Cultural practices and host resistance:
Two IPM strategies for control of Phytophthora ramorum in nurseries

D. Michael Benson, Department of Plant Pathology,
North Carolina State University, Raleigh

In many ways, control of Phytophthora ramorum in nurseries presents far fewer obstacles than management of this pathogen in the landscape and forests of infested areas in California, Oregon, and Europe. Nursery managers in the United States and Europe have been dealing with Phytophthora blight and dieback on nursery crops for many years, although until now P. ramorum was not the species of Phytophthora involved (Benson and Jones 1980). Thus, a wide range of management strategies is available for the disease. As part of an Integrated Pest Management (IPM) approach, cultural control and host resistance are two strategies that can be effective when combined with other IPM strategies (as addressed by other authors in this symposium.) Given the observation that once Phytophthora inoculum becomes established in a nursery, it is difficult to eradicate. The use of an IPM approach approach, however, can result in disease-free plants for market.

The goal of an IPM approach should be to reduce initial inoculum and/or slow the rate of infection. These two factors slow or stop an epidemic. Both cultural practices and host resistance have a role in reducing initial inoculum and slowing the infection rate of P. ramorum in nurseries. Adoption of effective cultural practices can reduce initial inoculum through nursery design and organization, sanitation, irrigation scheduling, and treatment of recycling water. Likewise, host resistance, as well as other cultural practices like fertility management, can reduce the infection rate and thus the number of plants infected. Over time a lower infection rate also contributes to a lower inoculum carry over in the nursery.

Splash dispersal. Before control strategies are discussed, the importance of splash dispersal should be recognized for Phytophthora pathogens causing blight and dieback. Splash dispersal of Phytophthora propagules from ground to foliage and from plant to plant can be a very efficient means of Phytophthora movement (Fig. 1). Kuske and Benson (1983b) demonstrated that a layer of gravel on the ground under nursery pots was effective in preventing splash dispersal of Phytophthora inoculum from the ground to plant foliage. When an impregnable plastic ground cover was used, inoculum splashed as high as 60 cm to infect rhododendron foliage (Kuske and Benson 1983a). Gerlach et al. (1976) demonstrated that inoculum of P. citrophthora could be splashed as high as 40 cm on 2-yr-old plants of Pieris japonica in 5 L pots but the most lesions were found on leaves 27 cm above the ground.

Nursery design and cultural practices. Nursery managers growing Phytophthora -susceptible crops know they need to design the nursery layout to avoid standing water and the potential for splash dispersal of inoculum during irrigation or rainstorms (Fig. 2). Nursery beds are crowned in the center to promote rapid drainage away from the pots after irrigation or storms. Gravel or porous fabric is used as a ground cover to avoid water puddles around pots as well as to establish a physical barrier between potentially infested soil and the pot. Growing areas and ditches can be laid out to collect and return irrigation or storm runoff to retention basins that include grass buffers or other vegetation barriers established so as to slow the return of water as well as filter out pathogen propagules and organic debris.

Grouping plants by age and cultivar susceptibility to P. ramorum in the nursery production area can aid in irrigation timing and other management practices to prevent disease development. Also, blocks of susceptible crops could be alternated with those of non-hosts to avoid rapid epidemic development and secondary spread of inoculum, should individual plants become infected with P. ramorum. With highly susceptible cultivars, pots of non-hosts can be placed around susceptible cultivars in a checkerboard pattern much the same as with the use of multilines for disease management in agronomic crops.

In some cases, Phytophthora species are introduced to a nursery on infected plant material purchased for growing to a size acceptable for retail sale. Growers should inspect shipments and quarantine plants on an independent irrigation system for several weeks to be sure they are free of blight and dieback before introducing them to the general nursery production area. In addition, nurseries located in forested areas where P. ramorum occurs should establish a very wide buffer zone of host free plants to avoid natural introduction of the pathogen. Given the eradication and monitoring program in place in the regulated area near Brookings, Oregon, 100 ft buffers of apparently disease-free host species around infection sites in the forest at the time of eradication were not wide enough to prevent disease from developing later in the buffer zone.

Other cultural practices such as fertility management can impact the severity of Phytophthora blight and dieback. For instance, excessive nitrogen to encourage lush rapid growth can result in foliage of rhododendron or other hosts that is more susceptible to disease (Hoitink et al. 1986).

Sanitation. In rhododendron, the foliage of newly rooted cuttings transplanted to pots may be killed by Phytophthora within a week or two, while on 3- to 4-yr-old plants only terminal shoot dieback may occur over several months. Thus, on larger plants, it may be possible to prune out infected shoots and save the plant, but small plants may be lost. Pruning shears should be disinfested between cuts. Plants should be observed for several weeks to confirm that the pathogen has been eliminated.

Stock plants for propagation should be segregated and examined keenly for any symptoms prior to taking cuttings. Shears should be disinfested. Disinfectants should be used for surfaces in propagation areas and well or municipal water should be used for misting cuttings.

Irrigation issues. Water management is a key issue in limiting the incidence of Phytophthora in nurseries. Production areas laid out with hosts of the same age on a schedule to deliver just enough water for growth can avoid the need to overwater young plants placed next to larger plants that need more water. Late afternoon irrigations should be avoided on susceptible hosts so that the foliage has time to dry off before sunset can limit favorable infection periods (Fig. 3). This is particularly effective because most species of Phytophthora require about an 8-to 12- hr wet period to form sporangia. Interrupting favorable wet periods can reduce production as well as dispersal of secondary inoculum.

The most troubling aspect of management of Phytophthora diseases in nurseries is the problem of inoculum in recycling irrigation water (Fig. 4). Many investigators have demonstrated this problem (see von Broembsen et al. 2001). Tjosvold et al. (2002) and Maloney et al. (2002) were able to bait P. ramorum from streams in California during the rainy season, so this pathogen could also be a threat in recycling irrigation water. Thus, nurseries in forested areas infested with P. ramorum should avoid taking any irrigation water from streams during the rainy season. For nurseries where P. ramorum or other Phytophthora spp. have been introduced and have become established in recycling water, several water treatment methods are available including slow sand filtration, chlorination, ozonation, and UV treatment. Von Boembsen et al. (2001) have described the advantages and disadvantages of these methods for nurseries.

Host resistance. Several cultivars of rhododendron, in particular English Roseum, Roseum Elegans, and Scintillation, and the species Rhododendron 'P.J.M.', have had a low incidence of Phytophthora dieback in nurseries caused by species of Phytophthora other than P. ramorum (Benson et al. 1982). Tooley et al. (2002) found that leaves of Rhododendron 'P.J.M.', R. maximum, and R. carolinianum dipped in sporangia suspensions of P. ramorum developed lesions on less than 10% of the leaf area compared to a highly susceptible cultivar like Cunningham's White that developed up to 50% leaf lesion area with the most virulent isolate tested. Given the ever-expanding host range of P. ramorum in California and Oregon forests and the potential host range determined experimentally on a number of common nursery crops (Linderman et al. 2002, Parke et al. 2002, Tooley et al. 2002), a cultivar-by-cultivar screen for host resistance will no doubt identify resistant cultivars that can be exploited for the nursery trade.

Management of P. ramorum in nurseries through IPM practices including cultural practices, fungicides, and host resistance will be paramount to producing disease- free plants as well as protecting the native plant environment around nurseries. As we learn more about the biology and ecology of the pathogen, and the epidemiology of diseases caused by P. ramorum, management strategies will continue to be improved.

References

Benson, D.M., and Jones, R.K. 1980. Etiology of rhododendron dieback caused by four species of Phytophthora. Plant Dis. 64:687-691.

Benson, D.M., Jones, R.K., and Barker, K.R. 1982. Disease loss assessment for azalea, rhododendron, and Japanese holly in North Carolina nurseries. Plant Dis. 66:125-128.

Gerlach, W. W. P., Hoitink, H. A. J., and Schmitthenner, A. F. 1976. Phytophthora citrophthora on Pieris japonica: Infection, sporulation, and dissemination. Phytopathology 66:302-308.

Hoitink, H. A. J., Watson, M. E., and Faber, W. R., 1986. Effect of nitrogen concentration in juvenile foliage of rhododendron on Phytophthora dieback severity. Plant Dis. 70:292-294.

Kuske, C.R., and Benson, D.M. 1983a. Survival and splash dispersal of Phytophthora parasitica, causing dieback of rhododendron. Phytopathology 73:1188-1191.

Kuske, C.R., and Benson, D.M. 1983b. A gravel container base for control of Phytophthora dieback in rhododendron nurseries. Plant Dis. 67:1112-1113.

Linderman, R. G., Parke, J. L., and Hansen, E. M. 2002. Relative virulence of Phytophthora species, including the sudden oak death pathogen, P. ramorum on leaves of several ornamentals. Phytopathology 92:S47.

Maloney, P. E., Kane, S. F., Jensen, C. E., and Rizzo, D. M. 2002. Epidemiology and ecology of Phytophthora ramorum in redwood/tanoak forest ecosystems of the California Coast Range. Page 11. In Sudden Oak Death, A Science Symposium. USDA Forest Service and University of California, Monterey, CA. 96 pp.

Parke, J. L., Linderman, R. G., and Hansen, E. M. 2002. Susceptibility of Vaccinium to Phytophthora ramorum, cause of sudden oak death. Phytopathology 92:S63.

Tjosvold, S. A., Chambers, D. L., Davidson, J. M., and Rizzo, D. M. 2002. Incidence of Phytophthora ramorum inoculum found in streams running through areas of high incidence of sudden oak death in Santa Cruz County. Page 52. In Sudden Oak Death, A Science Symposium. USDA Forest Service and University of California, Monterey, CA. 96 pp.

Tooley, P. W., Kyde, K. L., and Englander, L. 2002. Infectivity of Phytophthora ramorum on selected Ericacaeous host species. Phytopathology 92:S81.

von Broembsen, S. L., MacDonald, J. D., and Pscheidt, J. W. 2001. Disease management for nurseries using recycling irrigation systems. Pages 423-430. In Jones, R. K. and Benson, D. M. (eds). Diseases of Woody Ornamentals and Trees in Nurseries. APS Press, St. Paul, MN. 482 pp.