44 ILLINOIS NATURAL HISTORY SURVEY CIRCULAR 46 



circular to irregular dead areas with slightly depressed or raised 

 margins on slow-growing or old stems with smooth bark. Gir- 

 dling of stems by these areas results in death of the parts beyond 

 the affected regions. Usually trees die within 3 or 4 years after 

 they have become diseased. No effective control has been devel- 

 oped for chestnut blight. The varieties of chestnut which are 

 resistant to blight are more suitable for ornamental plantings 

 and nut production than for forest trees. 



Other Cankers. — Two other canker diseases of chestnut trees 

 have been observed in Illinois. One occurs on American chestnut 

 and is caused by the fungus Phomopsis castanea. The other oc- 

 curs on Chinese chestnut (Fig. 42) and seems to be associated 

 with a fungus belonging to the genus Fusicoccum ; possibly it is 

 Fusicoccum castaneiun (Cnjptodiaporthe castanea). Affected 

 branches should be cut 1 or 2 feet below any evidence of infec- 

 tion. In nursery and orchard plantings, trees should be spaced to 

 allow for free movement of air. 



CRAB APPLE 



Crab apple is susceptible to powdery mildew, leaf spot, scab, 

 rust, and canker diseases. Powdery mildew and leaf spot diseases 

 are described in the section "Types of Tree Diseases," Fire 

 blight is the most destructive canker disease of crab apple. 



Scab. — This disease of hawthorn and mountain ash, as well 

 as crab apple, is caused by the fungus Venturia inaequalis. It ap- 

 pears on both upper and lower surfaces of leaves ; frequently it 

 appears first on the lower. It produces olivaceous to sooty or 

 smudgy spots (Fig. 43). On young leaves, diseased areas fre- 

 quently appear as radiating spots with feathery edges. On older 

 leaves, the diseased areas appear as definite spots with contin- 

 uous, well-defined margins. In time, the leaf tissues in each spot 

 on the upper side of a leaf thicken and bulge upward until the 

 spot resembles a scab ; the tissues on the lower side become de- 

 pressed and cuplike in appearance. The disease may produce 

 similar spots on blossoms and fruit. On fruit, the spots enlarge 

 and become at first black. Later they become brown, with black 

 margins, and corky. 



Heavy scab infections may cause leaves to become curled, 

 distorted, and dwarfed. Leaf stems or petioles that become in- 

 fected show elongate lesions similar in color to the spots or le- 

 sions on leaves. A leaf with several of these lesions on its petiole 

 is likely to turn yellow and drop. Extensive petiole infection may 



