440 



ENCYCLOPEDIA OF PRACTICAL HORTICULTURE 



er. If a mature canker is examined 

 in midsummer, little elevations in tlie 

 bark are easily observed. They are at 

 first more or less conical in shape and 

 finally burst the outer layer of the bark 

 exposing a cream colored mass of fung- 

 ous tissue. These are the fruiting struc- 

 tures of the fungus and the spores are 

 produced in them in great abundance. 



In late summer or early fall the ma- 

 ture cankers are found and may be from 

 one-half inch in diameter up to eight 

 to 10 inches long by three to four broad. 

 Frequently large cankered areas may be 

 formed by several smaller ones becoming 

 confluent. Mature cankers have a definite 

 limiting crack separating them from the 

 healthy tissue; there is usually a definite 

 ridge surrounding the canker caused by 

 the slight formation of callus tissue at 

 the edge under the diseased bark. The 

 bark is sunken, dry and dead, and darker 

 in color than healthy bark. Thickly 

 scattered over the surface one finds the 

 little cracks described above which are 

 formed when the fungus bursts forth. 

 These cracks are transverse or triangular, 

 seldom if ever, vertical. (Fig. 1-A shows 

 a typical canker.) In old cankers the 

 fungus tissue exposed by the cracks turns 

 black and the bark gradually becomes 

 loose at the edges and drops out, leav- 

 ing ugly wounds. Bark may cling in the 

 cankers, however, for at least three 

 winters. The wounds, if not too large, 

 slowly heal over by the formation of cal- 

 lus. 



Injury f'aused by the Disease 



On account of the nature of the disease 

 it is diflicult to estimate the amount of 

 the injury resulting from its ravages. 

 XTnder ordinary conditions few to many 

 cankers may be found on the branches of 

 the trees in infected orchards. Some- 

 times twigs are girdled by the formation 

 of a small canker, which extends around 

 the stem as is shown in Fig. 2. Not in- 

 fi'equently larger branches are girdled 

 where several cankers grow together. 

 Sometimes trunks of young trees are 

 girdled in this wa.v. 



It is evident that the disease in any de- 

 gree of severity interferes with the nor- 



Fis. J. Apple Twia iJirdled h.v Small Canker 

 of Antlii-jicuose. 



mal function of the bark and so hinders 

 the proper distribution of elaborated food 

 in the tree. Branches are weakened by 

 the presence of the cankers and frequent- 

 ly break when heavily set with fruit. The 

 woolly aphis not uncommonly works un- 

 der the bark at the edge of cankers and 

 further saps the vitality of the tree and 

 interferes with the normal healing over 

 of cankers. Large cankers heal slowly 

 and the wood may be exposed for con- 

 siderable periods, thus affording oppor- 

 tunity for the entrance of fungi which 

 cause heart rot. 



Distribution 



As a serious orcliard disease the apple 

 tree anthracnose is peculiar to the Pa- 

 cific Northwest. It is known to occur in 

 British Columbia. Washington, Oregon, 



