Effect of Six Fungicides, Applied as Soil Drenches, on Mycorrhizae
of Two Nursery-Grown Rhododendron Cultivars
S. Moore-Parkhurst, Graduate Research Assistant, and L. Englander, Associate Professor
Department of Plant Pathology-Entomology, University of Rhode Island, Kingston, RI
Contribution #2074 of the Rhode Island Agricultural Experiment Station
supported in part by an assistantship grant by the Rhode Island Nurserymen's Association.
Nurserymen often use fungicides in the commercial production of Rhododendrons to reduce losses from various stem and root rots. However, some fungicides have been reported to affect other members of the soil's population in addition to the fungal pathogens they were developed to control. We were particularly concerned about the possible inhibitory influence of these fungicides on beneficial mycorrhizal fungi. (For more background information on the subject of mycorrhizae consult the following reference: Englander, L. 1980. Mycorrhizae and Rhododendron. Quart. Bull. Am. Rhod. Soc. 34(1): 8-12.)
From previous research we had conducted we knew that nursery-grown Rhododendrons in Rhode Island were mycorrhizal and that these mycorrhizae may give plants an advantage when they are planted out in the often harsh environment provided by the average homeowner.
To test the possible effects of fungicides on mycorrhizae in Rhododendrons we applied fungicides as soil drenches to rooting cuttings of Nova Zembla in the greenhouse and to field-grown plants of Francesca. Cuttings were drenched immediately after sticking and monthly thereafter with Subdue, Truban and an experimental compound, SN 66752. After approximately 11 months root samples were taken from the treated plants to examine them for mycorrhizae. In field tests the fungicides Subdue, Truban, SN 66752, Terrachlor, Benlate, and Banrot were applied to test plots of undisturbed and newly regenerated root systems once each month during June, July and August for 2 consecutive growing seasons. Roots were examined for mycorrhizae at the end of the second season. Control plants in both the rooting and field tests were drenched with an equivalent amount of water.
Samples were carefully washed and stained to determine the presence of fungal coils in the root cells which are indicative of Rhododendron mycorrhizae.
With the exception of Truban, none of the fungicide treatments on cuttings were significantly different in percent of mycorrhizal roots than the control. Truban, however, produced a phytotoxic response resulting in restricted root system development, similar to that observed by other workers, hindering comparison of mycorrhizal development with other treatments.
In the field test, roots from all treatments were mycorrhizal. The evaluation of the degree of mycorrhizal infection in each root sample showed no significant differences in the fungicide-drenched test plots among either undisturbed roots or with regenerated root systems.
Results of this study indicate that most of the fungicides evaluated could be used to control soil borne pathogens with minimal effects on development of mycorrhizae in Rhododendron spp. Of the 6 fungicides tested, only Truban had a significantly deleterious effect on mycorrhizae, and this occurred only in the propagation phase. We applied Truban at a rate and frequency greater than recommended by the manufacturer, yet comparable to applications commonly used commercially for Phytophthora prevention or control. A lower dose or frequency of application possibly would not limit root or mycorrhizal development. Truban apparently had no phytotoxic effects on field-grown plants. Other research has shown that rooting cuttings exhibit a phytotoxic response to Truban, while 2 year-old plants were not affected.
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