Avenacin Production in Creeping Bentgrass (Agrostis stolonifera) and Its Influence on the Host Range of Gaeumannomyces graminis.

Avenacinase activity has been shown to be a key factor determining the host range of Gaeumannomyces graminis on oats (Avena sativa). G. graminis var. avenae produces avenacinase, which detoxifies the oat root saponin avenacin, enabling it to infect oats. G. graminis var. tritici does not produce avenacinase and is unable to infect oats. G. graminis var. avenae is also reported to incite take-all patch on creeping bentgrass (Agrostis stolonifera). It is unknown whether creeping bentgrass produces avenacin and if the avenacin-avenacinase interaction influences G. graminis pathogenicity on creeping bentgrass. The root extracts of six creeping bentgrass cultivars were analyzed by fluorimetry, thin-layer chromatography, and high performance liquid chromatography for avenacin content. Avenacin was not detected in any creeping bentgrass cultivars, and pathogenicity assays confirmed that both G. graminis var. avenae and G. graminis var. tritici can infect creeping bentgrass and wheat (Triticum aestivum), but only G. graminis var. avenae incited disease on oats. These results are consistent with the root analyses and confirm that avenacinase activity is not required for creeping bentgrass infection by G. graminis.

Greenhouse and Field Evaluation of Biological Control of Fusarium Head Blight on Durum Wheat.

Fusarium head blight (FHB) is a devastating disease that causes extensive yield and quality losses to wheat and barley. In durum wheat, the pathogen-produced toxin deoxynivalenol (DON) is retained in semolina at ˜50%, and the causal agent of FHB, Gibberella zeae, has a strong adverse effect on pasta color. Two bacteria and two yeast strains with known efficacy against G. zeae on hexaploid wheats were produced in liquid culture and assayed on two cultivars of durum wheat in greenhouse bioassays. All antagonists reduced FHB severity on cultivar Renville, and three of the four reduced severity on cultivar Ben, with Bacillus subtilis strain AS 43.3 decreasing FHB severity by as much as 90%. In separate greenhouse bioassays, the car-bon:nitrogen ratio of the medium used to produce antagonists did not consistently influence antagonist efficacy. All antagonist/production medium combinations but one were effective in reducing disease on both durum cultivars. Of six antagonists tested at field sites, Cryptococcus sp. OH 71.4 and C. nodaensis OH 182.9 reduced disease severity by as much as 57% in Peoria, IL, while Cryptococcus sp. OH 181.1 reduced disease severity by as much as 59% in a trial at Langdon, ND. Antagonists did not influence the DON content of grain in the Peoria trial. Relative performance indices for four antagonists calculated from greenhouse and field results on the two durum cultivars demonstrated that the bioassay location, but not the cultivar of durum, influenced the relative performance of antagonists. Yeast antagonists OH 71.4, OH 181.1, and OH 182.9 appear to have the highest potential for contributing to the reduction of FHB on du-rum wheat in the field.

Selection and Evaluation of Microorganisms for Biocontrol of Fusarium Head Blight of Wheat Incited by Gibberella zeae.

Gibberella zeae incites Fusarium head blight (FHB), a devastating disease that causes extensive yield and quality losses to wheat and barley. Of over 700 microbial strains obtained from wheat anthers, 54 were able to utilize tartaric acid as a carbon source when the compound was supplied as choline bitartrate in liquid culture. Four tartaric acid-utilizing and three nonutilizing strains reduced FHB in initial tests and were selected for further assays. Antagonists were effective against three different isolates of G. zeae when single wheat florets were inoculated with pathogen and antagonist inoculum. All seven antagonists increased 100-kernel weight when applied simultaneously with G. zeae isolate Z3639 (P ≤ 0.05). Bacillus strains AS 43.3 and AS 43.4 and Cryptococcus strain OH 182.9 reduced disease severity by >77, 93, and 56%, respectively. Five antagonists increased 100-kernel weight of plants inoculated with G. zeae isolate DAOM 180378. All antagonists except one increased 100-kernel weight, and four of seven antagonists reduced disease severity (P ≤ 0.05) when tested against G. zeae isolate Fg-9-96. In spray-inoculation experiments, Bacillus strains AS 43.3 and AS 43.4 and Cryptococcus strains OH 71.4 and OH 182.9 reduced disease severity, regardless of the sequence, timing, and concentration of inoculum application (P ≤ 0.05), though 100-kernel weight did not always increase when antagonists were applied 4 h after inoculum of G. zeae. Overall, 4 of 54 isolates that utilized tartaric acid in vitro were effective against G. zeae versus only 3 of 170 isolates tested that did not utilize tartaric acid (P ≤ 0.05, χ-square test of goodness of fit), demonstrating the potential benefit of prescreening candidate antagonists of FHB for their ability to utilize tartaric acid. Biological control shows promise as part of an integrated pest management program for managing FHB.

First Report of Cucumber mosaic virus in Garlic Mustard in Ohio.

Garlic mustard (Alliaria officinalis) is a common weed species associated with woodland borders, hedge rows, and suburban gardens. Garlic mustard plants expressing foliar symptoms of leaf mosaic and vein banding were collected from Franklin and Cuyahoga counties in Ohio. Analysis of symptomatic tissue using viral-associated double-stranded RNA (dsRNA) analysis on 5% polyacrylamide gels and stained with ethidium bromide resulted in the production of a banding profile (four dsRNA bands with molecular weights of 2.6, 2.0, 1.5, and 0.7 × 106 daltons) similar to that of Cucumber mosaic virus (CMV) (1). Symptomatic tissue suspected of being infected with CMV was analyzed with an indirect enzyme-linked immunosorbent assay (iELISA) employing commercially produced antiserum (Agdia Inc.) against the common strain of CMV antiserum confirmed the presence of CMV. Nonsymptomatic tissue reacted negatively to CMV. This is the first report of CMV in garlic mustard in Ohio. Due to the extensive range of this weed and the wide host range of CMV in ornamental and food-plant species, garlic mustard could serve as an alternate host for CMV in many commercially important plant species. Reference: (1) T. J. Morris et al. Plant Mol. Biol. Rep. 1:27-30, 1983.

Cross-Polarized Magic-Angle Spinning (sup13)C Nuclear Magnetic Resonance Spectroscopic Characterization of Soil Organic Matter Relative to Culturable Bacterial Species Composition and Sustained Biological Control of Pythium Root Rot.

We report the use of a model system that examines the dynamics of biological energy availability in organic matter in a sphagnum peat potting mix critical to sustenance of microorganism-mediated biological control of pythium root rot, a soilborne plant disease caused by Pythium ultimum. The concentration of readily degradable carbohydrate in the peat, mostly present as cellulose, was characterized by cross-polarized magic-angle spinning (sup13)C nuclear magnetic resonance spectroscopy. A decrease in the carbohydrate concentration in the mix was observed during the initial 10 weeks after potting as the rate of hydrolysis of fluorescein diacetate declined below a critical threshold level required for biological control of pythium root rot. Throughout this period, total microbial biomass and activity, based on rates of [(sup14)C]acetate incorporation into phospholipids, did not change but shifts in culturable bacterial species composition occurred. Species capable of inducing biocontrol were succeeded by pleomorphic gram-positive genera and putative oligotrophs not or less effective in control. We conclude that sustained efficacy of naturally occurring biocontrol agents was limited by energy availability to this microflora within the organic matter contained in the potting mix. We propose that this critical role of organic matter may be a key factor explaining the variability in efficacy typically encountered in the control of pythium root rot with biocontrol agents.

Effect of organic matter decomposition level on bacterial species diversity and composition in relationship to pythium damping-off severity.

Rhizosphere bacteria were isolated from root tip segments of cucumber seedlings grown in a suppressive, slightly decomposed light-colored peat mix, a conducive, more decomposed dark-colored peat mix, and a suppressive dark peat mix amended with composted hardwood bark. The bacteria were identified by a gas chromatographic fatty acid methyl ester analysis. The total number of taxa recovered from a single root tip segment ranged from 9 to 18. No single taxon predominated on all root tip segments harvested from any of the mixes. The highest relative population density reached by a given taxon on any root tip segment was 45%. Hill's first and second diversity numbers, the modified Hill's ratio, and Hurlbert's rarefaction method, which were used as measures of species diversity, indicated that the organic matter decomposition level of the potting mixes did not affect bacterial species diversity. Bray-Curtis polar ordination and Dice resemblance functions, however, indicated that the organic matter decomposition level of a mix significantly influenced the composition of bacterial species in the rhizosphere. Pseudomonas spp. and other taxa capable of inducing suppression of pythium damping-off predominated in the suppressive mixes. These organisms were absent from the conducive mix, in which Arthrobacter and Bacillus spp. predominated. Although effective bacterial biocontrol agents were isolated from both the suppressive mixes and the conducive mix, the majority were isolated from the less decomposed suppressive mixes. Finally, the efficacy of strains was significantly greater in the slightly decomposed light peat mix than in the decomposed dark peat mix. Natural disease suppression within these mixes was associated with the organic matter decomposition level and the bacterial species compositions of the mixes.