CARTER: ILLINOIS TREES: THEIR DISEASES 71 



bark on the trunk. These areas vary from a few inches to more 

 than a foot in diameter and occasionally form a band around the 

 trunk. The fungus Aleu)vdiscus oakesii is associated with this 

 disease. It grows in the outer diseased bark and produces very 

 small, cream-colored, cup-shaped, and somewhat leathery, cush- 

 ion-like fruiting bodies, which appear during winter and spring. 

 Since the fungus is confined to the outer, rough, dead bark, the 

 disease does not retard growth of affected trees. 



Dothiorella Canker. — This stem disease, caused by the fungus 

 Dothiorella quercina, is the most destructive canker disease of 

 oak in Illinois. It affects twigs, branches, and occasionally the 

 trunks of trees in both the white oak and red oak groups. Can- 

 kers develop as dark brown, oval to elongate, sunken areas in 

 the bark (Fig. 65). Frequently, cracks form between the living 

 and dead bark along the margins of the cankers. Soon after the 

 bark has died, fruiting bodies of the fungus develop as pustules 

 in the diseased bark. In time, they erupt through irregular open- 

 ings in the bark (Fig. 66), and spores (Fig. 2) of the fungus ooze 

 out on the surface of the bark. The young sapwood beneath the 

 diseased bark is dark brown to black and shows streaks of dis- 

 coloration that extend into the living wood beyond the area of 

 diseased bark (Fig. 67). 



Other fungi which may cause canker and dieback, especially 

 of weakened oaks, include species of the following genera : Bul- 

 garia, Coniothijrimn, Coryneum, Cytospora, Diatrype, Fusicoc- 

 cum, Nummidaria, Phoma, Phomopsis, Pyrenochaeta, Sphaerop- 

 sis, and Strumella. 



Oak Wilt. — This vascular disease, caused by the fungus 

 Ceratocystis fagacearum, affects all of the important native oaks 

 in the Midwest (Fig. 68). Also it has been associated with bush 

 chinquapin, Chinese chestnut, European chestnut, and tan-bark 

 oak. It is the most destructive disease in oak woodlots and forest 

 areas in the United States. 



Wilt of leaves usually appears first on branches in the upper 

 portion of the crown of an affected tree. The wait progresses 

 downward and inward until all of the foliage is affected. Leaves 

 on trees in the red oak group become dull or pale green in color, 

 and the margins may curl upward. These symptoms are followed 

 by yellowing or bronzing of the leaf tissues ; the discoloration 

 spreads from the margins toward the midribs of affected leaves, 

 which may fall at any stage of wilt. Mature leaves usually re- 

 main stiff and fully expanded during the different stages of wilt 



