Review of Fungicides for Control of Phytophthora and Pythium Diseases on Potted Ornamentals

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A. R. Chase, Ph.D.

University of Florida, IFAS,
Central Florida Research and Education Center - Apopka,
2807 Binion Rd. Apopka. FL 32703.
CFREC-Apopka Research Report, RH-93-3

Some of the most common and costly diseases of ornamental plants are caused by Phytophthora and Pythium spp. These diseases can affect cuttings, seedlings, and all other stages of the crops. Pythium spp. primarily attack plant roots. Diseases caused by Phytophthora can occur on any part of the plant. In Florida, aerial leaf spots caused by Phytophthora parasitica occur on a number of ornamental crops although stem rots are probably the most common problems. The roots of some types of plants, such as schefflera (Brassaia actinophylla), are almost always infected with Pythium spp. despite preventative use of fungicides.

Fungicides remain an important method to control losses due to Phytophthora and Pythium spp. The following review was developed to summarize the fungicides which are available for ornamentals to assist in control of Phytophthora and Pythium spp. The table gives a summary of studies performed at the CFREC-Apopka since 1981.

Currently, there are relatively few fungicides available for controlling Phytophthora or Pythium. Propamocarb (Previcur N), etridiazole (Terrazole and Truban), fosetyl aluminum (Aliette), and metalaxyl (Subdue) have been tested on one or more ornamental crops. All of these fungicides are applied to the potting medium with the exception of Aliette which can be applied as a drench or a foliar spray. Although some differences in efficacy occur from plant to plant, some general conclusions can be drawn.

The best control of Phytophthora and Pythium spp. on ornamentals occurs when Subdue is applied as a soil drench. Aliette drenches are nearly as effective and sometimes more effective than Subdue. When Aliette is applied as a foliar spray, efficacy is usually a little lower than a drench of either Subdue or Aliette. Terrazole and Truban drenches can also be effective but are usually a little less effective than Subdue drenches. It is perhaps a little confusing to find Subdue listed as both "excellent" and "good" in the table for a single plant. This variability in the degree of control can be explained by the effect of disease pressure since under very severe conditions a fungicide generally does not work as well as it can under more normal conditions. Overall, Subdue gives good to excellent control of these diseases depending on disease pressure. Only a few tests were performed with Previcur N but they showed that this compound had a significantly lower efficacy than the other fungicides. This conclusion has been supported by researchers working on other ornamentals.

This information may allow growers to choose the best fungicide for Phytophthora and Pythium spp. Schefflera requires preventative fungicide applications since Pythium root rot is common on all stages. Other diseases can be controlled following diagnosis of the problem. Finally, some diseases, such as Phytophthora aerial blight on vinca are rarely controlled with any fungicide treatment and cultural control methods must be used for these diseases.

Pesticides should be applied according to label directions.

Regardless of the pesticide or mixture of pesticides used, it is
strongly recommended that the effects be evaluated on a few
plants, under your particular conditions before treating all plants.

Mention of a commercial or proprietary product in this paper
does not constitute a recommendation by the authors,
nor does it imply registration under FIFRA as amended.

Reference to University of Florida/IFAS Pest Control Guides

References

1. Chase, A. R. 1981. Evaluation of Several Pesticides for Control of Damping-off of Schefflera Seeds. Agricultural Research Center, Univ. of Fl. ARC-Apopka Research Report RH-81-15.
2. Chase, A. R. 1982. Influence of Soil Drench Fungicides on Rooting of Two Foliage Plants. Agricultural Research Center, Univ. of Fl., ARC-Apopka Research Report RH-82-12.
3. Chase, A. R. and L. S. Osborne. 1982. Pesticide Evaluations in Support of Registrations on Tropical Foliage Plants. Agricultural Research Center, Univ. of Fl., ARC-Apopka Research Report RH-82-18.
4. Chase, A. R. 1984. Subdue 5S - Phytotoxicity on Three Foliage Plants. Agricultural Research and Education Center, Univ. of Fl., AREC-Apopka Research Report RH-84-21.
5. Chase, A. R. 1984. Some New Soil Fungicides for Control of Pythium Root Rot of Foliage Plants. Agricultural Research and Education Center, Univ. of Fl., AREC-Apopka Research Report RH-84-23.
6. Chase, A. R., D. D. Brunk, and B. L. Tepper. 1985. Fosetyl Aluminum Fungicide for Controlling Pythium Root Rot of Foliage Plants. Proc. Fla. State Hort. Soc. 98:119-122.
7. Chase, A. R. and R. T. Poole. 1985. Irrigation Frequency and Subdue® Rate Affect Pythium Root Rot of Pothos. Agricultural Research and Education Center, Univ. of Fl., AREC-Apopka Research Report RH-85-14.
8. Chase, A. R. 1986. Fungicide Trials of Foliage Plants - Summary 1986. Agricultural Research and Education Center, Univ. of Fl., AREC-Apopka Research Report RH-86-21.
9. Chase, A. R. 1988. Efficacy of Aliette 80WP in Controlling Pythium Root Rot of Begonia and Phytophthora Stem Rot of Aphelandra. Central Florida Research and Education Center, Univ. of Fl., CFREC-Apopka Research Report RH-88-5.
10. Chase A. R. 1988. New Fungicides or Fungicide Uses for Foliage Plants - 1988. Proceedings of the 4th Conference on Insect and Disease Management on Ornamentals. Society of American Florists. pp. 148-152.
11. Chase, A. R. 1988. Chemicals for Control of Pythium Root Rot - 1988. Central Florida Research and Education Center, Univ. of Fl., CFREC-Apopka Research Report RH-88-13.
12. Chase, A. R. 1989. Fungicide Trials for Cylindrocladium, Helminthosporium, Pythium and Rhizoctonia Diseases of Ornamentals - 1989. Central Florida Research and Education Center Univ. of Fl., CFREC-Apopka Research Report RH-89-15.
13. Chase, A. R. 1990. 1990 Fungicide Trials for Control of Alternaria, Helminthosporium, Phytophthora and Rhizoctonia Diseases of Ornamentals. Central Florida Research and Education Center, Univ. of Fl., CFREC-Apopka Research Report RH-90-25.
14. Chase, A. R. and T. A. Mellich. 1992. Fungicides Tests for Control of Phytophthora and Pythium Diseases of Ornamentals. Central Florida Research and Education Center, Univ. of FL., CFREC-Apopka Research Report, RH-92-9.