dvocado ROOT ROT By George A. Zentmyer • Albert O. Paulus • Robert M. Burns CALIFORNIA AGRICULTURAL Experiment Station Extension Service CIRCULAR 511 THE COVER PHOTO illustrates an avocado tree affected by avocado root rot. This circular tells you how to recognize the disease, how to detect the fungus that causes it, and how to prevent and control the root rot. PHYTOPHTHORA ROOT ROT OF AVOCADO is a disease caused by a soil fungus, Phytophthora cinnamomi, known com- monly as the avocado root rot fungus or as the cinnamon fungus. This soil organism, first described on cinnamon trees in Sumatra in 1922, is now known to attack more than 115 species of plants. Avocado root rot is the most serious disease affecting avocado trees in many parts of the world. It has affected an estimated 4,500 acres of avocado trees in California, and has been found in Fresno, Los Angeles, Orange, Riverside, Santa Rarbara, San Rernardino, San Diego, Tulare, and Ventura counties. This circular tells how to recognize the disease and what you should know about the fungus (page 3); how to detect the fungus (pages 4 to 6); what plants the fungus attacks (page 6). It outlines what is known to date about measures that will prevent develop- ment and spread of the disease (pages 6 to 11) and what can be done to con- trol root rot after it has invaded the grove (pages 12 to 18). MARCH, 1962 THE AUTHORS: George A. Zentmyer is Plant Pathologist in the Citrus Research Center and Agricultural Experiment Station; Albert O. Paulus is Extension Plant Pathologist; Robert M. Burns is Extension Horticulture Technologist — all at the University of California, Riverside. This circular replaces Circular 465, Phytophthora Avocado Root Rot. GEORGE A. ZENTMYER ALRERT O. PAULUS RORERT M. RURNS Avocado ROOT ROT An avocado planting will develop root rot if the soil contains the fungus Phytophthora cinnamomi, and if there are periods of excess soil moisture. Both of these conditions must be present, or root rot will not develop. The fungus commonly becomes active as a result of poor internal soil drainage, but may also become troublesome on the better-drained soils because of breaks in water lines, overirrigation, or excessive rainfall. Drainage may be poor because of an impervious layer under the surface soil or because the entire soil profile contains considerable clay, or because strata of different textures interfere with internal soil drainage. HOW TO RECOGNIZE ROOT ROT Leaves of infected trees are smaller than normal, usually pale or yellow green instead of dark green, often wilted; they tend to drop, giving the tree a sparse appearance as shown in the photo below. New growth is often absent; if new leaves are formed, they do not develop normally and are pale green. In advanced stages of the disease, branches die back and fruit is often small. Diseased trees will fre- quently set an abnormally heavy crop of fruit because the loss of many roots has a partial girdling effect. Trees of any size, from nursery trees to large, old trees, may be affected. Many of the small feeder roots on diseased trees are blackened, brittle, and dead. In advanced stages of the disease it is difficult to find feeder roots. The absence of the feeder root- lets prevents the uptake of moisture, hence the soil under diseased trees tends to stay wet. WHAT YOU SHOULD KNOW ABOUT THE FUNGUS The avocado root rot fungus is known as a water mold because it thrives in wet places. It is a minute form of plant life which needs wet soil for the best development of its three spore stages: Sporangia, which release swimming spores (zoospores); Resistant spores (oospores); Chlamydospores. These spore types are too small to be seen by the naked eye. (See en- largement on page 4.) Approximately ten million zoospores could be placed in an area one inch square. The fungus requires water to form and liberate its spores, to germinate, and to infect Young avocado tree in advanced stage of root rot. the roots. Sporangia, the most impor- tant spore-forming bodies, are formed at relatively high soil temperatures, mainly between 77° F and 87° F. This indicates that the major infection takes place during the warmer months of the year. The fungus makes no growth below 50° F or above 93° F. The fun- gus can build up very rapidly because sporangia are produced in great abundance in the soil and many swim- ming spores are released from each sporangium. Recent research showed that avocado roots susceptible to root rot exude a chemical that attracts the small swimming spores of the fungus. This substance obviously plays an im- portant role in infection and may have some relation to resistance. Oospores have been produced in the laboratory but little is known about their role in the disease. They are thick-walled spores resistant to ad- verse conditions. Chlamydospores are non-motile, spherical spores that ger- minate readily in water. HOW TO DETECT THE FUNGUS The fungus can be identified in the laboratory by making laboratory cul- tures of small feeder roots on corn- meal agar or other selective media. Small blackened feeder roots are se- lected from a soil sample, placed in a beaker of tap water, dipped briefly in 70 per cent ethyl alcohol, blotted on a paper towel, and put on the agar medium in petri dishes. (See photo on page 5.) A practical method of isolating the fungus is to place a mature, firm avo- cado fruit (Fuerte or similar green Left, most cultures of the avocado root rot fungus look very similar. Culture SB 216 comes from avocado roots in California, SD 815 from macadamia trunks in California, F 2 from avocado roots in Florida, PR 2 from avocado roots in Puerto Rico. Left, sporangium, enlarged approximately 1,000 times, the principal spore stage of the avocado root rot fungus. It releases from 20 to 30 zoospores which swim through water in the soil, germinate on and infect avocado roots. Right, laboratory method of detecting avo- cado root rot fungus: Ten small avocado roots were placed in cornmeal agar. Five of the 10 rootlets developed fan-shaped growth indicating the presence of root rot fungus. fruit) in a waxed paper cup containing a soil sample. Flood the surface of the soil with water and leave the fruit on the soil for four days (see photo be- low). Then remove the fruit from the soil, wash it, and leave it at room temperature for several days. If the root rot fungus is present, firm brown to purplish-brown spots will develop at the water line on the fruit in four to six days after placing the fruit in the infested soil. Positive identifica- tion of the fungus requires placing bits of tissue from these discolored areas on cornmeal agar in petri dishes, but so few other fungi will attack firm unwounded avocado fruit that devel- opment of spots as pictured will show for all practical purposes that root rot fungus is present. In many cases the fungus may be present on healthy-appearing trees on the margin of infested areas, and its presence can be detected only by tak- ing root and soil samples for testing. To check a suspected tree Take several cupfuls of soil from three or four locations around the tree. Mix these samples and take out two A practical method of detecting the fungus consists of plac- ing an avocado fruit in a soil sample as shown at the left, the fungus is present, brown spots will appear on the fruit four to six days. Below, a healthy fruit (left) is compared with two that indicate the presence of the fungus. o ««< to three cupfuls to be sent to a labora- tory. Samples should include soil and small feeder roots taken from a moist part of the root zone from the upper 6 inches of the soil. Place samples in small polyethylene bags to prevent drying out before the cultures are made. Keep samples in a cool place until cultures or fruit tests have been made. Several commercial laboratories are available for making tests for the root rot fungus. Your University of Cali- fornia Farm Advisor office will be able to tell you where such tests may be made. Caution: Remember that positive detection of this soil fungus is diffi- cult, especially where infestation is light. A very small amount of the fun- gus may be present even though the laboratory test is negative. If in doubt whether fungus is present, recheck questionable trees. WHAT PLANTS DOES THE FUNGUS ATTACK? In addition to the avocado, the fungus attacks a wide variety of other plants which can serve as sources of infec- tion for the avocado grove. For ex- ample, if you bring a camellia affected with the fungus to your property, the fungus could spread from the diseased camellia and start root rot in your avo- cado grove. Following are some of the major plants severely attacked by the root rot fungus: azalea, camellia, cinna- mon, cypress, Douglas fir, heather, Lawson cypress, oak, papaya, pine (several types), pineapple, plane tree, rhododendron. Many more plants are hosts of the fungus — contact your Farm Advisor for the complete list. HOW TO PREVENT ROOT ROT The best control for avocado root rot is to prevent the introduction of the fungus into your orchard. Diseased nursery stock has undoubtedly been mainly responsible for the wide distri- bution of the fungus throughout the avocado-producing areas of southern California. A soil fungus of this type can be readily transported with balled or container-grown plants. It can also be spread or introduced to new areas by infected seed, movement of in- fested soil and movement of water containing spores. Research indicates that the avocado root rot fungus is not native to southern California soils. You can prevent its invading your nursery or orchard by several ways discussed here. GROW DISEASE-FREE NURSERY STOCK In July 1958 a program was estab- lished to certify avocado nursery stock as being free from avocado root rot. This program was initiated by the avo- cado growers of California and devel- oped through cooperation between the State Department of Agriculture, Division of Nursery Service, and the University of California. The basic program as outlined below consists of three parts: • Heat treatment of seeds. • Fumigation or steam treatment of the soil. • Good sanitation. For further information contact your local Agricultural Commissioner or Farm Advisor. Use clean seed The fungus can be spread in avo- cado seed if the fruit from which the seed is taken is allowed to remain for several days on soil infested with the »»» 6 Avocado seed can be treated in various con- tainers with hot water to kill any root rot fungus that may be present. Water tempera- tures of 120°F to 122 °F will kill the fungus within 30 minutes. fungus. Take all seed used for plant- ing from fruit picked from the tree, or treat it with hot water (see photo above). Immerse all avocado seed that is picked off the ground or was placed in boxes that may contain the fungus in a hot-water bath at 120° F to 122° F for 30 minutes. Be certain that your ther- mometer is accurate, because temper- atures of 125° F and greater will re- duce germination. Hot-water treatment is simple and inexpensive, and does an excellent job of killing the fungus because the plant can stand more heat than the fungus. A 100-200 gallon tank will allow you to control the temperature easily, but for small numbers of seed the kitchen sink or some similar container is use- ful. Use a circulating pump or a wooden paddle to agitate the water bath. One nursery has used an old bathtub with a circulating pump and two water heaters linked together. A trickle of steam or water 20° F to 25° F above that of the treatment is used to keep the temperature within the required range. Coarse cloth bags or wire screen containers may be loosely filled to hold the seeds during immer- sion. Bags or -wire containers that are too full will prevent the hot water from adequately circulating around all the seeds. After the hot-water treatment, im- mediately rinse the seeds with clean, cold running water, then spread them out to dry thoroughly on a clean sur- face not in contact with the ground. Several nurseries have had excellent success with the hot-water avocado seed treatment and have shown that it is commercially feasible. Fumigate or steam nursery soil for container- grown plants Container-grown material, particu- larly with open bottoms such as tar- paper bands, should be grown on clean benches or raised slabs to avoid any possible infection from the soil. It would be worse than useless to treat seed and soil and then place plants on soil infested with the fungus since the pathogen will grow through treated soil more rapidly than un- treated soil providing it gets there be- fore other competing organisms. Soil fumigation or steaming will in- sure soil free of the fungus. Methyl bromide is an effective fumigant when used under a plastic cover at a dosage of 3 pounds per 100 cubic feet of soil for 24 hours. Steaming soil to 180° F for 30 minutes is also effective in kill- ing the fungus. The treatment can be V ' ! --*«»« Soil fumigation with methyl bromide is effective in eliminating the root rot fungus from nursery soil. Upper photo shows nursery area with cans of soil for use in planting avocado seed on raised nursery beds. Lower photo shows entire area being fumigated under a plastic tarpaulin by injecting methyl bromide. done before or after the soil is placed in the containers. A very satisfactory method has been to treat the filled containers in place on the slabs or raised benches with methyl bromide. In this procedure the whole area, con- tainers and walkways, is covered by a polyethylene plastic tarp, and the gas is injected underneath the tarp. A profitable side effect from fumigation is the killing of many weed seeds that are present in the soil mix. Seed can be planted from one to two weeks after removing the tarp. »»» 8 Practice sanitation In addition to the seed and soil treatments, strict adherence to sani- tary growing conditions is absolutely necessary. To comply with the regula- tions for production of certified nurs- ery stock it is also necessary to fence the entire area. Because an area can be infected in many ways, such as mud on shoes, tools, or hoses, all personnel in the nursery area must practice the utmost cleanliness possible. Keep the walkways clean; keep hose nozzles and equipment off the ground and in the fenced nursery area. Use equal care in selecting orna- mental plants for planting on avocado property. As previously mentioned, the fungus attacks many other woody plants and can be brought to a prop- erty in infected stock. PREVENT MOVEMENT OF SOIL OR WATER FROM INFESTED AREAS Take all possible steps to prevent movement of soil or water from dis- eased areas into noninfested groves. The fungus can be moved by any means by which moist soil is moved — on cultivation equipment, trucks, cars, shovels, soil augers, and shoes. The fungus has been recovered from mud scraped from shoes worn in an area of infested soil. The fungus also can be spread downhill from an infected area by surface or subsurface drainage water, because the swimming spores are readily moved in water. Watertight drains should be installed to take care This diagram illustrates an actual case of fungus having spread downhill in water in a San Diego County grove. Black dots in the diagram are new infections resulting from spread from two infected trees (X). Move- ment is more gradual in other directions than it is downhill with drainage water. 9 ««« of surface runoff if a diseased area lies above a healthy grove. If gophers are active in a grove their runs may serve as good avenues of movement of the fungus, particularly its movement downhill in water. This is another good reason for gopher control. Where possible, make cooperative arrangements with neighboring avo- cado growers so that drainage from diseased trees does not run over into healthy groves owned by someone else. Small pieces of equipment, such as shovels, augers, trowels, should be washed well after use around diseased trees, and dipped in alcohol or formal- dehyde solution. A 70 per cent solu- tion of methanol, ethanol, or rubbing alcohol can be used. Dilute commer- cial formalin or formaldehyde to make a 5 per cent solution. Always use culti- vation equipment in the healthy por- tion of the grove before using it in the diseased portion. Wash and allow equipment to dry thoroughly after use in the diseased section. KNOW YOUR AVOCADO SOIL A grove will usually show much less tree damage from avocado root rot if trees are planted on soils having good internal drainage. These are soils in which water moving through the soil is not stopped within 5 feet of the sur- face. Clay layers in the subsoil, a dense rock substratum within 2 or 3 feet of the surface, or clay texture from the surface to 3 or more feet will cause a soil to become poorly drained. Statewide surveys have shown that avocado root rot damage is closely correlated with soil series. A soil series is a group of soils with similar char- acteristics, usually named for the area where first described, such as Ramona, Porter ville or Vista series. Root rot has been most severe on those soils with Root rot damage started in the low areas of the Merriam series with dense claypan, hav- ing poor drainage, and subsequently moved slowly into the better-drained Fallbrook series. Black dots in the diagram represent diseased trees. »o ~- O O O O Q O Jb i).pj-oe-e-Q.q o 0,6 6' ©JD OOOOOOOOOO-, s o ots^o 000000 0^6 d # %-*^\°/6 o/f So oods^oooo 0,-0 o dm • * < ^\& up dvo 000 'cr«-Q-o''o_o,a-4,% # # ^' * # / \