Biological control options for Phytophthora species

Robert G. Linderman
USDA-ARS Horticultural Crops Research Laboratory, Corvallis, Oregon

The genus Phytophthora contains many species that are pathogenic on a wide range of host plant species, causing diseases of all plant parts including roots, crown, stems, branches, leaves and fruits. Some species are restricted to belowground infections, some to aboveground infections, some to both. The newly described pathogen, Phytophthora ramorum, has been found causing only aboveground infections on an increasing number of woody host plants. Although this pathogen appears to infect only aboveground plant parts, its life cycle could have a soilborne phase as well. P. ramorum causes a range of symptoms from tree death due to the formation of large cankers, to stem or crown cankers leading to shoot die-back, to leaf spots and branch dieback. There has been no research reported to date using biologically based control strategies for P. ramorum. Thus, I will discuss here biological control concepts and approaches relating to other species of Phytophthora that could be applicable to P. ramorum in nurseries.

Biological control of Phytophthora. Biological control of Phytophthora species generally is the result of interactions between the pathogen and microbial antagonists at the site of infection. Antagonistic bacteria, fungi, or actinomycetes can produce antibiotics or toxic metabolic byproducts that inhibit vegetative growth, spore germination, or sporulation by the pathogen; directly parasitize the pathogen; or compete with the pathogen for some limited resource it needs to cause infections. Whatever the mechanism, it is necessary to establish and maintain threshold levels of antagonists at the infection site, whether above or below the ground, for antagonists to suppress the pathogen. If the numbers and/or activities of the antagonists fall below that threshold, then the pathogen may progress and cause infections, if environmental conditions are conducive. All species of Phytophthora are favored by free water in which spores can germinate and usually produce sporangia that release motile zoospores that swim or that encyst and are moved passively in moving water to new infection sites. Secondary sporangia and zoospores are likely produced on infected tissues, and these can disperse the pathogen to other host plants in water. If biological suppression is to occur, antagonists must actively block critical stages of pathogen development to prevent infections. In some cases, the activity of the antagonists can reduce the population of the pathogen by lysing mycelium or spores or by inhibiting sporulation. In cases of documented suppressiveness of soils to Phytophthora, increased microbial populations are correlated with reduced disease incidence or severity, and usually those populations are supported by high organic matter content in the soil that serves as a nutrient substrate for the antagonists.

Another mechanism of biological control that does not require direct interaction of the antagonists and pathogen at the infection site is induced resistance. Some microbes can be applied to belowground or aboveground portions of the plant. They then stimulate host defense reactions that suppress disease at some other site on the plant. Induced resistance can be the result of treatment with microbes unrelated to the pathogen or with related but non-pathogenic strains of the pathogenic species. There is increasing interest and evidence to support the strategy of inoculating roots or foliage with microbes or composts or compost extracts/teas that carry microbes or microbial byproducts that induce greater host plant resistance to infections, mainly by foliage pathogens. The mechanistic details of the induced resistance process are still being developed.

Strategies for implementing biological control. For Phytophthora species to be inhibited or suppressed biologically, the microbial agents generally must come in contact with the hyphae or spores of the pathogen. Some fungal agents are known to parasitize the pathogen and thereby destroy its biomass; others may produce enzymes that lyse the pathogen mycelium or spores. Bacterial or actinomycete antagonists may produce antibiotics, other toxic metabolites, or enzymes that also may inhibit growth or sporulation and/or lyse hyphae or spores of the pathogen. These antagonists are thought to be attracted to and nourished by substances exuded by the pathogen hyphae or spores. Once present, these same microbes can produce lytic enzymes that destroy the pathogen. In some suppressive soils, certain bacteria are inhibited that normally produce sporangia-stimulating metabolites, thus eliminating a critical stage of the pathogen needed for dispersal and infection.

Some microbes can produce metabolites that enter plant tissues and induce defense reactions by the host plant without direct contact with the pathogen. Such microbes and/or their metabolites have been reported to occur in composts used to amend soil or soilless growth media. The induced systemic resistance approach holds promise for the suppression of pathogens like P. ramorum in that microbes may be applied to the roots to induce resistance on the aboveground portions of host plants.

To manage Phytophthora diseases biologically, one must consider methods for introducing antagonists or enhancing resident antagonists. For root-infecting species of Phytophthora, adding significant quantities of organic matter that stimulates antagonistic microbial activity seems to be the key. However, not all organic materials work that way. Organic matter that accumulates on the soil surface and encourages root growth into the decomposing material prevents the roots from becoming infected. The pathogen may still be present below the organic layer but cannot cause root rot of the "protected" roots. The antagonists in such situations appear not to be introduced in the organic material but are nutritionally encouraged to multiply. Amendment with organic materials with high cellulose content will encourage the development of high populations of cellulase-producing microbes that lyse the cellulosic walls of mycelium or spores of Phytophthora species. If the background microbial populations of the soil or soil-less potting media used in the nursery industry are not high in antagonists, then organic amendments may not be able to stimulate higher numbers of antagonists. Most soils contain some antagonists that can be increased by enrichment with organic materials. Their presence or absence can be demonstrated by treating them with increasing levels of heat that selectively eliminates bacteria, fungi, and actinomycetes, and then by inoculating them with Phytophthora and observing the increased disease where antagonists have been eliminated (Figure 1).

In cases where antagonists may not be present naturally (as with aboveground plant parts), or their numbers and distribution are not enough to interact with the pathogen and suppress its growth and activity, then antagonists need to be applied at levels to effect biological control. In the nursery industry where environmental conditions can be manipulated better than in the field, heavy applications of antagonists could work, provided they are timely in relation to the infection times for the pathogen, and that some nutritional support is provided to maintain high antagonistic potential. With a pathogen like P. ramorum that is favored by cool, wet/rainy conditions, it will be difficult to apply and maintain high antagonist populations on aboveground plant surfaces where infections will occur. On the other hand, if infections normally occur during propagation stages under cover, then applications of biological control agents might be very effective. Such control strategies are good prospects for the future.

References

Cook, R. J. and Baker, K. F. 1983. The Nature and Practice of Biological Control of Plant Pathogens. The American Phytopathological Society, St. Paul, MN.

Downer, A. J., Menge, J. A., and Pond, E. 2001. Association of cellulytic enzyme activities in eucalyptus mulches with biological control of Phytophthora cinnamomi. Phytopathology 91:847-855.

Hoitink, H. A. J., Inbar, Y, and Boehm, M. J. 1991. Status of compost-amended potting mixes naturally suppressive to soilborne diseases of floricultural crops. Plant Disease 75:869-873.

Linderman, R. G., Moore, L. W., Baker, K. F., and Cooksey, D. A. 1983. Strategies for detecting and characterizing systems for biological control of soilborne plant pathogens. Plant Disease 67:1058-1064.

Malajczuk, N. 1983. Microbial antagonism to Phytophthora. pp. 197-218 in: Phytophthora: Its Biology, Taxonomy, Ecology, and Pathology. D. C. Erwin, S. Bartnicki-Garcia, and P. H. Tsao (Eds.) The American Phytopathological Society, St. Paul, MN.