Fungicide Tests for Control of Phytophthora and Pythium Diseases on Ornamentals

University of Florida, IFAS,
Central Florida Research and Education Center - Apopka
CFREC-Apopka Research Report, RH-92-9

Fungicides have continued to be an important element of the disease control techniques used by many ornamental producers. A quaternary ammonium compound (Greenshield from Whitmire) and a numbered compound from ISK Biotech (ASC-66825, fluazinam) are two products with unknown activity ranges on diseases of ornamental crops. In addition, tests included metalaxyl (Subdue 2E) and fosetyl aluminum (Aliette 80WP) to provide standard controls as well as provide information for label expansion. Some of the most important diseases caused by soilborne fungi are root and stem rots caused by Phytophthora and Pythium species. The specific diseases which were researched during the past year were: Phytophthora stem rot of zebra plant, Phytophthora stem rot and aerial blight of vinca, and Pythium root rot of schefflera.

Phytophthora stem rot of zebra plant (Aphelandra squarrosa) is caused by Phytophthora parasitica which is a pathogen of many flowering, foliage, and bedding plants. Treatments included noninoculated and inoculated controls, metalaxyl (Subdue 2E at 3.5 oz per 100 gallons), ASC-66825 (fluazinam from ISK Biotech at 4 and 16 oz per 100 gallons), and a quaternary ammonium compound (Greenshield at 1, 2, and 4 oz per 100 gallons). Subdue was applied as a drench on 6 December and ASC-66825 treatments were applied as drenches on 6 and 19 December 1991. Greenshield treatments were sprayed onto the plant base on a daily schedule from 3 to 19 December 1991. Plants were inoculated with Phytophthora parasitica (87-10) on 9 December and the degree of stem rot and plant collapse was evaluated on 20 December 1991. Disease severity was rated on the following scale: 1 = no disease; 2 = slight wilt or rot of one leaf; 3 = moderate wilt or rot of 2 to 4 leaves; 4 = severe wilt; and 5 = dead. Subdue and ASC-66825 drenches gave good control of Phytophthora stem rot on zebra plant (Fig. 1). Applications of Greenshield on inoculated plants gave more severe disease symptoms than the inoculated control plants and grew more severe as the rate of Greenshield increased. In a separate test, healthy (noninoculated) zebra plants were treated with the same rates of Greenshield and developed no symptoms of disease or phytotoxicity. These results indicate that Greenshield increased severity of Phytophthora stem rot on zebra plant.

Phytophthora stem rot of vinca (Catharanthus roseus) is also caused by Phytophthora parasitica. Treatments included noninoculated and inoculated controls, fosetyl aluminum (Aliette 80WP at 16 and 32 oz per 100 gallons), metalaxyl (Subdue 2E at 2 oz per 100 gallons), and ASC-66825 (4 and 16 oz per 100 gallons). Subdue was applied as a drench on 17 October and other treatments were applied as drenches on 17 and 31 October 1991. Plants were inoculated with Phytophthora parasitica (91-37A) on 21 October and the percentage of the plant showing wilt and stem rot was evaluated on 4 November 1991. A11 of the fungicide drenches gave excellent control of this disease on vinca (Fig. 2). No evidence of phytotoxicity was noted.

Pythium root rot of schefflera (Brassaia actinophylla) is caused by Pythium splendens which is the most common species of Pythium infecting foliage crops. Two tests were completed with this disease of schefflera. Treatments in the first test included noninoculated and inoculated controls, metalaxyl on both noninoculated and inoculated plants (Subdue 2E at 2 oz per 100 gallons), and ASC-66825 (4 and 16 oz per 100 gallons). All treatments were applied as drenches on 20 December 1991 and 17 January 1992. Plants were inoculated with Pythium splendens (461) on 23 December 1991. Plant height, percentage of the potting medium with healthy appearing roots and fresh weight of tops were recorded on 31 January 1992. Plant height was significantly lower for inoculated controls and the two ASC-66825 treatments. The ASC-66825 applications did not result in any disease control. Excellent disease control was achieved with the Subdue drench for both roots and top growth (Fig. 3). Applications of Subdue to noninoculated plants did not result in growth improvement over the noninoculated water controls indicating that these plants did not have any measurable degree of root infection with pythiaceous fungi when we obtained them for the test. No evidence of phytotoxicity was noted.

The second test employed different fungicides as well as different application methods. Four spray treatments with Aliette 80WP and Aliette 80WG (each at 2.5 and 5.0 lb per 100 gallons) were included. Aliette 80WG was also applied as a drench at 2.5 and 5.0 lb per 100 gallons alone and in combination with iprodione (Chipco 26019 50WDG) as drenches. Combination treatments were Aliette 80WG and Chipco 26019 (2.5 and 1.0 lb per 100 gallons, and 0.4 and 0.5 lb per 100 gallons, respectively). Noninoculated and inoculated controls and a metalaxyl drench (Subdue 2E at 2 oz per 100 gallons) were also included. Treatments were applied on 6 February and 4 March 1992 and plants were inoculated with Pythium splendens (461) on 10 February 1992. Plant height, percentage of the potting medium with healthy appearing roots and fresh weight of tops were recorded on 17 or 18 March 1992. Plant height was not significantly affected by treatment in this test (Table 1). Best roots were produced on plants treated with the lower drench rate of Aliette 80WG or the Subdue. Other drench applications of either formulation of Aliette with or without Chipco 26019 gave good disease control as well. Spray applications did not provide good disease control (Table 1). Additionally, examination of the roots demonstrated that the plants had become naturally infected with a pythiaceous fungus prior to use in this test (note low rating for noninoculated-controls).

The best top growth was found on plants treated with the Subdue drench (Table 1). Good growth also occurred on plants treated with the high drench rate of Aliette 80WG and the combination drench of Aliette 80WG and Chipco 26019. Other treatments gave a lesser degree of control with the spray treatments providing less control than the drench treatments. No evidence of phytotoxicity was noted.

These tests indicate that Subdue remains one of the best fungicides for controlling both Phytophthora and Pythium diseases on ornamentals. In addition, Aliette provides good control of Pythium root rot on schefflera when used as a drench but not as a spray. The use of Greenshield on zebra plants inoculated with Phytophthora resulted in worse disease than if they had not been sprayed. Always be careful when trying new products on your crops since this type of problem can occur even when overt symptoms of phytotoxicity are absent. The numbered compound ASC-66825 has proven to have activity against a stem rot caused by Phytophthora but not Pythium root rot. Further testing with this product is required before its true range of activity is known.

*Professor of Plant Pathology and Biological Scientist, Central Florida Research and Education Center - Apopka, 2807 Binion Road, Apopka, FL 32703-8504.

Table 1. Effect of Aliette 80WG, Subdue 2E, Chipco 26019 50WDG, and Aliette 80WP on height and root and top growth of schefflera (Brassaia actinophylla) infected with Pythium splendens.

Treatment^a	Rate/100 gallons	Height (in) 17 Mar	Percent roots 18 Mar	Top weight (g) 18 Mar
Noninoculated	---	7.5 ab	35.5 abc	34.4 abc
Inoculated	---	7.5 a	34.5 abc	34.6 abc
Aliette 80WG spray	2.5 lb	7.0 a	56.0 abcd	37.9 abc
Aliette 80WG spray	5.0 lb	8.0 a	63.5 cd	40.7 abc
Subdue 2E	2 oz	8.7 a	72.0 d	55.0 d
Chipco 26019 50WDG	1.0 lb	7.7 a	32.0 ab	31.5 a
Aliette 80WG	2.5 lb
Chipco 26019 50WDG	0.5 lb	8.6 a	61.5 bcd	44.0 bc
Aliette 80WG	0.4 lb
Aliette 80WP spray	2.5 lb	7.6 a	47.5 abcd	31.7 a
Aliette 80WP spray	5.0 lb	7.7 a	29.0 a	32.7 ab
Aliette 80WP spray	0.4 lb	8.3 a	73.4 d	43.1 abc
Aliette 80WG	0.8 lb	8.2 a	59.0 bcd	45.2 cd

^aSpray treatments were applied to the point of drip on 6 February and 4 March 1992. All other treatments were applied as a drench at about I pint/ft2 of pot surface area on the same dates.
^bNumbers in the same column followed by different letters were significantly different using Duncan's New Multiple Range Test, P = 0.05.

Figure 1. Effect of Subdue 2E, ASC-66825, and Greenshield on severity of Phytophthora stem rot of zebra plant (Aphelandra squarrosa) caused by Phytophthora parasitica.

Figure 2. Effect of Aliette 80WP, Subdue 2E, and ASC-66825 on Phytophthora stem rot and aerial blight of vinca (Catharanthus roseus) caused by Phytophthora parasitica.

Figure 3. Effect of Subdue 2E and ASC-66825 on root and top growth of schefflera (Brassaia actinophylla) infected with Pythium splendens.