Two Root Rots of Fruit Trees

H. Earl Thomas, Stephen Wilhelm, Neil Allan MacLean.

A disease known as oak fungus disease, oak root fungus disease, mushroom root rot, shoestring root rot, and armillaria root rot is responsible for large annual losses in most varieties of tree fruits in the western part of the United States.

Fig, persimmon, northern California black walnut, and French pear are the only orchard trees or rootstocks now known that will live a normal life span in the presence of Armillaria mellea, the fungus that causes the disease. All other orchard varieties or rootstocks show varying degrees of susceptibility.

Infected trees show until they are killed a progressive yellowing and wilting of the foliage and a stunted growth. Some fruit tree species, peach and apricot among them, may be killed within a year or two after planting. Others, such as the myrobalan 29 and Marianna 2624 plums, may live for 10 years or more in the presence of the fungus.

The fungus infects the roots and lower part of the trunk. It is there that a diagnosis of the disease can be made. Infected bark tends to become moist, spongy, and somewhat stringy. At times it will slough off, leaving the wood exposed. The characteristic fans or mycelial plaques are formed in the bark and between the bark and the wood. These fans, of coarse, white or yellowish-white, radiating hyphal strands, are the most characteristic signs that the disease is present.

After fall and winter rains, a second stage in the life cycle of the fungus is to be found at and around the bases of infected trees the mushroom or toadstool stage. In rainy weather the fungus may produce large numbers of yellow-brown or honey-colored mushrooms, from which we get the names mushroom root rot or toadstool root rot. The mushrooms appear only for a few weeks each year and therefore are not a good year-around sign of the disease.

Other structures of this fungus are the rhizomorphs rootlike growths of compact fungus strands, dark outside, white inside and somewhat smaller than the lead in a pencil. They spread the fungus from root to root for short distances through the soil and these structures appear to be the only ones which are able to penetrate the roots to start new infections. They are not formed at high temperatures (above 80 F.), a fact that no doubt is related to the scarcity of this root rot in warmer areas. The name shoestring root rot has been derived from them.

Armillaria is very sensitive to drying in or on the soil but can live as a saprophyte on dead woody material in moist soils for many years. It resumes its parasitic activity when trees susceptible to it are planted again. Susceptible roots may become infected by growing into or near an already infected or decayed root and coming into contact with the rhizomorphs. The rhizomorphs invade the roots by means of pressure and dissolving enzymes. The fungus grows through the outer cortex and to a lesser extent the wood of the root to the crown of the plant. The death of a tree follows the killing of the roots and the girdling of the lower trunk. In an orchard in which many trees are infected one finds groups of dead trees in circular areas evidence of how the fungus spreads outward from a center.

The fungus may be carried into new areas by floodwaters and perhaps infrequently by irrigation and cultivation. But the great majority of infections trace back to earlier cropping to susceptible plants or to the presence of the fungus on native vegetation notably oak trees in the case of new land. The fungus does little apparent damage to most native trees and shrubs in natural sites.

The use of resistant rootstocks and soil fumigation help control Armillaria in deciduous orchards.

As we have noted, rootstocks differ greatly in susceptibility to Armillaria. Only four are very resistant. We list five groups of rootstocks from the very susceptible to the very resistant: Group 1 almond, peach, apricot; replants may be killed in 1 or 2 years. Group 2 mahaleb cherry, Morello cherry, quince; susceptible, but somewhat less so than group 1.

Group 3 myrobalan plum, mazzard cherry, apple; unwise to plant in known oak root fungus spot.

Group 4--myrobalan 29, Marianna 2624; selected for greater resistance than ordinary plum roots.

Group 5 fig, persimmon (three species (kaki, lotus, virginiana), French pear, black walnut (Juglans hindsii); usually live a normal life span in the presence of the fungus.

Of the many chemicals that have been tested as soil fumigants against Armillaria, only one, carbon bisulfide, can be recommended. It does not give complete control, but if it is applied correctly it does give commercial control in lighter soils.

Other chemicals that have been tested may give a better kill of the fungus but are not satisfactory because they do not penetrate deeply enough or they are too expensive. A fungicide to be effective against Armillaria has to move to a depth at least of 6 or 7 feet. Carbon bisulfide usually goes only 5 or 6 feet deep in many soils. It is relatively cheap, but it is inflammable, poisonous, and corrosive.

Carbon bisulfide can be applied with a hand applicator or with a power-drawn applicator. The hand applicator is used when only a small space is to be treated. Power-drawn equipment is used when areas of an acre or more are to be treated.

Treatment is usually done in the fall after the crop has been harvested. The soil should be warm and porous. It should have a uniform and low moisture content, but it should not be dry. The dead trees are pulled, all the large roots are dug out, the land is leveled, and the fumigant is injected. When the fungicide is applied with a hand applicator, 2 ounces by weight (1 3/5 ounces by volume) of the material are injected at a depth of 6 to 8 inches at points 18 inches apart. The rows are also 18 inches apart and the injections are staggered. Injections should not be made closer than 6 to 8 feet from any living tree (or up to the tree drip) as the material is toxic to living roots. A gallon of carbon bisulfide will treat approximately 180 square feet. To treat a single tree space 24 x 24 feet takes 05 gallons.

The power equipment is in the form of a 3-standard subsoiler with automatic measuring valves. In operation the injector standards are submerged to the desired depth, at which point the pumps automatically begin to dispense the liquid chemical in prescribed dosages.