Department of Plant Pathology
Department of Horticulture and Landscape Architecture, Oklahoma State University, Stillwater, OK 74078
Research involving biological control of soilborne plant pathogens has received much attention during the past ten years. Most current research utilizes the genus Trichoderma (form-class Deuteromycetes), which is a common soil-inhabiting saprophyte.
In 1972, Wells et al. (1) found Trichoderma harzianum. Rifai present in sclerotia of Sclerotium rolfsii Sacc., and used this isolate to reduce damage of S. rolfsii in tomatoes under greenhouse and field conditions. Others isolated strains of T. harzianum from soil which were antagonistic to S. rolfsii and Rhizoctonia solani Kuhn (2,3,4,5).
Although Trichoderma harzianum is a potential biological control agent under experimental conditions, there are problems associated with its commercial use. A variety of carriers have been developed for delivering the biocontrol agent to the soil (1,5,6), but all are commercially impractical because of the excessively large amounts of carrier required for application. However, recent findings by Harman et al. (7,8) indicated that using T. hamatum (Bon.) Bain as a seed treatment may eliminate this problem. They also found that T. hamatum was a superior antagonist compared to T. harzianum for control of disease caused by Pythium spp. and R. solani.
Another area of investigation is the application of fluid drilling or gel seeding technology to biological control problems. This technique involves the addition of germinated seeds to a gel carrier and subsequent sowing into the soil. The major advantage of sowing germinated seed compared to dry seed is earlier and more uniform emergence (9). The gel protects the exposed radicle from mechanical damage and also provides the growing seedling with an initial water source. Unfortunately, the gel tends to attract microorganisms, including soilborne pathogens which may result in an increased incidence of disease. Conway et al. have used fungicides as adjuvants to the gel matrix to decrease damping-off disease caused by R. solani in chili peppers (10). Fluid drilling offers an ideal system for delivery of a biocontrol agent such as Trichoderma for control of soilborne disease problems.
Trichoderma isolates which were antagonistic to R. solani were identified according to Dennis and Webster (11). Typical reactions involved the coiling of Trichoderma hyphae around the R. solani mycelium.
Initial studies indicated that there are no significant increases or decreases in the viable populations of the fungus in gel over a period of 14 days (Conway, unpublished data). Therefore, preliminary field studies were conducted to determine survivability of fluid-drilled Trichoderma in the soil. Experimental plots were located in four field sites in two locations in the state, Bixby and Stillwater. At each field site, eight treatments were evaluated using a randomized complete block design with four replicates. Along with germinated hot chili pepper seed, two different species, T. hamatum and T. harzianum, were incorporated into the gel at two rates, 1 × 105 conidia/ml and 1 × 107 conidia/ml. Additional treatments consisted of growing the Trichoderma on oats and incorporating the oats into the soil at time of planting, a fungicide gel
treatment, and a gel control with no Trichoderma spp. added. Soil samples were taken from the planting rows at a depth of 10 cm., three to four weeks after planting. Trichoderma counts were made from soil dilution plating on Trichoderma-selective medium (12). Duncan's multiple range test at the 5% level of significance was used to compare treatments. Significantly higher numbers of Trichoderma propagules were detected in soil samples taken from treatments which utilized Trichoderma on oats and Trichoderma incorporated into the gel at 1 × 107 conidia/ml (Table 1). An exception to this was the results from the T. hamatum treatments at Stillwater, where there was no significant difference among treatments. Other gel seeding studies measuring Trichoderma survivability in soil (Conway, unpublished observations) have shown similar variations. These differences could be due to the resident populations of microorganisms antagonistic to Trichoderma or to varying soil environment.
These preliminary studies have shown some promise that Trichoderma can be incorporated into a fluid drilling system, and remain viable in the gel if planting is delayed. Additionally, high populations of Trichoderma can be maintained through the period in which seedlings are most susceptible to damping-off.
Environmental chamber and additional field studies are in progress to further evaluate the incorporation of Trichoderma into a fluid drilling system, for reduction of damping-off losses due to soilborne pathogens.
The authors thank Bryan Brown, David Ferris, Zohreh Zoofan and Nancy Stacey for their technical assistance.
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