Three Rust Diseases of the Apple 17. 



These measurements were taken from cells rather toward the tip than 

 toward the base of the peridium. There is a marked difference in size 

 and shape between cells from the base and those from the tip of the same 

 peridium, especially in G. juniperi-virginianae. This fact may account 

 for the difference between these figures and those of Kern (1911). 



The question of the existence of pathogenic strains among these fungi 

 has received little attention, and no certain evidence bearing upon it 

 has come out of this work. However, in view of the circumstantial evi- 

 dence, including the variation in the results of inoculations with telial 

 material from several sources, it seems desirable that this question be- 

 studied further. 



Inoculation tests 



It has been pointed out that this work was stimulated chiefly by the 

 appearance on Mcintosh apple fruits of a serious disease which for a 

 time was not identified with certainty. When it seemed evident that 

 the disease was a "cedar rust," several urgent questions arose. Chief 

 among these was the question of determining which of the three rust 

 diseases common on cedars in the Hudson Valley was blamable for the 

 condition on Mcintosh fruit. Accordingly, inoculations were made 

 with the three fungi G. juniperi-virginianae, G. glooosum, and G. ger- 

 minate, on foliage of the Mcintosh and Wealthy varieties in the green- 

 house and on Mcintosh fruit and foliage in the field. 5 



In seven fruit-inoculation trials involving several hundred fruits, no 

 conclusively positive results were obtained. Most of these inoculations 

 were made at least ten days after the blossoms had fallen, and were 

 apparently later than the optimum period for natural infection. 



On foliage, no infection was produced by G. germinate on Mcintosh 

 and Wealthy in the greenhouse (four trials) or on Mcintosh in the field 

 (four trials). 



G. juniperi-virginianae was used to inoculate Mcintosh foliage in the 

 greenhouse and in the field in nine trials. Three inoculations were 

 made on Wealthy in the greenhouse, for comparison. In two of the 

 trials in the field, natural infection was not entirely excluded and hence 

 no conclusive results were obtained from these. Of the remaining 

 seven tests with Mcintosh, six yielded infection varying from a few 

 spots to as high as 74 spots on one leaf. In one test in the greenhouse, 

 the infection on ;i Mcintosh tree was sufficiently heavy to cause the 

 dropping of several leaves in four weeks. Those spots rarely reached 

 a diameter of 1 millimeter, however, and for the most part merely 

 developed into necrotic "flecks." Fukushi (1925) has shown that 

 Mcintosh foliage responds in a somewhat similar manner to infection 



5 As in the case of other workers (Kern, 1911), the most satisfactory infection was 

 obtained by suspending tbe inoculum above the parts to be inoculated in a humid but 

 not stagnant atmosphere. 



