This mildew affects, in addition to the apple, the pear, quince, cherry, plum, thorn and juneberry. Seedling nursery - stock, wherever apples are grown, are likely to suffer to some extent from this disease. In localities where the relative humidity at times runs high such varieties as the Yellow Newtown and Yellow Bellflower are liable to be severely injured by this disease. In the Pajaro Valley of California no variety is immune, although there is some variation in this respect. The most susceptible varieties include the Smith, Missouri, Esopus and Gravenstein in addition to the two already mentioned. Those less seriously affected are: White Pearmain, Winter Pearmain, Red Astrachan, Rhode Island and Langford.
Apple Powdery Mildew has a wide geographical range over the world. It has been reported from Europe (Germany, Austria-Hungary and Russia), Asia (Japan), Australia, New Zealand and North America (United States and Canada). In the United States it has for many years been a serious nursery pest, and recently the disease has caused injury to bearing orchards. This mildew is most prevalent and most serious west of the Rocky Mountains, particularly along the Pacific Coast where climatic conditions apparently are very favorable to the pathogene. The peculiar weather conditions in the Pajaro Valley of California favor Powdery Mildew, and here commercial apple orchards suffer more than those of any other district in the United States. Bearing trees I elsewhere in California also suffer extensively from the effects of this disease.
The nature of the injury induced by Powdery Mildew makes an estimate of the financial loss difficult. The leaves and twigs are directly affected, while the fruit escapes almost entirely. Damage may result to the affected host in one or more of the following ways: (1) the leaves are so affected that it is difficult to find normal foliage in humid regions; this is especially true of susceptible varieties; (2) affected shoots are stunted; the terminals as well as the leaves die; (3) the floral organs are reduced in size; (4) affected trees may fail to form blossom - buds; (5) diseased trees obviously cannot produce well; (6) trees which are affected year after year decline in normal appearance, owing to the cumulative effect of the disease.
Early in the summer the foliage and young twigs may show signs of Powdery Mildew. Leaves are most commonly affected on the lower surface, where a whitish or grayish powder develops. These powdery spots may reach nearly an inch in diameter; several may occur on a leaf, and as a result of their coalescence the lesions involve considerable area. Affected leaves crinkle, become stunted, and are particularly more narrow than normal leaves. Later the mildew may be seen on the upper surface of the foliage. Diseased twigs may be practically covered with mildew. Like the leaves they are stunted, the internodes of a single year's growth sometimes reaching not more than one or two in length. In the winter the terminal shoot may die back; the following spring a new shoot arises from a lateral bud. This phenomenon may be repeated year after year. In the latter part of the summer black fruiting bodies appear as specks scattered through the powdery surface growth. No direct injury is done to the fruit and infections thereon are extremely rare. When the floral parts are affected, they are dwarfed and deformed.
The pathogene is a fungus known as Podosphcera leucotricha [ = Sphcerotheca Mali (Duby) Burr.]. Sometimes a closely related species, P. Oxyacanthae (Dc) DeBary, attacks the apple, causing much the same difficulty as that just described. But in the apple regions of New York and California where the disease has been carefully studied it has been found that P. leucotricha is by far the more common form present.
The mildew that is found on the leaves and twigs is made up of interlaced fungus hyphae. Certain of these threads, or hyphae, become erect and bear chains of conidia. In mass they give to the affected organ a powdery appearance. These spores, or conidia, are blown to other leaves and twigs throughout the growing season, causing infections. A spore falling on a leaf or twig soon germinates by protruding a germtube. From the latter, mycelium develops copiously, growing out in a radiating fashion. At frequent intervals the mycelium sends haustoria into the epidermal cells of the attacked part; through these organs the parasite obtains food. Soon a new crop of conidia is developed, as already described. During the latter part of the summer dark-brown, globose perithecia are formed among the hyphal threads. These appear to the naked eye as irregular, smoky patches on the twigs. The fungus is carried over winter by these bodies. Each perithecium has several appendages, and a single ascus containing sexual spores. These ascospores are capable of producing infection with the advent of the growing season. It is said that in the Pajaro Valley where Powdery Mildew is so prevalent on the apple the ascospores do not commonly bring about first infections in the spring. In that region as well as in others a dormant-bud infection occurs. The lateral and terminal bud - scales are penetrated by the fungus; here it remains dormant until spring, at which time growth is resumed. Hence infected shoots appear as soon as the leaves develop. Conidia mature rapidly on these shoots and soon the parasite has extended itself to other twigs and leaves in a manner conducive to certain perpetuation.
The application of certain fungicides supplemented by the eradication of heavily mildewed twigs are satisfactory remedial measures. But the disease is not easy to control. In those cases where copper sprays are still in use, little benefit may be expected so far as the control of apple Powdery Mildew is concerned. The chief difficulties lie in the fact that copper compounds produce leaf and fruit injury, and that the mildew fungus is not as susceptible to bordeaux mixture and similar copper sprays as it is to the sulfur sprays. The sulfur compounds are effective under certain conditions of preparation. Sulfur in some very finely divided state is the most efficient fungicide available for use against this disease. Such fungicides as ground sulfur, sulfur flour and flowers of sulfur are all said to be too coarse to yield satisfactory results. On the other hand, precipitated sulfur is particularly effective against apple Powdery Mildew. This may be obtained by the use of lime sulfur solution, diluted one gallon of the concentrated solution to fifty gallons of water to which is added three pounds of iron-sulfate. By stirring the two together a black, muddy precipitate is formed; this contains sulfur in a finely divided condition, iron-sulfid and calcium-sulfate. It has been demonstrated that this spray is effective against apple mildew both in New York State and in the Pajaro Valley, California. In the nursery the first application should be made soon after the seedlings have developed the first new shoots. Three or four sprayings should follow at intervals of about two weeks. According to experience in the Pajaro Valley, orchard trees should be sprayed as follows: (1) apply the above fungicide in conjunction with the first spraying for codlin-moth; (2) inconjunction with the second application for codlin-moth; (3) three weeks after the second; (4) three weeks after the third. These recommendations, while based on experiments made in the above-named region where the weather conditions are somewhat unusual, should serve as a guide in other localities. Doubtless it will be found desirable to vary the schedule tabulated above to suit the conditions obtaining in those apple districts where it becomes necessary to give attention to the control of apple Powdery Mildew.
Ballard, W. S. Apple powdery mildew and its control in the Pajaro Valley. U. S. Agr. Dept. Bul. 120: 1-26. 1914. Stewart, V. B. Some important leaf diseases of nursery stock. Apple powdery mildew. Cornell Univ. Agr. Exp. Sta. Bul. 358: 181184. 1915. Galloway, B. T. Experiments in the treatment of pear Leaf Blight and the apple powdery mildew. U. S. Agr. Dept. Veg. Path. Sec.
Circ. 8:5-11. 1889. Jackson, H. S. Apple diseases. Powdery mildew. Oregon Crop Pest and Hort. Bienn. Rept. 1911-1912: 236 - 238. 1913. Pammel, L. H. Powdery mildew of the apple. Iowa Acad. Sci.
Proc. 1899: 177-182. 1900. Grout, A. J. A little - known mildew of the apple. Bul. Torr. Bot.
Club, 26: 373 - 375. 1899.