Filberts and Persian Walnuts

Paul W. Miller.

The Persian, or English, walnut (Juglans regia) is prey to several diseases that harm its growth and reduce the quality and yield of nuts. Some, called parasitic diseases, are caused by micro-organisms that live as parasites on the host plant. Other disorders, known as nonparasitic diseases, result from unfavorable weather conditions, unsuitable soil, or deficiency or excess of one or more mineral elements in the soil.

Bacterial blight of walnuts, caused by Xanthomonas juglandis, is of greater economic importance than all other diseases of walnuts together. Its prevalence and destructiveness vary from year to year. It is usually much worse in the foggy coastal districts of the Pacific coast than in inland localities. More than 50 percent of the crop in unsprayed orchards has been lost in years of epidemic outbreaks.

The leaves, nut, catkins, buds, and shoots of current growth are attacked. The disease first appears in the spring on the young leaves as small, reddish-brown spots at the margins or in the tissues between the veins. On the stems the disease causes black, slightly depressed spots, which often girdle the shoots and cause them to die back. Young leaf and catkin buds turn dark brown or black and die. It is most serious and destructive on the nuts, on which it causes black spots of varying size.

The causal organism lives over from one year to the next primarily in diseased buds and to a lesser extent in hold-over cankers on twigs of the previous year's growth.

The bacteria are disseminated principally by raindrops from hold-over sources of infection. The bacteria enter the tissues of the current growth through pores (stomata). Long and frequent rains just before and during blossoming and for about 2 weeks thereafter may bring serious outbreaks, as the nuts are most susceptible then. Infections that occur after the nuts are about three-fourths grown are of little economic importance because they seldom reach the shell. Infected pollen grains from diseased catkins also sometimes distribute bacteria.

The blight can be controlled by timely and thorough applications of certain copper compounds.

Any one of the following spray materials, properly applied, is effective: (1) Bordeaux 4-2 100 (or 6-3 100 in exceptionally wet districts). A summer oil emulsion, at the rate of 1 pint of oil emulsion to every too gallons of bordeaux mixture should be added to prebloom applications to reduce the severity of leaf injury in the event that conditions favor. its development. (2) Yellow cuprous oxide (Yellow Cuprocide) at the rate of 1 pound in 100 gallons of water (1 1/2 100 in exceptionally wet districts). (3) Tetra-copper-calcium oxychloride (Copper A Compound) at the rate of 2 pounds in 100 gallons of water (3 pounds to 100 in exceptionally wet districts).

Any one of the following dust formulations is effective if enough timely treatments are applied: (1) Copper-lime-sulfur-oil dust composed of 15 percent monohydrated copper sulfate, 30 percent hydrated lime, 10 percent dusting sulfur, 39 1/2 percent talc, 2 percent bentonite, 2 percent diatomaceous earth, and 1 1/2 percent light mineral oil. (2) Yellow cuprous oxide-sulfur dust composed of 5 percent yellow cuprous oxide, 15 percent dusting sulfur, 78 percent talc, and 2 percent diatomaceous earth.

If the walnut orchard is interplanted with filberts, the sulfur should be omitted from the formulas I have given, as it sometimes is toxic to filbert foliage. The following dust mixtures are recommended in a mixed orchard of walnuts and filberts: (1) 15 percent monohydrated copper sulfate, 30 percent hydrated lime, 49 1/2 percent talc, 2 percent bentonite, 2 percent diatomaceous earth, and 1 1/2 percent light mineral oil. (2) 5 percent yellow cuprous oxide, 93 percent talc, and 2 percent diatomaceous earth.

The number of spray applications needed to control the disease satisfactorily varies with the season. In years when the rainfall is sparse during the infection period, one properly timed application will control it. If the rains are heavy and frequent during this period, as many as three properly timed applications are required to control the disease satisfactorily. Since it is impossible to predict the weather during the infection period, the wisest course is to apply the maximum number of applications, which should be made in (a) the early preblossom stage; (b) the late preblossom stage; and (c) the early postblossom stage of pistillate flower development.

Four to six dust applications will be needed to control walnut blight, depending on the amount of rainfall during the critical period for infection. In years when the rainfall is light during the infection period, four properly timed applications will control the disease. In seasons of heavy rainfall during this period, as many as six properly spaced applications are required. The safest course is to apply six applications every year, at approximately 7-day intervals, beginning in the early preblossom stage.

CROWN ROT is caused by the fungus Phytophthora cactorum, a "water mold" that thrives best under wet conditions.

Crown rot attacks the bark below the ground line and produces dark-brown or black, irregular cankers or decayed areas at the crown. The infected areas are soft or spongy and usually are confined to the bark. The wood is not attacked, although the disease may discolor it inwards a short space. A black fluid often accumulates in cavities in the cambial region and eventually finds its way to the surface. In more advanced stages the disease extends outward on the lateral roots and kills the taproot. The cankers usually stop at the graft union but occasionally extend into the trunk beyond the union.

The first indication of infection in the above-ground parts of the tree is usually a somewhat stunted appearance, accompanied by sparse, yellowish-green foliage and poor twig growth. The affected trees often set an unusually heavy crop of nuts. In more advanced cases, the trees appear drought-stricken in midsummer, the leaves turn yellow, wither, or drop prematurely; the twigs and smaller branches die back. The tree generally succumbs the following year.

The disease occurs most frequently on trees located in poorly drained or wet soils or in soils where excessive flood irrigation is practiced.

The most effective method of control is to grow walnuts that are grafted on Persian or Paradox hybrid rootstocks.

In orchards of trees grafted on susceptible roots, it is best to expose the crown and basal parts of the main roots to the sunshine and air. That can be done by digging a hole about 18 inches wide and 1 to 2 feet deep around the base of the tree. Leaves and other material should be kept out of the trench so that the crown will be kept dry. Irrigation water should never be permitted to come closer than 4 feet from the tree. The crowns should be exposed when the disease is first noted.

In regions of cold winters it is advisable to cover up the crowns before cold weather begins; cold injury may otherwise result.

Sometimes one can control the disease when the lesions are still small or localized by cutting out the infected areas. The wounds should be disinfected with bichloride of mercury 1 1,000, and then covered with bordeaux-linseed oil paint or some other good wound dressing that contains a fungicide.

BRANCH WILT is caused by the fungus Hendersonula toruloidea (Exosporina fawcetti).

Its first manifestation is a sudden wilting of the leaves on some, branches in midsummer or late summer. The disease usually appears first on the smaller twigs and branches, but later may involve large limbs. It may appear in any part of the tree, though it is more common on the south or southwest sides. All leaves on an affected branch wilt at once and then dry to a brownish color but remain attached to the branches until late winter. The bark and wood of the wilted limbs are dark brown or almost black. Similar dead areas may also be found on branches not yet wilted. The discolored areas, or cankers, commonly occur on the upper side of the branch and are frequently centered around sunburned areas or injuries made by harvesting hooks or poles. The discoloration in the wood is caused by a dark-brown deposit, which fills the cells and prevents the normal upward flow of water to the leaves.

A noticeable feature is the loosening and sloughing away of the outer layer of corky bark over the diseased areas. A dark-brown or black powdery deposit is then exposed. The deposit is composed of numerous, dark-brown spores of the causal fungus which are disseminated by wind and rain.

The varieties of walnuts most commonly affected are Franquette and Mayette. Meylan, Eureka, Blackmer, Payne, and Concord varieties also have been found affected. Concord is the most resistant.

Since a lack of vigor increases Susceptibility to the disease, trees should be kept in as healthy a condition as possible by proper fertilization, adequate irrigation, and proper culture. All diseased branches should be removed as soon as they are discovered. When diseased branches are removed, the cut should be made well back of the most advanced margins of the affected areas. All wounds should be covered with a good wound dressing such as bordeaux-linseed oil paint. One should try not to injure the branches with harvesting hooks and poles. Experiments indicate that bordeaux mixture (10 10 100) applied after leaf fall helps prevent infection.

MELAXUMA (black sap) is caused by the fungus Dothiorella gregaria. It attacks the twigs, branches, and trunk, and generally produces an inky-black liquid, which oozes out of the surface of infected areas. The bark beneath is discolored. It eventually dies and forms a slightly sunken canker.

All cankers on the larger limbs and trunk should be cut out. All of the discolored bark and some of healthy bark around the advanced margins also should be removed. The wounds thus made should be disinfected with bichloride of mercury, 1 1,000, and subsequently painted with bordeaux paste or paint.

BLOTCH is caused by the fungus Gnomonia leptostya (Marssonia juglandis). It attacks the leaves, young shoots, and nuts. On the leaves, it produces roundish, reddish-brown spots, that have grayish-brown centers and are up to three-fourths inch in diameter. On the stems of shoots of current growth, the lesions appear as oval to irregularly circular, sunken, light grayish-brown, dead areas, which have dark, reddish-brown margins.

Blotch is most serious and destructive on the nuts. On them it causes depressed, circular to irregularly circular, dead spots in the husk. They The are not quite so large as those on the leaves. Very young nuts that are infected do not develop normally and drop to the ground prematurely. The disease may reduce the yield greatly when it is exceptionally severe.