlarge and 

 unite to form large, angular to irregular dead areas. Minute 

 black dots, fruiting bodies of the fungus, may appear embedded 

 in tissues of the diseased areas. On leaves of some trees, spots 

 may have red to reddish-brown margins, as on leaves of chest- 

 nut, linden, oak, poplar, and redbud. On leaves of other trees 

 the spots may be bordered by purple, as on ash, crab apple, and 



