CARTER: ILLINOIS TREES: THEIR DISEASES 



11 



should be made at 14- to 21-day intervals. The first spray should 

 be applied approximately 3 weeks before spots appear on leaves, 

 about June 15 for most leaf spot diseases in Illinois. 



Infectious leaf diseases that affect one or a very few spe- 

 cies of trees include leaf blotch, leaf blight, and leaf rust. Each 

 of these diseases is described in the discussion of one of the trees 

 on which it occurs. 



Scorch. — This noninfectious disease may occur on any kind 

 of tree. Ash, elm, and maple are frequently affected by it. 

 Scorch develops as yellowing or browning of tissues between 

 the veins or along the margins of leaves (Fig. 14), or as brown- 

 ing and withering of entire leaves. It may be caused by internal 

 physiological disturbances, such unfavorable weather conditions 

 as low temperature or drought, girdling roots, or soil area too 

 limited for good growth. Frequently, scorch develops in July and 

 August when the roots are unable to furnish sufficient water to 

 compensate for the moisture lost from the leaves during pro- 

 longed dry periods. Drying winds when the temperature is high 

 will increase the amount and severity of scorch. Trees affected 



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W^ 



Fig. 11. — Scorch and chlorosis are noninfectious diseases associated 

 with unfavorable environmental conditions. Scorch {oak leaf at right) de- 

 velops as browning or yellowing between the veins or along the margins 

 of leaves, or as complete browning and withering of leaves. Chlorosis {center 

 oak leaf) is indicated by a pale yellowish-green color of the leaf tissue and 

 normal green color of the midribs and veins. A normal oak leaf is shown at 

 the left. 



