The most effective method for the control of mushroom root rot would be to plant walnuts grafted on resistant rootstocks. The eastern black walnut is reported to be highly resistant to mushroom root rot. For many years Hinds black walnut was believed to be highly resistant, but Armillaria mellea has been found on its roots in California and Oregon.

The discovery of the fungus in black walnut roots makes it inadvisable to recommend without reservation Persian walnuts grafted on Hinds black walnut roots for land contaminated with the mushroom root rot fungus. Using Hinds black walnut as a stock will not necessarily guarantee freedom from mushroom root rot.

It is possible to kill the fungus in the soil by treatment with powerful soil disinfectants, among the most effective of which are carbon disulfide and chloropicrin. Carbon disulfide is preferable as it is less dangerous to use and not so costly. The method used in applying carbon disulfide is to inject 2 ounces by weight (3/5 ounce liquid measure) of the material in holes 8 inches deep and spaced on an equilateral triangle 18 inches apart each way. The holes must be plugged immediately after application for the gas fumes to be effective. The 2-ounce dose is enough to kill the fungus to a depth of 5 or 6 feet. Since the gas produced is heavier than air, the liquid should not be applied at too great a depth; otherwise, satisfactory kill of the fungus will not be had at the surface of the ground.

For light, sandy soils, the depth of application should be about 8 to 10 inches; in heavy soil types, not more than 6 to 8 inches. Carbon disulfide is highly inflammable and should be handled carefully. All living trees and other vegetation, as well as the fungus in the treated area, are killed, but the fumes soon disperse, and the area can eventually be replanted. All infected trees should be taken out before the soil is treated.

All the affected soil area must be thoroughly treated if the spread of the fungus is to be stopped. Treatment of the hole made in the removal of a diseased tree is not enough. The exact determination of the extent to which the fungus has penetrated the soil is difficult. For safety, consequently, the treatment should be extended beyond the probable limits of fungus distribution. Sometimes it may be necessary to sacrifice apparently healthy trees around the area to exceed the apparent limits of contamination.

Walnut and filbert orchards should not be planted on newly cleared land where infected native oak trees formerly stood. Care should also be taken not to injure the basal part of the trunk or the main roots of walnut trees during cultivation, for such wounds increase the chance of infection by providing an easy avenue of entry for the fungus into the tissues.

FILBERT bacterial blight, caused by Xanthomonas corylina, is the most destructive parasitic disease of the filbert. It Is present in practically all filbert orchards in Oregon and Washington. Its prevalence and destructiveness vary with the season. It causes considerable damage in years of heavy rainfall in fall and winter.

The organism attacks the buds, leaves, branches, and trunk. Occasionally it attacks the nuts, but seldom does it invade the roots.

The disease manifests itself as small, angular or round, pale yellowish-green water-soaked spots in the blades of the leaves. The lesions later turn reddish brown.

The first evidence of infection of the stems of shoots of the current season's growth consists of dark-green, water-soaked areas in the bark. They later turn reddish brown. The lesions often encircle the stems and cause the leaves on the distal parts to turn brown and die. The dead leaves generally cling to the girdled stems for some time and look as though they had been killed by fire.

One- and two-year-old twigs are also attacked and killed. Infection of such twigs takes place indirectly through wounds or by invasion of the bacteria from blighted buds and diseased shoots of the current season's growth. The lesions frequently girdle and kill the twigs. Twig infection is of considerable economic importance, because many of the killed twigs are potential nut bearers.

The formation of cankers on the tree trunk is the most serious aspect of the disease, especially when they girdle and kill the trees, as often happens in orchards up to 4 years old. The trunk is seldom infected after the fourth year, but many buds and nut-bearing twigs in the tops of older trees are attacked and killed.

The original source of infection is diseased nursery stock, from which it is spread by cultural operations and rain. Shears or knives used in pruning or suckering may be readily contaminated with the bacteria by accidentally cutting through an active canker. Unless a disinfectant is used on the tools, the germs are carried to adjoining trees, where other centers of infection may be established.

While bacterial blight can and does attack healthy, vigorous trees, the damage is accentuated when the trees have been weakened or injured by sunscald, cold, drought, improper drainage, or other adverse factors. The tissues of devitalized or injured trees offer much less resistance to infection and the subsequent extension of the diseased areas than do those of strong, vigorous trees. It is therefore important that the trees be kept healthy and vigorous to lessen the damage in the event of infection.

Tree losses from blight traceable to contaminated tools can be prevented by sterilizing the tools with a good germicide, such as a 95-percent solution of methyl alcohol (methanol). The use of such a sterilizing agent on the tools is particularly advisable when suckering and pruning young trees, 1 to 4 years old, as lesions on the trunks during that period frequently girdle and kill the trees.

The incidence of bud and twig blight due to bacterial blight can be reduced in both young and old orchards by timely spraying with bordeaux mixture (6-3 100). The addition of a commercial wetting and sticking agent, such as Dupont spreader-sticker, to bordeaux mixture will increase its effectiveness. In a normal season one application of bordeaux mixture, made in August before the first fall rain, is generally enough to give good control. In seasons of heavy rainfall in fall and winter, supplementary applications in late fall when the leaves are about three-fourths off the tree and in the early spring when the leaf buds are breaking open are necessary to hold the disease in check.

It is possible to combine in one mixture the spray materials recommended for the control of bacterial blight with that recommended for control of the filbert worm and thus save time and labor required to make separate applications. This combination spray consists of bordeaux mixture (6-3-100) plus lead arsenate, 3 pounds in 100 gallons of mixture. To this should be added an efficient, compatible spreader-sticker, such as Dupont spreader-sticker. In preparing this combination spray, the bordeaux mixture should always be made up first, then lead arsenate and the spreader-sticker added in that order. This spray material should be applied after the moths of the filbert worm begin to lay eggs but before any eggs hatch. In a normal season in the Pacific Northwest, the first moths generally emerge in early July and continue doing so for a month or more.

The filbert is very susceptible to wood and heart rots caused by a variety of wood-rotting fungi.

Those organisms cause a rotting and decay of the heartwood and sapwood. The disease generally begins around pruning cuts and other injuries and works its way back into healthy tissues. Most of the organisms causing wood rots gain access to the tissues through injuries from pruning, careless cultivation, cold injury, sunscald, windstorms, and ice storms. Stubs of branches left in pruning are one of the common avenues of entry.

All wounds one-half inch or more in diameter, particularly if made in the fall or winter, should be painted at once with a paste made by mixing equal parts of a commercial dry bordeaux mixture and water. Later, after the wound has dried out and started to heal, a semi-permanent wound dressing containing a fungicide, such as bordeaux-linseed oil paint, should be applied to the surface to prevent the entrance of bacteria and fungi.

All large pruning cuts should be made close to the main branch or body of the tree to expedite healing. In removing limbs, care should also be taken not to tear the bark. This danger will be avoided if the branch is cut nearly half through from the under side first and then finished off from above.

PAUL W. MILLER, a graduate of the Universities of Kentucky and Wisconsin, is a plant pathologist in the Bureau of Plant Industry, Soils, and Agricultural Engineering. He joined the Department of Agriculture in 1929 after being an instructor in plant pathology and research assistant at the University of Wisconsin. Dr. Miller has conducted research on the diseases of apples, vegetable seeds, strawberries, walnuts, and filberts.