Powdery Mildew on Apples

Roderick Sprague.

Apple powdery mildew, caused by the fungus Podosphaera leucotricha, is widely distributed in the United States. It is serious mainly in the far West, notably in the warm, dry valleys of Washington (in the neighborhoods of Wenatchee and Yakima) and Oregon and in some valleys of California.

Many of the old susceptible varieties Black Ben, Grimes, Spitzenberg, and to some extent Yellow Newtown have all but disappeared from commercial production in the West and no longer cause problems. Jonathan and Rome Beauty, which are still widely grown, however, are highly susceptible to powdery mildew.

Jonathan is used as a pollinizer for Delicious and Red Delicious. It carries mildew in the mixed orchards. Jonathan is being replaced in the West by less susceptible and more profitable varieties, but its value for juice processing and its recognized merit as a fruit will keep it in continued use for many years. Therefore the control of apple powdery mildew on those useful but susceptible varieties remains an important problem.

Powdery mildew attacks the leaves, twigs, blossoms, and fruit of apples, pears, and sometimes quince, hawthorn, serviceberry, cherries, and prunes. Usually the mildew found on cherries is a closely related species (P. oxyacanthae) which is reported now and then on apples.

Powdery mildew first forms off-white or pale-gray, feltlike patches on the margins of leaves, usually on the under side. The fungus then gradually spreads until it covers the whole leaf surface and all the leaves in a terminal cluster. The mildew grows down the twig, which it covers with a gray felt. Dark-gray or black patches later form on the gray felt after the many tiny, globe-shaped winter fruiting bodies of the fungus develop in the summer.

Young infected leaves become somewhat narrowed and folded longitudinally, and their under sides are exposed. Some become curled, crinkled, and stiffened. Many fall off before autumn. Terminals attacked early in the year often fail to survive or have tiny, dried remnants of leaves left on them at harvest. Such twigs and their buds are easily injured in winter. Mildewed terminals tend to enter the winter with open buds. Jonathan is particularly subject to terminal killing because of this inability to close the buds over with bud scales. Because so many terminals are killed or stunted by mildew, the trees are forced to send out twigs from side buds; twig bunching or staghorn growths often result. They deform the tree and reduce its fruiting surface.

Blossom infection sometimes occurs when the fungus overwinters in the dormant blossom bud. Such infection usually causes blighted clusters in which no fruit survives. Because the infected buds emerge late in spring, the chances of widespread seasonal blossom infection are slight. Sometimes young fruits are attacked soon after the blooming period, however. The fruit infection does not last very many weeks but sometimes causes an etching or russeting of the fruit, especially at the calyx end.

Russeting of fruit has been the cause of most of the complaints about loss from mildew. Heavy cullage from fruit russet in 1948 in eastern Washington led the industry to ask for further research of the problem. A great deal of work was done by D. F. Fisher, of the Department of Agriculture, about a quarter of a century ago at Wenatchee. Later E. L. Reeves continued the investigations. Mildew continued to be abundant on susceptible varieties in eastern Washington but very little fruit russeting attributable to powdery mildew was seen between 1948 and 1953. Mildew injury to the fruit in California is not common.

Mildew on such resistant varieties as Delicious or Winesap is usually from infections of the current season. Very little overwinters, except under favorable conditions in thickly planted orchards. A few twigs of Winesaps appear infected in very early spring.

The infection on Delicious from the current-season spores usually starts as obscure marginal lesions. They may show a pale, wine-colored or lavender-tinted border. The leaves usually roll inward. In orchards where Jonathan and Delicious are closely inter-planted, 50 percent of the leaf clusters of Delicious sometimes are infected, but the lesions usually are restricted or only a leaf or two in one cluster is diseased.

Mildew appears in the Wenatchee district before blossoming time. It develops rapidly until June. After hot weather comes, the rate of its spread slows down, but the total infection slowly increases until harvest. During severe infestations about 75 percent of unsprayed leaf clusters of Jonathan are infected at harvest. The next spring 10 to 40 percent of the emerging buds show mildew, which arises from overwintering mycelium in the buds.

The severity of the winter determines how many infected buds will survive. The mildew fungus is relatively winter hardy,. but severe cold will kill many of the diseased buds and with them the mildew. Mildew is a parasite that cannot survive without its host plant. Trees that are not sprayed for mildew show excessive twig injury and in time will succumb to winter injury.

THE FUNGUS that causes mildew grows on the surface but it sends longsuckers (haustoria) into the leaf cells. Perhaps that is why it is hard to kill it with fungicides.

The fungus produces enormous numbers of spores in chains on the fungus threads. Wind carries the spores great distances. The spores require little moisture to germinate the dew in irrigated orchards is enough to allow the fungus to develop in dry regions.

A survey in Chelan, Douglas, and Okanogan Counties in north central Washington in 1949-1951 showed that mildew was nearly as prevalent in orchards irrigated by rills along the ground as in ones irrigated by sprinklers. There has been a tendency to blame the sprinklers for an increased amount of mildew.

Overhead sprinklers tend to favor mildew in resistant varieties or in some young orchards of Golden Delicious. On susceptible varieties such as Jonathan, the type of irrigation did not appear to have any great influence on mildew although sprinkling at frequent intervals tended to increase mildew.

In Jonathan the mildew is usually worse on the southwest and west sides of the trees and in the treetops. In a tree that shows about 75 percent of the leaf clusters mildewed the tops will usually average about 95 percent mildew.

DR. FISHER mentioned that lime-sulfur sprays to control apple scab would also control mildew. For that reason mildew was seldom serious in a region where scab was serious and an efficient spray program was carried out for scab. Lime-sulfur has been used less and less in the West, and outbreaks of scab have occurred in several places in Washington.