JARS v38n4 - New Fungicides Offer Protection Against Phytophthora Root Rot

New Fungicides Offer Protection Against Phytophthora Root Rot
Dr. D.M. Benson

Dr. D.M. Benson is an associate professor, department of plant pathology, North Carolina State University, Raleigh.

Phytophthora root rot caused by Phytophthora cinnamomi is a serious problem for many ornamental species, including azaleas that are container, field and landscape grown.

Several approaches to controlling phytophthora have been suggested. They include using a gravel base under containers, introducing resistant cultivars into the nursery trade, using composted growing media of pine or hardwood bark and applying preventative fungicides.

Recently, two new systemic fungicides, Aliette (fosetyl A1) and Subdue (metalaxyl), have been approved for use on selected ornamental species. Research in North Carolina and elsewhere during the past several years has demonstrated the effectiveness of these two fungicides for controlling phytophthora diseases.

These systemic fungicides offer enhanced protection against phytophthora because they are acquired by the root system. Most studies with Aliette and Subdue demonstrate that these fungicides prevent root infection by phytophthora, and thus application must be made prior to the onset of the disease.

In a recent test of Aliette and Subdue for control of phytophthora root rot on container grown evergreen azaleas, a granular formulation of these fungicides was compared with other formulations. The effectiveness of Aliette and Subdue was compared with Truban (Ethazol) longstanding, nonsystemic fungicide used for phytophthora root rot control. Ten month old 'Hinodegiri' azaleas, growing in 3 inch pots, were transplanted to a medium of three parts pine bark and one part sand by volume in 1 gallon containers.

Nursery Testing
Plants were set out in a container nursery on the campus of North Carolina State University on May 26, 1983. Plants were top dressed with 1 teaspoon each of a sulphur-coated fertilizer (16-9-12) and a micronutrient fertilizer. Every two weeks, additional nitrogen, phosphorus and potassium was applied in a 21-7-7 solution at a rate of 2.6 grams per liter.

The azaleas were drenched (250 milliliters per plant) or top dressed with the granular fungicide on June 3. Seven days later, the plants were inoculated with oat grains infested with Phytophthora cinnamomi.

Irrigation from an overhead sprinkler (0.9 centimeters per day) supplemented rainfall, which was below normal in North Carolina during July and August. A randomized complete block design with eight replications per treatment was used. It included control plants left untreated with fungicide and either inoculated or not inoculated. Aliette and Truban were reapplied on July 5, Aug. 1 and Aug. 31 for a total of four applications. A second application of Subdue was made on July 29. Data for top weight and root rot rating were collected Sept. 29.

Distinct differences in the plants began to be noted about mid July, when symptoms of phytophthora developed on inoculated plants not treated with fungicide. In addition to generally low vigor, symptoms included yellowing, wilting and plant stunting.

In general, the granular formulations of the three fungicides were no more or less effective in controlling phytophthora root rot than the other formulations of the same fungicide. Aliette and Subdue provided the most effective control, although top weight and root rot ratings with Truban were not always significantly different.

Root rot ratings and top weights of plants to which Aliette, Subdue and Truban 30WP had been applied were significantly better than for plants in the untreated, inoculated group.

Acceptance of these new fungicides should provide growers with increased profits as crop losses due to phytophthora root rot are reduced.

Comparison of fungicide formulations for control of phytophthora root rot
Treatment Application Rate Top
rot rating 1
Aliette 80WP D 9.7 oz. / 100 gal. 59.1 ab 1.9 cd
Aliette 10G TD 1.5 gal. / 0.19 sq. ft. 67.9 a 1.1 d
Subdue 2EC D 2.0 oz. / 100 gal. 60.5 ab 1.9 cd
Subdue 5G TD 0.115 gal. / 0.19sq.ft. 62.4 ab 1.8 cd
Truban 30WP D 10.0 oz. / 100 gal. 59.8 ab 2.2 bed
Truban 5G TD 2.3 gal. / 0.19 sq. ft. 36.4 be 3.0 ab
Control 1 22.0 c 3.3 a
Control 2 70.4 a 1.6 cd
1 1 = healthy roots, 2 = fine roots necrotic, 3 = coarse roots necrotic, 4 = crown rot and 5 = dead plant.