Fungicides for managing Phytophthora species in nurseries

Steven N. Jeffers
Department of Plant Pathology & Physiology,
Clemson University, Clemson, SC

On an annual basis, Phytophthora spp. cause some of the most economically important diseases on ornamental crops grown in nurseries worldwide. Root and crown rots are the most devastating because they affect an extremely wide array of plant species, genera, and families, and these types of diseases usually result in plant mortality. Phytophthora spp. also attack aerial parts of ornamental plants causing leaf and stem blights as well as shoot dieback. Aerial blights usually are not lethal and occur on a more limited range of hosts. A number of species of Phytophthora attack ornamental crops—either below ground, above ground, or both—and symptoms produced by individual species are similar. P. ramorum causes sudden oak death (SOD) in the coastal forests of California and Oregon and is capable of causing aerial blight and stem dieback on nursery crops. To date, P. ramorum has been found on nursery crops almost exclusively in Europe, but the risk of moving this species into nurseries in the United States is considerable. The host plants attacked and the symptoms caused by P. ramorum are similar to those caused by the other species of Phytophthora already present in nurseries around the world.

Fungicides for Phytophthora spp. Managing Phytophthora diseases in nurseries is challenging and requires an integrated approach, as pointed out in the other three papers in this session. Chemical fungicides should be an integral part of any integrated disease management strategy developed for Phytophthora species, including P. ramorum. However, very little research has been conducted to date to evaluate the efficacy of fungicides for P. ramorum. So, for the time being, we must assume that the fungicides that are effective against other species of Phytophthora also will be effective against this species.

A number of fungicides are labeled to control Phytophthora spp. on ornamental crops (Table 1), and these can be grouped into three general categories: those specifically active against Phytophthora spp. and other oomycetes; strobilurins—a relatively new class of fungicide active against a wide range of fungi; and broad-spectrum protectants—well-established traditional fungicides that also are active against a wide range of organisms. These fungicides vary in several key characteristics (Table 1). Some function merely as protectants and, therefore, have to be present on the plant surface before inoculum arrives. Others have protectant and curative action; curative action means that the fungicide can stop an established infection inside plant cells if applied soon after infection occurs—usually within 24 to 72 hours. However, all of these fungicides work best when applied before infections occur because it is too risky to rely on curative action to prevent disease establishment. These fungicides also vary in their systemic nature—i.e., their ability to move within plants. The broad-spectrum protectants are non-systemic and are active only on plant surfaces. The mefenoxam and metalaxyl family of compounds, propamocarb, dimethomorph, and azoxystrobin are upwardly systemic and can move from treated organs upward to other plant tissues and organs. Fosetyl-Al and related compounds are unique among fungicides in that they are truly systemic. They can move both upward and downward from the point of application, which gives them an advantage over the other chemicals.

Fungicide resistance. Fungicide resistance results when the target fungus changes genetically to become less sensitive to the toxic active ingredient. Many factors determine the potential for an organism to develop resistance to a given fungicide, including the biochemical mode of action of the fungicide and the biology of the target fungus. The fungicides labeled for Phytophthora diseases on ornamental crops vary in their potential for resistance development (Table 1), and this information must be considered when a fungicide application strategy for the nursery is designed. Misuse or overuse of fungicides with high or moderate risks of resistance in nurseries will likely result in resistance development, which will severely limit or eliminate the effectiveness of the fungicides to which resistance has developed. To date, among the species of Phytophthora that attack ornamental crops, there are very few documented cases of resistance to the fungicides used on ornamental crops. These all have involved resistance in P. nicotianae to the mefenoxam and metalaxyl fungicides.

Managing aerial blights vs. root rots. When managing Phytophthora diseases in nurseries, it is important to recognize that there are different fungicides labeled for foliar spray applications to control aerial blights and for soil applications (e.g., drench, soil-directed spray, or incorporation into the soil) to control root rots. Most of the products targeting root rots have been developed and formulated to move through soil and container mixes so they can reach the infection court—i.e., the roots and root-crown. Likewise, most of the products used for aerial blights have been developed and formulated to be applied as foliar sprays and usually target an array of foliar fungus pathogens, including Phytophthora spp. Some of these products are very active against the species of Phytophthora that cause root rots, but they are not mobile in the soil profile and, therefore, cannot get to and protect the roots and root-crown (e.g., chlorothalonil, mancozeb, maneb, most copper compounds).

Of the products labeled for foliar application to control aerial blights (like those caused by P. ramorum), fosetyl-Al has been very effective over the years. Recently, a number of chemically related compounds that are based on similar phosphorus acid chemistry (e.g., phosphonate, phosphite, etc.) have been developed and currently are being tested in trials around the world. Overall, these compounds show great promise for effective management of Phytophthora spp. on ornamental crops and have been effective on selected tree species inoculated with P. ramorum in preliminary trials. Another relatively new product that shows very good activity against aerial blights caused by Phytophthora spp. is dimethomorph; however, it is still too new in the marketplace to be widely accepted. The broad-spectrum protectant fungicides also are effective against foliar infections by Phytophthora spp. These products have been used for years to control late blight on potato and tomato (caused by P. infestans) and downy mildews (caused by closely related genera of fungi) on various crops. Azoxystrobin is another relatively new compound that has shown reasonably good activity against Phytophthora spp. on a variety of crops, but its usefulness on ornamental crops has yet to be established. In the southeastern United States, tank mixtures of fosetyl-Al and mancozeb have provided excellent control of Phytophthora aerial blight on Madagascar periwinkle (Catharanthus roseus), caused by P. nicotianae, in both research trials and in landscapes. Tank mixes of fungicides take advantage of different mechanisms of action and should be considered in management strategies for P. ramorum.

Fungicide label constraints. The fungicide label is a legal document that must be followed and, therefore, can limit a potentially effective use of a fungicide. For example, mefenoxam and metalaxyl fungicides are very effective against both root rots and aerial blights but are not labeled for foliar applications to ornamental plants. Therefore, they cannot be applied to combat aerial infections. In addition, some fungicides are limited by the scope of the label. A fungicide may be labeled for use only on specific plants even though it is effective on a wide range of plants. When this situation occurs, one label may allow uses on plants not specified on the label but other labels may not allow such uses. Application intervals and resistance management statements also may limit the number of applications or the type of applications allowed to certain crops. These constraints must be followed. As always, the end-user must read and follow the label on any pesticide applied and used in the nursery.

In summary, fungicides continue to play an important role in the integrated management of Phytophthora diseases on ornamental plants in nurseries. The products currently available to the nursery industry should be effective against P. ramorum and should help limit the establishment and spread of this new threat to ornamental crop production.

References

Benson, D. M., and von Broembsen, S. 2001. Phytophthora root rot and dieback. Pages 52-56 in: Diseases of Woody Ornamentals and Trees in Nurseries. R. K. Jones and D. M. Benson, eds. APS Press, St. Paul, MN.

Brent, K. J., and Hollomon, D. W. 1998. Fungicide Resistance: The Assessment of Risk. FRAC Monograph No. 2. Global Crop Protection Federation, Brussels, Belgium.

Cohen, Y., and Coffey, M. D. 1986. Systemic fungicides and the control of Oomycetes. Annu. Rev. Phytopathol. 24:311-338.

Jeffers, S. N., Miller, R. W., and Powell, Jr., C. C. 2001. Fungicides for ornamental crops in the nursery. Pages 409-416 in: Diseases of Woody Ornamentals and Trees in Nurseries. R. K. Jones and D. M. Benson, eds. APS Press, St. Paul, MN.

Schwinn, F. J., and Staub, T. 1995. Oomycetes fungicides. Pages 323-346 in: Modern Selective Fungicides: Properties, Applications, Mechanisms of Action. 2nd edition. H. Lyr, ed. Gustav Fischer Verlag, Jena, Germany.