■Jffs UN IVERSITY OF CALIFORNIA %" Armillaria Root Rot of Deciduous Fruits, Nuts, and Grapevines H. J. O'REILLY -X \. m IS & : : : :*: • Recogn izing the disease • Identifying the fungus • Principal method of tree-to-tree infection by rhizomorphs • The mushroom (spirophore) stage • Survival and spread ofthefungus • Controlling the disease CALIFORNIA AGRICULTURAL t\x periment Station .tension Service CIRCULAR 525 Glossary of terms Crown rot — a common fungus disease in which the infected plant or tree develops decay at or near the ground level; also referred to as "collar rot." Fungus — (Plural, fungi) a lower plant form lacking chlorophyll and hence subsisting on other plants, plant remains, or animals. Mycelium — a group or mass of fungus filaments. Parasite — an organism living on or in, and getting its food from, another living organism with or without fatal effects to the organism. Pathogen — a parasite able to cause disease. Plaque — a flattened, visible group of mycelium strands, often arranged in a characteristic fan shape. Rhizomorph — a visible strand or cord of compacted mycelium, often dark-colored, usually found on or between bark and wood. Saprophyte — an organism or a plant which lives upon dead organic matter and commonly causes its decay. The Author: H. J. O'Reilly is Agriculturist, Agriculturial Extension Service, Davis. NOVEMBER, 1963 In order that the information in our publications may be more intelligible it is sometimes necessary to use trade names of products or equipment rather than complicated descriptive or chemical identifications. In so doing it is unavoidable in some cases that similar products which are on the market under other trade names may not be cited. No endorsement of named products is intended nor is criticism implied of similar products which are not mentioned. H. J. O'Reilly Armillaria Root Rot of Deciduous Fruits, Nuts, aud Grapevines Recognizing the disease Armillaria root rot, a disease caused by a soil fungus, has killed great numbers of orchard trees, grapevines, forest trees and other plants in California and throughout the world. While Armillaria root rot is the most common name of the disorder, it is also known as "oak root fungus disease," "shoestring fungus rot," and "mushroom root rot"- — the last name has been given because the causal fungus, Armillaria mellea, produces a honey- yellow or tan-brown to blackish mush- room. Several hundred different types of plants, including trees, shrubs and vines are known to be affected by this pathogen, and a survey in the Santa Clara Valley in 1945 showed that 10.7 per cent of the orchard land planted to susceptible crops was infested with oak root fungus. Thirty- four of the 65 properties surveyed had one or more infested sites. Symptoms. One of the first notice- able symptoms of Armillaria root rot is a reduction in growth rate. Affected limbs stop growing and leaves are small and yellow, often dropping prematurely. Symptoms may appear on only one or two limbs at first, but usually develop on the opposite side of the tree in a year or two; when the disease is well-advanced the tree has the appearance of being stricken with crown rot. Trees suffering from rodent injury may also bear a re- semblance to A. mellea victims, particu- larly if the injury has occurred on only one side of the crown. In new infection sites the diseased trees or vines may be scattered, but older sites are roughly circular in area owing to the spread of the fungus from its initial infection point. Diseased trees in such areas usually die in 1 to 3 years, with their rate of decline depending largely upon the type of tree involved. The stress of summer heat and crop production often hastens death of the trees. Peach trees in the light, sandy soils found in parts of the San Joaquin Valley some- times die within 2 or 3 months after de- cline is first noticed; infected almonds and apricots in the same type of soil take slightly longer to die. Identifying the fungus Diagnosis of Armillaria root rot de- pends upon locating and identifying the fungus pathogen. The crowns of sus- pected trees or grapevines should be care- fully examined at the ground level. Re- moval of bark may expose flattened whitish-yellow plaques made up of fun- gus mycelium and an identifying pattern [31 Figure 1. Armillaria fungus mycelium on peach roots. Bark removed to expose flat- tened whitish-yellow plaques which have developed between wood and bark. may be seen within each fungus plaque: at the margins of the advancing fungus, the mycelium takes on characteristic fan- like shapes each about % to 1 inch in diameter. Groups of these fans form the constantly enlarging plaques, which are located between the wood and the bark as well as in the bark; eventually, they spread and involve all the root system. (Infected wood decays but remains fairly firm in texture.) The plaques occur largely between the bark and wood, and this helps to distinguish A. mellea from certain other wood-rotting soil fungi, some of which produce white threads of fungus mycelium mostly on the outer bark surface and in the surrounding soil — these fungi are common in soils high in organic matter. A fungus known as Porta is often con- fused with A. mellea but differs in that it produces a soft, crumbly wood rot and does not develop fan-shaped plaques- In certain tree species attacked by A. mellea, a gelatinous or gummy material may also be associated with the fungus plaques. In some susceptible hosts, particularly forest trees, the margins of the invaded bark gum copiously. Gummy exudates do not occur on diseased grapevines, but are fairly common at the ground level on infected stone-fruit trees. As the fungus gradually girdles and kills the tree, the texture of the infected wood changes from firm and slightly moist to somewhat soft and dry. In the more advanced stages a pungent mushroom smell is also notice- able. A. mellea primarily involves the under- ground parts of the tree and seldom ex- tends more than 2 feet above ground level, although occasionally found as high as 5 feet, particularly in apple trees. The fungus is highly drought-sensitive, soon desiccating and dying when exposed to dry, warm conditions. Exposure and dry- ing out of diseased roots, particularly when trees are in early stages of infec- tion, may occasionally kill the organism in individual trees and shrubs, but this is of litde practical value in large or- chards or vineyard plantings (although walnut orchards may be an exception). Fungus mycelium detected in the early stages can sometimes be scraped off and the diseased areas left exposed to dry out. I 4 J Principal method of tree-to-tree infection by rhizomorphs A tentative diagnosis of Armillaria root rot may be based upon the presence of fungus fans, or plaques, but a positive identification can best be made when structures called rhizomorphs are devel- oped by this fungus. Rhizomorphs are brown to black flat string-like strands of mycelium often extending along the sur- face of infected roots; they resemble flattened shoestrings, hence one of the older common names, shoestring fungus rot. Rhizomorphs are often mistaken for rootlets but closer examination shows that they both adhere to and penetrate directly into infected root surfaces, branching in a somewhat different man- ner than do tree rootlets; they occur largely on and near infected root sur- faces, seldom more than an inch or so from the diseased roots. A moist soil is most suitable for rhizo- morph development. When the rhizo- morph's growing tip comes in contact with a nearby susceptible root it adheres and penetrates through the bark to the cambium region; typical mycelial fans and plaques soon develop and the new root becomes invaded and parasitized. Rhizomorphs are seldom found in grape- vines. Growth along the diseased root and penetration of nearby healthy roots is the principal means by which the fungus moves and thus expands the area of diseased trees. Figure 2. Flattened, string-like rhizomorphs with small whitish fungus plaque in center. Rhizo- morphs are often mistaken for rootlets, but close examination shows that they branch differently. ... * w f *V *i!" '*** % m * ^vvi.^; j Figure 3. Typical Armillaria mushrooms. C shows undersurface of cap. The mushroom (sporophore) stage Armillaria fungus occasionally pro- duces a cluster of mushrooms around the base of infected trees or stumps a few days after a rain. The clusters are seen from October to February, generally in moist coastal areas, on dead stumps or diseased trees. The top of the mushroom cap is honey-yellow to almost black and may have a covering of brown scales. The mature cap is 2 to 5 inches in diam- eter and occasionally wider; the cap's lower surface has whitish gills radiating like the spokes of a wheel. Great numbers of colorless spores (white when in large masses) are produced on the gills, and are eventually released and moved about by air currents; it is unlikely that they are an important source of infection. Survival and spread of the fungus The fungus can survive for several years on dead roots, or can live from year to year on the living roots of Cali- fornia live oak, aspen, and several fir and willow species without rapid spread of the diseased area. From such infected roots of native trees, rhizomorphs may cause new infection in nearby roots of any susceptible species with which they come in contact. The spread of A. mellea from place to place is associated with the activities of both man and nature. Cultivating equip- ment working in infected sites may move diseased root pieces large enough to cause new infestations in adjoining healthy areas, and rhizomorphs arising from such diseased roots may then attack nearby healthy roots. After heavy rainfalls, flooded streams or rivers often dislodge infected roots and carry them consider- able distances downstream. Control of Armillaria root rot Two methods have been used with moderate success in controlling the dis- ease: soil fumigation, and the use of resistant rootstocks. SOIL FUMIGATION Fumigating soil prior to replanting trees is fairly successful when the fumi- gant is applied properly and soil condi- tions are favorable, but it is always pos- sible that a few diseased roots may escape the lethal effects of the gas. When this happens, the fungus survives and may attack young trees replanted in the dis- eased site. Orchardists must be prepared to re-treat such areas, and when this is done promptly the fungus can usually be eliminated. Such re-treatment, often done with hand equipment, may be required periodically for 9 or 10 years before all diseased spots are eliminated. Fumigants. Carbon bisulfide is the only chemical soil fumigant for control of oak root fungus which has been widely used for many years (it is also known as carbon disulfide, but will be referred to here as CS 2 ). University of California research indicates that CS 2 is the most effective soil fungicide for oak root fun- gus control. Under favorable conditions it is capable of killing the fungus to a depth of 5 or 6 feet, but probably does not penetrate to this depth where "plow pans" (impervious clay layers) exist; the ideal soil fumigant should probably go down to a depth of 7 feet or more. CS 2 is a heavy liquid which converts to a gas 2% times heavier than air when released into the soil. It is inflammable, explosive (its vapor ignites at 300°F), poisonous, corrosive to metal, and ill-smelling. Fires or even explosions can occur when appli- cation equipment strikes a rock and pro- duces a spark. CS 2 may be handled with confidence and security if one takes the same pre- cautions as with gasoline. Safety rules are: (1) Store in a cool place. When using from a container in the field dur- ing warm weather, cover the container with wet burlap or cloth to prevent ex- pansion and excessive evaporation of ma- terial. Preferably, use CS 2 when air tem- peratures are below 85°F. (2) Do not light matches or smoke near this chem- ical. (3) Never use a hammer and chisel to loosen a bung; use a wrench to fit the counter-sunk hole, or cover the bung with wet burlap and open with a pipe wrench. Allow gas pressure to slowly become equalized before completely opening the bung. (4) To preclude accumulation of [7] static electricity, do not use glass or crockery containers when drawing out the liquid. Rest the drum on the ground or, when on a truck, ground the drum by means of a chain or wire one end of which is buried in moist soil. (5) If applicator chisels strike rocks and sparks ignite the CS 2 in the ground behind the chisels, soil quickly shoveled over the fire will usually smother it. (6) Do not inhale con- centrated fumes continuously. (7) To avoid metal corrosion, rinse all equip- ment with kerosene, and coat equipment surfaces with oil after use. Methyl bromide has recently shown promise of performance equal to CS 2 when used as a fumigant in light sandy soils, but needs further evaluation with regard to other soil types before it can be recommended. Diseased tree or vine removal. This is most effectively carried out during the winter months when soil conditions are better for removal of roots. All roots should be removed, even the smaller ones (1 to 2 inches in diameter). Diseased roots and stumps should be burned in that part of the orchard where the disease is present — this prevents scattering of diseased root pieces. In addition to the removal of all trees in the infected site, an entire row of healthy-appearing trees should also be removed in all directions around the mar- gin of the site and the roots and crowns should be examined for evidence of Armillaria. Unless these apparently non- diseased trees are pulled and the soil fumigated, any fungus surviving on their roots may reinvade newly planted trees. With grapevines, two healthy-appearing rows should be removed on all sides of the diseased ones, thus permitting exten- sion of the fumigated area beyond the visibly infected vines; a study conducted in San Joaquin County proved that if only one row were removed the disease reappeared a few years later in many replanted vines. Removal of diseased trees or grape- vines in the months of August or Septem- ber followed immediately by fumigation is not recommended. Because of dry, hard, soil conditions common in the sum- mer, stumps tend to break off and leave infected roots in the soil. After tree removal, an entire summer- growing season without irrigation is needed for soil to dry out and open soil- pore spaces for adequate diffusion of gas to desired depth. During the next grow- ing season the soil should also be planted to a deeply rooted unirrigated cover crop such as mustard or safflower. This helps WARNING ON PESTICIDE RESIDUES These recommendations for pest control are based on the best information currently available for each pesticide listed. Treatments based upon these recommendations should not leave residues that will exceed the tolerance established for any particular chemical. To avoid excessive residues, fol- low directions carefully with respect to dosage levels, number of appli- cations, and minimum interval between application and harvest. The grower is legally responsible for residues on his crops as well as for problems caused by drift from his property to other properties or crops. [8] reduce soil moisture to the low level nec- essary for optimum diffusion of the gas, and also increases soil porosity. In heavy soils harder to dry out, such a deeply rooted cover crop is especially beneficial. Before fumigating, soil-tube or auger testing to a depth of at least 5 or 6 feet is essential to determine if lower soil layers are well dried out; this is espe- cially important in heavier soils. When to fumigate. Soil fumigation with CS 2 is frequently done in the fall after harvesting is completed but while soil is still warm, porous, and low in moisture. Summer fumigation is also satisfactory when soil conditions meet these requirements, although high soil temperatures can cause loss of a portion of fumigant through evaporation. Spring applications of CS 2 have the disadvantage of lower soil temperatures and, fre- quently, too much soil moisture. Soil preparation before fumigation. Thorough preparation of soil is impor- tant. If a plow pan has developed below the normal cultivation depth, subsoiling or deep chiseling will be necessary. Soil should be loosened to 12 to 14 inches, if possible; preparation can be started in spring, when there is still enough soil moisture to facilitate deep cultivation. Just before fumigation, soil should be prepared to a seedbed consistency by disking and dragging (this helps confine the fumigant). Following final cultiva- tion, the top 6 to 8 inches of soil should be moistened lightly, as this aids in the penetration of the soil fumigation equip- ment and also helps confine the CS 2 . Minimum areas to fumigate. In single- tree sites the minimum fumigated area suggested is the midway point between the tree spaces on all four sides or, prefer- ably, to the drip line of the adjoining healthy trees. To avoid leaving unfumi- gated spots if a delay occurs during fumigation, overlap the application slightly when treatment is resumed. Hand equipment for fumigation. — The Mack fumigant injector (often referred Figure 4. Hand-operated soil fumigant guns. to as the "Mack weed gun" or "Mack gun") is a convenient hand tool used for treating small plots with CS 2 . The Mac- lean Fumigun may also be used — this is a more precisely built hand injector capable of injecting other fumigants such as chloropicrin. Both of these injectors are lightweight and consist basically of a soil injector probe to which a 1 to 1% gallon tank is attached. When placed at the desired soil depth of 6 to 8 inches, 2 ounces of CS 2 can be accurately ejected in a single operation. A metal funnel Table I ESTIMATED SAFE MINIMUM FUMIGATION DISTANCES FROM ADJOINING FULL-SIZED HEALTHY TREE OR VINE Crop Distance from nearest healthy tree or vine Grapes Apricots, peaches, plums, prunes and pears Almonds and apples Walnuts 3- 5 feet 8-10 feet 10-12 feet 15-25 feet [9] ?_;&»' '*<'■£ "' m Figure 5. Homemade rake with tines spaced 18 inches apart to mark spots for hand injections of soil fumigant. X's indicate how injections are alternately staggered at and between intersections of lines to give more uniform gas diffusion. equipped with a fine screen should be used when filling the gun in order to keep out small particles of foreign matter, otherwise valves may stick and cause inaccuracies. Injections are made at 18-inch inter- vals in each direction. To obtain maxi- mum soil diffusion of the gas, injections should be placed in staggered rows so that the pattern of injection between ad- joining rows is diagonal or diamond- shaped. The area to be fumigated can be marked out beforehand with a handmade wooden rake having tines at 18-inch intervals or with a rope or wire having marks at 18-inch intervals. When using wire or rope, one row is fumigated at a time, inserting the weed gun at the 18- inch intervals with each succeeding row staggered. Two-ounce (45 cubic centimeter) in- jections with the fumigation gun are made to a depth of 8 inches in sandy soils (6 inches in heavier soils) tamping the soil with the foot afterwards to prevent gas escape. This 2-ounce injection is equivalent to 1% ounces by volume per injection. A simple method for computing the amount per injection is to use a medicine prescription bottle calibrated in cubic centimeters (cc) or milliliters (ml) — the two measurements are identi- cal. Guns may be calibrated at the fac- tory to discharge this amount, but the Maclean Fumigun must be calibrated at the time of usage (use of these guns makes it possible to cover a small area about three times more quickly than when CSo is poured directly into holes spaced 18 inches apart). I 10] WP^*?r