APPLE POWDERY MILDEW. 15 



amount of mildew on definite branches of each tree, thus presenting 

 a fair average of conditions over the plat. This method removed 

 the personal equation and allowed the results to be placed on a per- 

 centage basis for comparison. In detail the method employed was as 

 follows: An equal number of representative branches were in- 

 spected in the tops and around the lower parts of the trees in each 

 plat; each leaf cluster was taken as a unit, and if any of its leaves 

 showed active mildew infection the unit was so counted. Several 

 thousand such units were examined in each plat about a month after 

 the last spraying. The results, as shown in Table I, indicate the ef- 

 fectiveness of the sulphur sprays, and particularly of lime-sulphur 

 solution, in controlling apple powdery mildew. On trees sprayed 

 with lime-sulphur solution the mildew infection was reduced to a 

 negligible quantity, while the check trees were badly diseased. The 

 less favorable results obtained from the use of the other materials 

 can be attributed partly to the fact that they did not possess as ef- 

 ficient wetting power as the lime-sulphur solution, a point which will 

 be more fully discussed in a later paragraph. However, in the case 

 of the sodium-sulphur sprays the poor results were doubtless in- 

 fluenced largely by the relatively low concentration of polysulphids 

 and free sulphur, which can be regarded as the active ingredients. 



SPRAYING EXPERIMENTS IN 1916. 



The results of the spraying experiments conducted in 1916, to- 

 gether with the resulting spray injury and the control established, 

 are shown in Table II. In 1916 most of the Pryor Eed trees were 

 sprayed with the same materials as in 1915, but the Jonathan and 

 Black Ben Davis trees, which were not included in the 1915 experi- 

 ments, were given two applications of iron sulphicl, formula 7 (see 

 p. 11) by the owner of the orchard. 



In the spraying experiments in 1916 only a negligible amount of 

 foliage injury developed early in the season. Only about 5 per cent 

 of tip burning was found prior to June 22 in plat 13. On that date, 

 however, some injury appeared in nearly alLplats. Sulphur spotting 

 of the fruit began to appear on June 18, and by June 20 a consider- 

 able portion of the exposed fruit which had been sprayed with sul- 

 phur preparations was severely burned. As in 1915, the injury was 

 confined to the portions of the tree receiving the direct rays of the 

 sun in the hottest part of the clay. The extent of the injury was esti- 

 mated by actual counts of several hundred apples on the affected por- 

 tion of each tree. It appeared to be most severe where the deposits 

 of spray material were heaviest. In considering the fruit injury it 

 was found that high temperatures had again prevailed between the 

 time of the completion of the last spraying (June 10) and the date of 

 the first appearance of the sulphur spotting on the fruit (June 18). 



