Gary Y. Yuen

Evaluation of Stenotrophomonas maltophilia strain C3 for biocontrol of brown patch disease

Abstract Bacterial strains isolated from grass foliage were tested for inhibition of brown patch disease, caused by Rhizoctonia solani Kuhn, on detached blades and potted seedlings of tall fescue ( Festuca arundinacea Schreb. cv. Kentucky 31). Stenotrophomonas maltophilia strain C3 prevented growth of the fungus on leaf blades and reduced the severity of necrosis on seedlings. When strain C3 was tested on tall fescue cv. ‘Kentucky 31’ planted at three seeding densities, biological control was not achieved. When the strain was applied to field plots of six tall fescue cultivars, the severity of brown patch disease was reduced in one cultivar, but increased in another. A rifampicin-resistant mutant strain of C3 applied to four cultivars was recovered from all of the canopies at approximately 10 6 CFU/g foliage throughout the experiment. This population level is over 10-fold lower than that associated with effective antagonism under laboratory conditions.

Bean rust biological control using bacterial agents

Abstract Over 120 bacterial strains were evaluated in a greenhouse for control of bean rust caused by Uromyces appendiculatus . The strains, found previously to be antagonistic to some fungal pathogens, were isolated from dry edible bean ( Phaseolus vulgaris ) and other hosts. Only Pantoea agglomerans B1, from a bean blossom, and Stenotrophomonas maltophilia C3, a chitinolytic strain from a Kentucky bluegrass leaf, were effective in multiple experiments in reducing bean rust severity. The addition of colloidal chitin to C3 cell suspensions and treatment with chitin broth cultures of C3 were evaluated as methods to improve biocontrol efficacy of C3. While chitin amendments increased rust control in the greenhouse as compared to C3 cells in buffer, chitin broth cultures gave the highest- and longest-lasting level of control. In four field experiments, treatments with C3 suspended in buffer, with and without chitin amendment, reduced rust severity in only one experiment. Strain B1 was not effective. In three other field experiments, C3 chitin broth cultures were comparable to multiple applications of thiophanate methyl or thiophanate methyl combined with manganese ethylenebisdithiocarbamate (maneb) in reducing bean rust severity.

The microclimate in tall fescue turf as affected by canopy density and its influence on brown patch disease

Microenvironment was monitored within tall fescue (Festuca arundinacea) experiment plots in which the severity of brown patch disease, caused by Rhizoctonia solani AG-1-IA, was found to correlate with canopy density. Canopy density was varied either by planting cultivars that produced different densities, or by planting a single cultivar at three seeding rates. Air and foliage temperatures within the canopy differed by only approximately 1°C between low- and high-density canopies during 1993 and 1994 in both studies. In a wet year, 1993, leaf wetness and relative humidity did not differ significantly between low- and high-density canopies. In 1994, which was more typical in regards to weather, a denser canopy promoted leaf wetness and high relative humidity in both studies. Leaf wetness duration averaged over 10 days was 0.8 h longer in the high-density cultivar Arriba than in the low-density cultivar Fawn. In addition, the period of relative humidity above 90% was 2.3 h longer in Arriba than in Fawn. Canopies of tall fescue with different plant densities were inoculated in the laboratory with R. solani and placed under uniformly high humidity and temperature. Hyphae grew between leaf blades separated by up to 8 mm. Interblade hyphal growth occurred more frequently in high-density canopies because of the closer proximity of leaf blades, and as a result, mycelia and necrosis spread more rapidly from inoculation sites in high-density canopies. It was concluded that microenvironmental conditions and the physical proximity of leaf blades in high-density turfs can be more favorable for brown patch disease.

Biological control of Rhizoctonia solani on tall fescue using fungal antagonists

A binucleate Rhizoctonia sp. (isolate GM 460) from soil and Gliocladium virens (isolate TRBG) from creeping bentgrass consistently inhibited Rhizoctonia blight on tall fescue in laboratory bioassays. When R. solani was inoculated onto tall fescue seedlings treated with either antagonist, 20-60% of the foliage was blighted after 10 days, whereas 80-100% of the foliage was affected in the controls. Suppression of blight was associated with reduced growth of R. solani on grass blades. GM 460 and TRBG differed in their tolerance of decreasing relative humidity. At 100% relative humidity, TRBG grew on grass blades to a greater extent than GM 460; at 95% relative humidity, GM 460 grew on grass blades but TRBG did not

A site-specific sensor for measuring leaf wetness duration within turfgrass canopies

Leaf wetness is a key factor in the initiation and development of many plant diseases. Measurement of leaf wetness duration in turfgrass requires sensors that do not disrupt the canopy environment and are representative of leaf surfaces. Therefore, a small (5 mm head width) sensor was developed that can be inserted into grass blades with only minor modification of the canopy and can capture the spatial variability of leaf wetness. The sensor detected wetness onset and depletion in a tall fescue turf canopy in the field with an average error of less than 20 min when compared with a tactile method. The sensor indicated leaf wetness over 1 h before moisture was visible and detected wetness depletion 1 h after moisture was no longer visible, suggesting that the sensor is sensitive to microscopic water films. When the sensor was compared with a leaf latent heat flux. model in tall fescue canopies, no significant differences between the two methods were found in detecting wetting and drying episodes and in measuring leaf wetness durations. However, the sensor was sufficiently sensitive to record 1 h later wetness depletion (P < 0.01) and 1 h longer periods of leaf wetness (P < 0.01) in a high plant density canopy as compared with a low-density canopy, whereas no significant differences in moisture conditions between the two canopies was found using the leaf latent heat flux model.

Tall fescue canopy density effects on brown patch disease

Canopy density was investigated as a factor determining the severity of brown patch disease, caused by Rhizoctonia solani, in the field. In two separate experiments, tall fescue (Festuca arundinacea) cv. Fawn was seeded at 10 to 50 g/m 2 to create canopies with different levels of blade density (i.e., numbers of leaf blades per unit area) and verdure. Brown patch lesion development was more severe at the 50-g/m 2 seeding rate than at 10 g/m 2 in both experiments. Six cultivars of tall fescue, representing combinations of susceptibility (as determined in a growth chamber) and stature (tall, medium, or dwarf), were evaluated in the field for brown patch disease severity and canopy density. Disease severity measured over 2 years was highly correlated with blade density and verdure, but was not related to cultivar susceptibility. The tall cultivars produced the least dense canopies and sustained the least amount of disease. The medium and dwarf groups, however, could not be distinguished on the basis of canopy density or response to brown patch disease. These results show that canopy density directly affects brown patch disease severity under field conditions and is, in part, related to a cultivars stature.

Influence of canopy structure on tall fescue cultivar susceptibility to brown patch disease

Tall fescue (Festuca aurundinacea) cultivars were evaluated under growth chamber and field conditions for susceptibility to brown patch disease caused by Rhizoctonia solani. Percentage foliar necrosis varied among cultivars in growth chamber experiments. Kentucky-31 was the least susceptible cultivar (cv.), sustaining <25% necrosis 6 days after inoculation with the pathogen. Severity of necrosis in other cvs under the same conditions was as high as 88%. Brown patch disease development in the field was lowest in cv. Kentucky-31 among nine cvs evaluated over two years. Average blight severity in cv. Kentucky-31 in 1991 was 34%, compared with 60% in the most susceptible cv., Trailblazer. Similarly in 1992, cvs Kentucky-31 and Trailblazer had the lowest and highest levels, respectively, of brown patch lesion development. Leaf blade density and verdure in the field varied among the cvs. Blade density and verdure correlated (r = 0.78 and 0.68, respectively; p < 0.05) with the extent of lesions measured in 1992, but were not related to mean blight severity measured in 1991. Field disease levels observed in both years had low correlations (r = 0.59; p = 0.10) with levels of necrosis measured in the growth chamber.

Use of ELISA and isolation for determining the distribution of Rhizoctonia solani and other Rhizoctonia spp. in asymptomatic creeping bentgrass

Abstract A commercial Rhizoctonia -specific ELISA was compared with isolation on antibiotic-amended water agar for the detection of Rhizoctonia solani and other Rhizoctonia spp. in asymptomatic creeping bentgrass near active brown patch disease centres. A greater number of positive samples were indicated by ELISA than by isolation in tests of foliage and root tissues. Over 75% agreement was found between ELISA and isolation with foliage, but the presence of Rhizoctonia spp. in root tissues could not be confirmed by isolation from any of the ELISA-positive samples. There was poor agreement between the two methods in assays of organic matter from thatch. Greater numbers of thatch samples were positive for Rhizoctonia spp. by isolation than by ELISA. Pathogenic R. solani was found by both methods in asymptomatic foliage as far as 30 cm from the margins of brown patch disease areas. There was a more restricted distribution of the fungus in the thatch. Detection of the fungus in root tissues from asymptomatic areas by ELISA could not be confirmed by isolation.