Fusarium Species Associated with Tall Fescue Seed Production in Oregon

Seed samples were collected from 15 commercial tall fescue seed production fields and examined for Fusarium spp. The percentage of seeds from which Fusarium spp. were recovered ranged from 0 to 32%, while disinfesting seeds with 3% hydrogen peroxide reduced the recovery of Fusarium to 7% or less. The predominant Fusarium spp. isolated from the tall fescue seeds included F. avenaceum, F. culmorum, F. pseudograminearum, and F. sambucinum. Greenhouse inoculations of tall fescue panicles with F. avenaceum, F. culmorum, and F. pseudograminearum resulted in higher seedborne rates of each respective Fusarium sp. than that recovered from noninoculated plants. Seeds recovered from panicles treated with F. avenaceum or F. pseudograminearum had significantly lower germination rates relative to panicles sprayed with water or a suspension of F. culmorum. Our work confirms that Fusarium spp. decrease seed germination and expands the pathogen list to include F. avenaceum and F. pseudograminearum. Introduction Tall fescue (Festuca arundinacea Schreb.) seed production is an important industry in Oregon. In 2002, 65,862 ha of tall fescue in Oregon yielded 114 million kg of seed, valued at $86 million (11). Nearly all the hectares are planted as certified seed, requiring a high level of purity and germination. In 1998, the Oregon State University (OSU) Seed Laboratory noted reduced germination, below the 90% minimum germination requirement for seed certification, in tall fescue seed samples submitted for germination determination as required for seed certification. Fusarium sporodochia were observed on some seeds, and an association of Fusarium with reduced germination in tall fescue was suspected (S. C. Alderman, unpublished data). Fusarium heterosporum (4,9) and F. culmorum (5) have been previously reported on tall fescue seeds and roots. The impact of Fusarium spp. on viability of tall fescue seeds has not been established, nor has it been shown whether infection by a Fusarium sp. during flowering will lead to increased seedborne frequency of that species. Germination of grass seed is typically evaluated under 15/25°C night/day temperatures but in some cases an elevated temperature regime of 20/30°C night/day is used to shorten the duration of the germination test (2). It is unknown if either temperature regime favors Fusarium seed or seedling infection, but the 20/30°C regime was discontinued at the OSU Seed Laboratory due to observations of greater fungal infections of seeds during germination under the warmer temperature regime. The objectives of this research were to: (i) recover and identify species of Fusarium from commercial tall fescue seed produced in Oregon; (ii) determine if Fusarium infections can be controlled by surface disinfestation; (iii) determine if germination is decreased under an elevated incubation temperature; and (iv) evaluate the potential of Fusarium spp. as seed pathogens when introduced during flowering of tall fescue. Plant Health Progress 19 March 2004 Evaluation of Tall Fescue Seeds Produced in Commercial Fields During 1998, 15 25-g samples of tall fescue seed samples with reduced germination rates were obtained from the OSU Seed Laboratory. Each seed sample represented a separate field. From each sample, 50 nontreated seeds were picked at random and embedded into solidified Nash medium (7) supplemented with aureomycin (6). Plates were incubated at 24°C with only ambient light for 21 days. An additional set of 50 seeds from each sample was disinfested with 3% hydrogen peroxide for 60 min, triple-rinsed with sterile, distilled water, and cultured as above. Putative Fusarium colonies were transferred from the Nash medium to potato dextrose agar (PDA) and carnation leaf agar (CLA) (8). Cultures were incubated under fluorescent lamps supplemented with black light in a 12-h photoperiod at 24°C (8). Each putative Fusarium colony was identified to species according to the system of Nelson et al. (8), with the exception of F. pseudograminearum which was formerly known as F. graminearum Group I and lacks homothallic perithecial production (1). The experiment was repeated. Means were calculated and a paired t-test was used to compare disinfested to nontreated seed (SAS 8.02, Cary, NC). A subset of isolates was purified by the single-spore method and stored on silica gel at 5° C (10). Since the recovery of Fusarium spp. was similar between experimental replicates, data from experimental replicates were combined. The percentage of seed from which Fusarium spp. were recovered ranged from 0 to 32% (Table 1). Disinfestation with hydrogen peroxide reduced recovery of Fusarium from 0 to 32% to 0 to 7%. The predominant Fusarium sp. recovered from nontreated seeds was F. avenaceum (Table 2). Fusarium sambucinum, F. pseudograminearum, and F. culmorum also were commonly recovered from nontreated seeds; F. oxysporum, F. equiseti, F. semitectum, F. lateritium, and F. proliferatum were rarely isolated. Fusarium avenaceum and F. sambucinum were the predominant species recovered from disinfested seeds (Table 2). Since seed disinfestation significantly reduced the frequency of seedborne Fusarium spp. recovered, the majority of Fusarium spp. was likely present on seeds as surface contaminates or as superficial infections. Plant Health Progress 19 March 2004 Table 1. Germination percentage of tall fescue seeds and percentage that yielded Fusarium spp. a OSU Seed Certification Laboratory record number. b Determined by the OSU Seed Certification Laboratory. c Based on two replicates of 50 seeds per sample in each experiment. d Seeds were disinfested in 3% hydrogen peroxide for 60 minutes. Means of H2O2-treated seeds that are significantly different from nontreated seeds are labeled, according to paired t-test, with: for P = 0.005; ** for P = 0.01; and * for P = 0.05. Seed sample no.a Cultivar Germinationb Percent nontreated seeds that yielded Fusarium spp.c Percent H2O2-treated d seeds that yielded Fusarium spp. 163887 RG-93 (Bravo) 81 32 4 165016 Leprechaun 86 32 5 163222 El Dorado 89 30 3 163827 RG-93 (Bravo) 82 29 2 164654 Bonzai 82 29 3 162769 Titan 91 28 3 163891 RG-93 (Bravo) 78 22 7 162888 Titan 89 11 1 162687 Orygun 87 8 0 ** 163204 LRF-983 84 6 1 * 163226 Rebel III 86 5 1 162937 Avalon 82 3