Page 4 



BETTER FRUIT 



April 



Figure 3— Mildewed blossom clusters. Note the abundance of spores over the leaves and 

 blossoms and the deformity of the blossums. 



fungus spreads very rapidly and soon 

 the entire leaf becomes covered and the 

 mjcelium spreads down onto the twig 

 or spur and onto other leaves or apples 

 that may be attached. On the fruit the 

 fungus produces a characteristic russet- 

 ing — a disfigurement that lowers the 

 grade of the apple, and hence lowers 

 the return of the grower. This feature 

 of the disease is most serious on such 

 solid-color apples as Jonathan, Grimes, 

 Spitzenberg, Newtown and Black Ben 

 Davis, all of which appear to be very 

 susceptible to the disease. The russet- 

 ing is not as evident on the striped 

 varieties, of which the Stayman, Ben 

 Davis and Rome are most suscpetible. 

 No variety is immune, but the Winesap 

 and White Pearmain appear to be least 

 susceptible. 



About the middle of June tiny black 

 bodies begin to appear in the white 

 patches of mildew on the twigs. These 

 constitute another type of spore, which 

 at one time doubtless served to carry 

 the fungus over winter or other 

 adverse growing conditions, but this 

 function is now lost, and these "winter 

 spores" are known to play no part in 

 the essential life-history of the fungus. 

 The disease is carried over from year 

 to year by the mycelium which pene- 

 trates the buds formed on infected 

 branches and which remains dormant 

 with the buds until the following 

 spring. Then, as these infected buds 

 unfold, the meycelium resumes activity, 

 covering the leaves and twigs and soon 

 produces its first crop of summer 

 spores, whereby a new cycle of infec- 

 tion may be started. 



From these facts in the habits and 

 life-history of the fungus it is apparent 

 that there are two methods by which 

 the disease may be combated: (1) by 

 cutting out the infected twigs, and (2) 

 by destroying the fungus as it grows. 

 To the first there are practical diffi- 

 culties which render it impossible of 

 achievement on a commercial scale. In 

 the experimental work at Wenatchee, 



Washington, reported in this article, 

 it was found that many of the twigs 

 bearing infected buds could be detected 

 early in the spring, at the time the 

 normal buds were beginning to show 

 green. At this time the infected buds 

 remained dormant, being delayed about 

 ten days or two weeks behind the nor- 

 mal buds; too, the infected buds were 

 more slender and elongated, with a 

 peculiar reddish color. The persistent 

 mycelium on the stunted twigs bearing 

 these buds imparted a silvery appear- 

 ance in the sunlight. By extremely 

 careful work in pruning, all such twigs 

 were removed (in the case of badly 

 infected trees, only through the serious 

 mutilation of the young growth, how- 

 ever). Later, after all the buds had 

 opened, it was found that numerous 

 infections still existed. These could be 

 traced to isolated buds that had escaped 

 detection in the very careful pruning, 

 done with a thoroughness demanded in 

 investigational work. From this it was 

 apparent that pruning, as it ordinarily 

 must be done in commercial orchards, 

 could never be depended upon to eradi- 

 cate or materially affect the disease. 

 In subsequent spraying experiments it 

 was shown that the careful pruning 

 followed by proper spraying during the 

 growing season was a valuable supple- 

 ment to the spraying, and that less 

 trouble was experienced in controlling 

 mildew on the pruned trees. But at the 

 same time it was apparent that, since 

 adequate results follow a consistently 

 adhered to spraying program, the extra 

 time and expense involved in the very 

 thorough cutting out of infected twigs 

 in a badly-infected orchard is not justi- 

 fied from a commercial standpoint. 



Where apple scab is prevalent, as in 

 the more humid sections of the Pacific 

 Northwest, and where it is controlled 

 by spraying with lime-sulphur or other 

 sulphur materials, little trouble is ex- 

 perienced from apple powdery mildew, 

 since the treatment followed in control- 

 ling scab is likewise most efficient 



against mildew. It is in the hot interior 

 irrigated districts, where fungicidal 

 spraying is almost unknown, that most 

 difficulty is experienced in controlling 

 mildew — and where lies its greatest 

 menace. These districts are the highly 

 specialized and greatest prdoucing sec- 

 tions for apples in the West. They are 

 the districts, therefore, in which the 

 growers can afford to take the fewest 

 chances, although the nature of the 

 climate in these districts adds to the 

 difficulty of controlling the disease. 



In the experimental work at Wenat- 

 chee, Washington, it was not found 

 difficult to restrict the spread of the 

 disease with any one of several sulphur- 

 spray materials, applied during the 

 growing season (dormant sprays being 

 ineffective), and with lime-sulphur 

 solution good control was established. 

 However, it was found that the use of 

 any of the sulphur sprays during the 

 extremely hot weather which prevails 

 in the summer caused serious injury — 

 burning all of the apples exposed to the 

 direct rays of the hot midday or early- 

 afternoon sun. This injury is doubt- 

 less brought about by the volatilization 

 of the sulphur deposits on the fruit. It 

 does not necessarily occur immediately 

 after the spray is applied, but is de- 

 pendent on the intensity of the sun- 

 light. Sulphur burning of the fruit has 

 frequently been delayed fully two 

 weeks after the application of the 

 sprays, and has always followed the 

 advent of burning sunlight, which, 

 under Wenatchee conditions ' was 

 found to accompany shade tempera- 

 tures ranging above 90 degrees. Hence 

 the use of sulphur sprays should be 

 avoided when such temperatures may 

 be expected to prevail within a couple 

 of weeks. 



Other spray materials were tested in 

 an endeavor to find a substitute for use 



Figure 4 — Branch from a Jonathan tree, show- 

 ing a terminal spur killed by an infection of 

 the previous year and three spurs blighted 

 during the present season. This branch had 

 been sprayed with lime-sulphur solution and 

 the mildew ed leaves were severely burned, . 

 while the healthy foliage was unharmed. 



