Conifer and Hardwood Diseases

37. Phytophthora Root Rot Michelle M. Cram and Everett M. Hansen

Hosts Phytophthora root rot occurs on a broad range of conifers and hardwoods. Tree species most often affected by Phytophthora in forest nurseries include black walnut, Fraser fir, Douglas-fir, and red pine. Other Phytophthora hosts in forest nurseries include species of true fir, hemlock, larch, pine, spruce, white-cedar, yew, oaks, birch, and American chestnut. Species identified as highly tolerant of Phytophthora include western redcedar, western larch, lodgepole pine, ponderosa pine, and Chinese chestnut.

Figure 37.1—Phytophthora root rot develops on seedlings in low wet and poorly drained areas. Photo by Distribution Everett M. Hansen, Oregon State University. Phytophthora root rot is predominately a problem in bareroot forest nurseries infection on the root systems can result in seedlings have similar damage patterns, throughout the United States. The disease seedling losses in storage, transport, and but the infected tissue is brown or tan. develops in nurseries where susceptible outplanting. Whole nursery fields of high­ Positive identification ofPhytophthora seedlings are grown in saturated soils. ly susceptible hosts have been destroyed infection requires using the services of Damage by Phytophthora in bareroot for­ due to infestation by Phytophthora. a pathologist or commercially available est nurseries occurs most often in low wet test kits. Diagnostic kits for Phytophthora areas or poorly drained areas (fig. 37.1). Diagnosis provide a quick identification in the field. Damage in container nurseries occurs when container soil becomes contaminated General aboveground symptoms of Phy­ either by storing containers and potting tophthora root rot on seedlings include soil on infested ground or by irrigation chlorosis, stunting, wilting, and eventual from infested water sources. Some common mortality (fig. 37.2). The disease usually Phytophthora species that cause root rot occurs in patches as infection spreads and stem damage include P. cinnamomi, from seedling to seedling. The cambium P. citricola, P. cactorum, and P. drechsleri. tissue of infected conifers turns a reddish- In the Pacific Northwest conifer seedlings brown or butterscotch color at the root are also affected by P. sansomeana, collar and in the primary and lateral roots P. megasperma, P. pseudotsugae, (fig. 37.3). Fine feeder roots infected by P. cryptogea, and P. gonapodyides. Phytophthora are often black, decayed, or missing. Damage Black walnut is the hardwood species most susceptible to Phytophthora root Phytophthora root rot can extend to the rot. Black walnut infected tissue becomes root collars and stems of susceptible black, soft, and water-soaked at the conifer and hardwood seedlings, affecting root collar, extending into the stem and seedling quality and mortality. Seedlings Figure 37.2—Symptoms of Phytophthora root rot primary root (fig. 37.4). Oak and chestnut in a black walnut bed. Photo by Michelle M. Cram, USDA lifted with an incipient Phytophthora Forest Service.

126 Forest Nursery Pests Conifer and Hardwood Diseases

37. Phytophthora Root Rot

growing in flooded or saturated soil are further predisposed to infection from damage created by oxygen deficiency. Phytophthora species can survive dry conditions and temperature extremes by forming resting structures (chlamydospores and oospores) in damaged plants or soil. These resting structures will remain dor­ mant until activated by high soil moisture.

Control

Prevention

Select nursery fields with well-drained soils. Maintain good drainage by avoiding compaction and promptly fixing leaking irrigation pipes and sprinkler heads. Avoid transplanting seedlings from infested to disease-free nurseries or fields. Do not use diseased trees as mulch and Figure 37.3—Red-brown or butterscotch colored Figure 37.4—Blackened and water-soaked tissue of avoid moving equipment from infested to cambium tissue of a Douglas-fir seedling infected black walnut infected by Phytophthora. Photo by Michelle by Phytophthora. Photo by Everett M. Hansen, Oregon State M. Cram, USDA Forest Service. uninfested areas. Use Phytophthora-free University. water sources, such as well water, if possible. Irrigation water from sources Accuracy of these tests is high, but false that include surface runoff may have negatives and positives for Phytophthora Phytophthora contamination and require can occur. To increase test accuracy, take chlorination before use. multiple samples and use the diseased tissue along the leading infection edge to Cultural avoid secondary organisms that invade Reduce potential for saturated soil by dead tissue. Pathologists can provide a improving soil surface and subsurface very accurate Phytophthora species iden­ drainage. Drainage can be improved by tification with the use of selective culture building raised nursery beds or installing media; however, laboratory isolation underground drain tile systems and drain­ techniques require more time. age ditches. Correct compacted soils by deep ripping or chiseling. After seedbeds Biology are sown, wrenching can help reduce com­- paction. Prevent overwatering by moni­ Phytophthora species produce motile toring soil moisture and irrigation output. spores (zoospores) in sporangia (fig. 37.5) that can swim up to several hours in Patches or small areas of seedlings with saturated soil or flowing water before Phytophthora root rot should be lifted infecting roots or root collars. These separately from the noninfested areas motile spores can be produced under wet Figure 37.5—Sporangium and zoospores of and destroyed. Any equipment used in Phytophthora. Photo by Michelle M. Cram, USDA Forest conditions within 24 hours. Seedlings Service. infested areas must be cleaned of soil

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37. Phytophthora Root Rot

and plant debris before being used again. used to reduce disease progression in Hansen, E.M.; Hamm, P.B.; Julis, A.J.; Roth, When possible, avoid moving equipment an infested field. They seldom eradicate L.F. 1979. Isolation, incidence, and manage­ through areas affected by Phytophthora, an established infection, and may make ment of Phytophthora in forest tree nurseries or work in infested areas last and clean diagnosis more difficult by temporarily in the Pacific Northwest. Plant Disease Reporter. 63(7): 607–611. the equipment before moving to another limiting symptom expression on already field. Losses to Phytophthora root rot infected seedlings. Rotate between differ­ Hansen, E.M.; Wilcox, W.F.; Reeser, during storage and outplanting can be ent chemical families to avoid develop­ P.W. 2009. Phytophthora rosacearum and reduced by not lifting seedlings in infested ment of resistant strains of Phytophthora P. sansomeana, new species segregated from the Phytophthora megasperma “complex.” zones and culling seedlings that appear to systemic fungicides. Mycologia. 101: 129–135. diseased. Selected References Kuhlman, E.G.; Grand, L.F.; Hansen, E.M. In container operations, avoid Phytoph­ 1989. Phytophthora root rot of conifers. In: thora contamination by irrigating with Bruck, R.I.; Kennerly, C.N. 1983. Effects Cordell, C.E.; Anderson, R.L.; Hoffard, W.H.; uninfested water and keeping containers of metalaxyl on Phytophthora cinnamomi Landis, T.D.; Smith, Jr., R.S.; Toko, H.V., and soil off the ground. Use a potting root rot of Abies fraseri. Plant Disease. 67: tech. coords. Forest nursery pests. Agriculture mix that is highly porous and avoid soil 688–690. Handbook 680. Washington, DC: USDA Forest Service: 60–61. compaction in the containers. Cooley, S.J. 1987. Management of Phytoph­ thora root rot in conifer nurseries of the Pacific Lane, C.R.; Hobden, E.; Walker, L.; Barton, Chemical Northwest. Tree Planters’ Notes. 38: 37–40. V.C.; Inman, A.J.; Hughes, K.J.D.; Swan, H.; Colyer, A.; Barker, I. 2007. Evaluation of a Fumigation significantly reduces Green, R.J. 1975. Phytophthora root rot of rapid diagnostic field test kit for identification Phytophthora populations in the soil, black walnut seedlings. In: Cordell, C.E.; of Phytophthora species, including P. ramorum although it is less effective in poorly Anderson, R.L.; Hoffard, W.H.; Landis, T.D.; and P. kernoviae at the point of inspection. Smith, Jr., R.S.; Toko, H.V., tech. coords. drained soils where Phytophthora is most Plant Pathology. 56: 828–835. Forest nursery pests. Agriculture Handbook likely to be a problem. Systemic fungi­ 680. Washington, DC: USDA Forest Service: Zentmyer, G.A. 1980. Phytophthora cinnamomi cide use may also be required to control 99–100. and the diseases it causes. Monograph 10. St. Phytophthora root rot development, Paul, MN: The American Phytopathological especially with susceptible seedlings in Green, R.J.; Pratt, R.G. 1970. Root rot of Society. 96 p. black walnut seedlings caused by Phytoph­ nurseries affected by this pathogen in thora citricola. Plant Disease Reporter. 54: the past. Fungicides are more effective 583–585. if applied prior to infection and can be

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