A serious phase of the disease is found in the case of twigs stunted but not killed. New leaves are infected as rapidly as they appear. Buds formed on such twigs are infected by the overwintering mycelium, and in the spring the fungus resumes activity when the leaves unfold, thus performing a function vital to the spread of the disease in carrying the fungus over from season to season. On such twigs the internodes are much shortened and the lateral buds are crowded together. (Pl. II, figs. 3 and 4.) These buds are characterized by a purplish red color and an elongated shape, and they are always delayed in opening from a week to 10 days behind normal buds. (Pl. II, figs. 4 and 5, and Pl. III.) In cases of severe infection the formation of new fruit buds is prevented and the following season is one of a light crop or no crop at all. Vegetative growth, however, is favored by the absence of a fruit crop; hence, during such a season the trees are given an opportunity to recuperate their vigor and resist the mildew for the time being. But the following year an increased crop is produced, and mildew, again being favored by reduced vegetative vigor, resumes virulent activity and so tends to produce a periodicity of crops that is very undesirable. The effects of fruit infection are to dwarf the apple and produce a russeting of the skin beneath the mycelium. (Pl. I, fig. 2.) Fruit infection usually occurs early in the season. Active infections on apples after the skin has become hardened have not been observed, and it is therefore probable that danger of such injury is past before the apple is half grown. Infected apples often show elongated stems, while the basins are generally russeted. SUSCEPTIBILITY OF VARIETIES. The varieties of the apple found to be most susceptible to the disease in the Wenatchee Valley are Pryor Red, Jonathan, Newtown, Black Ben Davis, Grimes, Esopus (Spitzenburg), Fameuse (Snow), and Stayman. No variety appears to be immune, but among the least susceptible are Winesap and White Pearmain. Of these varieties the Pryor Red and Fameuse are not commercially important in the Wenatchee Valley. IMPORTANCE OF THE DISEASE IN THE PACIFIC NORTHWEST. Apple powdery mildew occurs in all of the apple-growing regions of the Pacific Northwest, but its economic importance in any of them varies from year to year. Investigations by the writer have shown that the disease is of little economic importance in the Willamette Valley of Oregon and the sections of Washington west of the Cascade Mountains. In these sections infection seems to be confined to watersprouts and terminals and has not been observed to be as severe as that often met with in the irrigated regions. Where apple scab is prevalent, as in these more humid sections of the Northwest, the ordinary sprayings with lime-sulphur solution for scab control are effective against mildew, and its control in these sections, therefore, presents no unusual difficulty. In such districts as the Spokane Valley, Wash., the Hood River valley, Oreg., and other sections where natural rainfall can be relied upon for part of the moisture necessary for tree growth and where spraying for apple scab is generally practiced, the growers and horticultural inspectors are agreed that apple powdery mildew is of little or no economic importance. However, in the hot interior valleys, where irrigation is entirely depended upon for moisture supply and where fungicidal spraying has been regarded as unnecessary, the disease has demanded serious attention, and the development of a safe and effective spraying schedule has been attended with peculiar difficulties. Reports of the State horticultural inspectors indicate that serious loss is often experienced. Mr. C. W. Gilbreath, State horticultural inspector in the Walla Walla (Wash.) district, is authority for the following statement regarding the disease: The damage from apple powdery mildew in this district grew heavier each succeeding year and reached its maximum in 1915. As a result many orchards showed a reduction in crop of 25 to 50 per cent. Jonathan and Newtown are the most susceptible varieties. That the disease is equally serious in the Yakima Valley, Wash., where the largest acreage of apple orchards in the Northwest is found, is indicated by the following statement of Mr. H. E. Waterbury, State horiticultural inspector for the district: Mildew is prevalent all over the valley on certain varieties, such as Jonathan, Spitzenburg, Newtown, and Rome. The loss occasioned is chiefly loss in vitality, which can scarcely be measured. I do not know of any orchards that do not have some mildew. There has been considerable loss from the so-called mildew scratches (russeting) on the fruit, which has reduced the grade. In some places the Black Ben Davis was so badly marked that fully 25 per cent had to be culled. Observations by the writer in the Wenatchee Valley indicate that orchards often have shown a reduction of fully 75 per cent of the crop as a result of mildew infection and that frequently nearly all of the new wood growth is attacked, while foliage infection may reach 75 to 90 per cent. Its damage was most severe in many parts of the valley in 1914 and 1915, but each year sees appreciable losses in some orchards due to mildew infection. In 1915 many crops of Jonathan and Black Ben Davis were complete failures, due to the severity of the 1914 infection, while many other varieties were seriously damaged through fruit russeting, foliage infection, and twig blighting. Alarm over impending damage became so general FIG. 1.-A late stage of foliage infection on Pryor Red watersprouts. Note that many of the leaves have been shed and that those remaining are covered with conidia and are somewhat folded longitudinally. The twigs are covered with mycelium, embedded in which may be seen the dark patches of perithecia, usually located at the base of a leaf and extending up on the petiole. FIG. 2.-Mildew russeting of a Black Ben Davis apple, August, 1916. FIG. 3.Mildewed blossom cluster from a Pryor Red tree. Note the abundance of conidia over leaves and blossoms and the deformity of the blossoms. FIG. 1.-Portion of the terminal growth of a Black Ben Davis tree, showing an early stage of foliage infection. FIG. 2.-Branch of a Jonathan tree, showing a terminal spur killed by an infection of the previous year and three spurs blighted during the current season. This branch had been sprayed with lime-sulphur solution and the mildewed leaves were severely burned, while the healthy foliage was unharmed. FIG. 3.-Branch of a Pryor Red tree, showing two years' growth, with lateral buds on the portion infected during the first year pushed out into spur growth. Note the shortened internodes. FIG. 4.-Terminal growth on a Pryor Red tree, showing shortened internodes on the infected portion. The noninfected buds are swelling, while those infected are still dormant. FIG. 5.-Portion of a branch of a Pryor Red tree, showing noninfected cluster buds opening and considerable expansion of healthy foliage, while infected leaf buds near the tip of the branch are still nearly dormant. FIG. 6.-An apple, showing injury resulting from the application of sulphur-dust sprays in the Yakima Valley in 1917. FIG. 7.-Pryor Red apple, showing sulphur injury, the type of injury which may be expected to follow applications of sulphur sprays after the advent of hot weather in the arid valleys of the Pacific Northwest. |