Tree Planter's Notes, Vol. 38, No. 4 (1987) Fall 1987/37

Management of Phytophthora Root Rot in Conifer Nurseries of the Pacific Northwest

Sally J. Cooley

Plant pathologist, USDA Forest Service, Pacific Northwest Regional Office, Portland, OR

and water movement, management era) Pacific Northwest nurseries Various cultural and chemical of water in the nursery is integral to routinely subsoil 1-year-old fields strategies to reduce the incidence and controlling the disease. Good water in the fall before the onset of severity of phytophthora root rot on management often involves the rainy weather. Similarly, conifers in bareroot nurseries are following practices: wrenching will enhance drainage discussed. Tree Planters' Notes in the bed itself. 38(4) :37-.40;1987. 5. Low, poorly drained areas in 1. Ideally, your nursery should fields should be noted, and after

be located on light, well the crop is lifted, these areas

drained soil; realistically, can be filled in or taken out of Phytophthora root rot has caused very few nurseries in the production until corrective work locally severe losses in many species Pacific Northwest have the can be done in them. of conifer and hardwood seedlings good fortune to be situated 6. Irrigation practices often throughout the United States. on light-textured soils. influence the development of Management of phytophthora root rot 2. Those nurseries with heavy, phytophthora root rot. Watering can be approached culturally and slow-percolating soils must needs should be coordinated to chemically. Both methods give some add drainage systems. Sub avoid constant saturation of control of the disease. The best surface drainage systems are soils, particularly during the strategy is to use good cultural common in Pacific Northwest spring, summer, and fall when practices and supplement, when nurseries; some were in the fungus is active. Irrigation for needed, with fungicides and stalled at the very onset cooling, pesticide application, or fumigants. when the area was initially following fertilization should developed and some were in Cultural Management coincide as much as possible stalled years later after the with routine irrigation. field has grown numerous Cultural practices that discourage Over-irrigation and alternating crops. the development of phytophthora root drying and saturation should be 3. Drainage of surface water rot include good water management, avoided, particularly in areas from nursery beds and onto sanitation, and use of tolerant or known to be infested with the roads or ditches can be en- resistant species in areas where fungus. hanced by crowning fields phytophthora root rot is or may be a and ensuring that all bed problem. Sanitation. Sanitation practices are areas slope downwards to a vital part of the management of any Water management. Because the wards roads or ditches. disease. Phytophthora root rot is no development and spread of 4. Beds can be raised above the different. Diseased seedlings should phytophthora root rot is very level of the tractor paths to be removed from nursery beds and dependent on high soil moisture allow water to run from beds disposed of, particularly if you do not into paths. When paths be plan to fumigate before planting the Paper presented at the Western Forest come compacted or beds are next Nursery Council Meeting, Tumwater, not raised, tractor paths can Washington, August 1215, 1986. be "subsoiled" to allow water to drain into the cut made by the subsoiler. Sev- 38/Tree Planters' Notes

crop. Rogued seedlings and to Phytophthora species found in Phytophthora root rot can occur in packing house culls should not be Pacific Northwest nurseries. Pine fumigated fields, however, if the fungus returned to the fields as organic species, spruce, larch, and incense is reintroduced. The fungus can be matter. cedar are moderately susceptible. reintroduced by transplantation of Resting spores are able to survive Western redcedar is resistant. These diseased stock, irrigation with in dead plant tissue and could serve as groupings are based both on field contaminated water, or movement of a source of infection if left in the field. observations and greenhouse contaminated soil during flooding or on Even composted culls are suspect. I pathogenicity tests. equipment. believe that composting will not kill In areas of your nursery that are Fungicides. A number of fungicides 100% of Phytophthora spp. inoculum poorly drained, prone to flooding, or are registered for use on conifer in seedling tissue; various fungi, where phytophthora root rot was seedlings for control of Phytophthora including Pythium (closely related to present in the previous crop, resistant and Pythium species: the genus Phytophthora), have been or tolerant species should be sown or 1. metalaxyl (e.g., Subdue®, a recovered from seedlings composted transplanted. Very susceptible species product of Ciba Geigy), for over 1 year (2). should be planted in areas that are 2. phosethyl AI (e.g., Aliette®, a Diseased seedlings and seedlings well-drained, and if possible, have not product of Rhone-Poulenc), from diseased areas of a nursery had a history of phytophthora root rot. 3. propamocarb hydrochloride should not be moved to other nurseries (e.g., Banol® a product of or to "clean" fields within the same Nor-Am Chemical Co.) nursery. Moving diseased stock will Chemical Management 4. fenaminosulf (e.g., Lesan®, a introduce the disease, and once there, product of Mobay), it is extremely difficult, if not Various fumigants and fungicides 5. ethazole-methyl thiophanate impossible, to get rid of. Also, are available to control phytophthora (e.g., Banrot®, a product of additional species of Phytophthora, not root rot. Mallinckrodt). present previously, could be Fumigants. Fumigants are volatile Fungicides should be used to introduced into a nursery by liquids, gases, or granular pesticides supplement cultural practices. They contaminated stock. used to treat fallow soil before sowing should not be depended upon to Similarly, equipment that has been or transplanting. The fumigants methyl protect or cure seedlings, especially in diseased fields should be washed bromide + chloropicrin, metam sodium, those that are growing in an free of soil to prevent contamination of and dazomet will reduce soil environment that promotes diseases, the next field it enters. This is very populations of Phytophthora spp. when such as a wet, poorly drained field. important for nurseries that borrow or used correctly. For nurseries with We have sufficient data and trade equipment. widespread, continuous problems with experience to demonstrate the Resistance. Tree species vary in phytophthora root rot and other effectiveness of metalaxyl. The other their susceptibility to phytophthora soildborne diseases, fumigants are an fungicides have been used with root rot. True firs, western and efficient and effective way to reduce success on other crops, but we have mountain hemlock, and Douglas -fir disease levels. not yet been able to show their are very susceptible effectiveness on conifers against Phytophthora spp. Fall 1987/39

There are several reasons for this: of degrading metalaxyl. Highly more disease-free than water from There have been relatively few field susceptible seedling species in open canals and agricultural districts. trials in conifer nurseries in which the high-risk areas (areas that are To determine if your water source is intensity of the disease has been high poorly drained, prone to flooding, or contaminated, Phytophthora spp. can enough to conduct a meaningful test. that have had disease in the be detected by "baiting," that is, In addition, application techniques and previous crop) should have the first placing unripe fruit (apples or pears) in timing, particularly with new products priority for treatment. the water. Spores of the fungus are such as Aliette, have not been optimal, One well-timed application of attracted to the fruit and will enter it so that the full potential of the fungicide metalaxyl per year can adequately and cause decay. The fungus can then has not been seen. control phytophthora root rot in most be isolated from the decayed tissue on Metalaxyl is a systemic material that cases. Several applications in 1 year a petri plate and identified by a plant is taken up by the roots and moved will give increasingly more benefit with pathologist. acropetally (upwards and outwards) in each additional application cost (3). In conclusion, I would urge all of you the plant. It can be applied as a liquid Application in the fall or spring prior to who have experienced phytophthora onto the foliage and then watered into the development of symptoms will be root rot in your nursery to keep at your the root zone (Subdue 2E) or it can be the most effective. Timing of fungicide drainage problems, continue to incorporated as granules into the soil applications, as well as cultural upgrade your field topography, be or onto the surface of the soil, with activities, are discussed in detail by conscientious about coordinating your release of the product occurring after Cooley et al. (1). water uses, and keep good histories of watering (Subdue 5G). Chlorination. In some locations, your fields s o that you can plan where Once in the plant, metalaxyl inhibits water supplies that are used for to sow or transplant your susceptible further development of Phytophthora irrigation are contaminated with species. I urge you not to depend on so that the progression of the disease Phytophthora spp. Inoculum in the fungicides; they are for emergencies, is halted. The fungus is not eradicated water can be eliminated or reduced to for marginal situations, not for routine from the seedling, however. The nondamaging levels by chlorination of applications. Fortunately, because consequences of non-eradication can the water before it enters the irrigation Phytophthora species are very be either minimal, such as when you lines . We are still learning to dependent on their environment, they outplant a treated but infected seedling adequately measure and interpret can be very easily controlled with good into a dry, well drained forest chlorine levels; for example, How cultural practices.

environment, or quite severe, such as much chlorine gas needs to be added to get 5 ppm free chlorine? How much when you transplant a treated, but free chlorine is required to reduce infected, seedling into a poorly drained References portion of another nursery. Phytophthora spp. to undetectable levels in the water? Other factors, such Metalaxyl should be used sparingly 1. Cooley, S.J.; Hamm, P.B.; Hansen, and thoughtfully, because strains of as pH and the amount of organic E.M. Management guide to phy- Phytophthora that are tolerant to material in the water, will also affect tophthora root rot in bareroot con- metalaxyl can become predominant in how chlorine reacts and how much is ifer nurseries of the Pacific needed to kill propagules of Northwest. Portland, OR: U.S. De- soils that have received repeated partment of Agriculture, Forest Serv- Phytophthora species. frequent applications of the fungicide. ice, Forest Pest management; 1985. Repeated use can also result in Some sources of water are more 2. Cooley, S.J. Unpublished data. decreased effectiveness because of likely than others to become 3. Hamm, P.B.; Cooley, S.J.; Hansen, the build-up of soil microbes capable, contaminated with Phytophthora E.M. Response of Douglas-fir and species. Well-water and municipal Phytophthora spp. to metalaxyl in forest tree nurseries in the Pacific water are probably Northwest. Plant Disease 68:671-673.