352 FUNGOUS DISEASES OF PLANTS 



may be exposed, or extensive wounds may result. The disease is 

 more common upon the larger limbs of older trees, but trunks and 

 twigs are not exempt, and young trees may suffer. When complete 

 girdling results, the limb is killed, yet serious consequences may 

 gradually develop without girdling. Fig. 1 70 shows an early stage 

 of this canker. 



Infection is probably most frequent in the spring. It is believed 

 upon good evidence that the worst wounds occur only when the 

 fungus gains entrance to the edge of the wood through wounds. 

 Trees which sunscald badly on the parts exposed to the direct 

 rays of the southwest sun are as a rule subsequently infested with 

 canker. In New York the Spitzenburg and Twenty Ounce are 

 mentioned as the most susceptible varieties of apple to the limb 

 canker, while Baldwin, Wagoner, Greening, and King follow in 

 the order given ; the Tallman Sweet was reported practically 

 resistant. The susceptible varieties of pear are not known. In 

 some cases, at least, the body blight of pear is also to be attributed 

 to this canker organism. 



Infrequently a Sphaeropsis has been found upon the leaves of 

 the pear, and this form appears to be similar to the canker fungus. 1 

 It is thought that general neglect, crowding, lack of pruning, 

 etc., encourage the canker, although it may appear in vigorous 

 orchards. There would appear to be absolutely no doubt that the 

 rot of apples, pears, and quinces is due to the one fungus. Trans- 

 fers of this organism are readily made. Paddock made many pure 

 cultures as well as many transfers of the canker strains to fruits 

 and vice versa, also to a variety of other hosts. The inoculation 



1 From extensive experiments made during 1907 by Scott and Rorer, it has 

 been demonstrated that the common leaf spot of the apple, as it occurs east of 

 the Rocky Mountains, is also generally traceable to Sphczropsis Malorum Pk. as 

 a primary cause. The other fungi which have been associated with the apple 

 spot, such, for instance, as Phyllosticta Pyrina Sacc. and Phyllosticta limitata Pk., 

 have not been found to induce leaf spots upon inoculation. From young spots 

 the Sphaeropsis colonies are constantly plated out, and the other fungi mentioned 

 were only present during the later stages of the disease. Moreover, inoculation 

 experiments with the former have almost invariably yielded positive results 

 within from five to ten days. These observers are of the opinion, therefore, that 

 neither Phyllosticta nor other forms which may be found upon those spots are of 

 any special importance in the apple orchard. (Scott, W. M., and Rorer, J. B., 

 Apple Leaf- Spot caused by Sphaeropsis Malorum. Bureau Plant Industry, U. S. 

 Dept. Agl. Built. 121 : 47-54. 1908.) 



