5.2.13
A NOVEL RANKING METHOD AND EVALUATION OF THE COMMERCIAL DEVELOPMENT POTENTIAL OF BACTERIAL ANTAGONISTS OF FUSARIUM DRY ROT

DA SCHISLER1, PJ SLININGER1, KD BURKHEAD1, G KLEINKOPF2, LE HANSON3, RJ BOTHAST1 and RC OSTROWSKI4

1USDA-ARS, National Center for Agricultural Utilization Research, Peoria, IL 61604, USA; 2University of Idaho, Kimberly Research and Extension Center, Kimberly, ID 83341, USA; 3Department of Plant Pathology, Cornell University, Ithaca, NY 14853, USA; 4United Agri Products, Greeley, CO 80632, USA

Background and objectives
Fusarium sambucinum (teleomorph Gibberella pulicaris) is one of the primary causal agents of dry rot of potato tubers. Lack of effective chemicals impedes disease control on tubers destined for processed and table stock use. When the microbiota from 29 agricultural soils were individually transferred to separate samples of a gamma irradiation-sterilized field soil enriched with potato periderm, samples differed in biological suppressiveness to Fusarium dry rot. 18 Gram-negative bacteria isolated from suppressive samples biologically controlled dry rot incited by F. sambucinum. Strategies and organisms for biologically controlling Fusarium dry rot continue to be developed at NCAUR, Peoria. We have demonstrated that both the growth kinetics and efficacy of microbial antagonist strains produced in liquid culture should be appraised to determine their commercial development potential [1]. We discovered specific pairs of microbial antagonists that reduced dry rot disease by 70% against controls while using 1/100 the dose required to give similar control using a single antagonist. Antagonist dose/disease response relationships have been determined for superior strains [2]. Antibiotic production by selected antagonists was characterized and demonstrated in situ [3]. Objectives of the current studies were (i) to evaluate, in a 3-year study, the efficacy of superior antagonists under commercial field storage conditions; and (ii) to determine the influence of multiple pathogen strains and different antagonist production media on the relative performance ranking of superior biological control strains.

Results and conclusions
(i) Several Gram-negative bacterial strains with high commercial development potential were selected for pilot studies at the UIKREC, Kimberly, and for bin trials at storage houses in Idaho and North Dakota. Fluorescent Pseudomonas strain S22:T:04 (1x108 c.f.u./ml) decreased the level of dry rot in year 1 trials when co-inoculated with the pathogen compared to controls and the fungicide Mertect 340F. In second-year studies, P. fluorescens strain P22:Y:05 and Enterobacter cloacae strain S11:T:07 (4x108 c.f.u./ml) controlled F. sambucinum (25 and 17% average disease decrease, respectively) but not F. coeruleum when antagonists were applied 24 h after pathogen inoculation. E. cloacae S11:T:07, produced in liquid culture, reduced naturally occurring levels of dry rot by an average of 21% for all year 3 bin trials compared to 14% for Mertect 340F.
(ii) 10 strains of F. sambucinum were isolated from potato tubers collected in the north-eastern USA. Five antagonists, produced in a semi-defined liquid medium representative of commercially utilized media, decreased disease incited by all F. sambucinum strains by 34%. However, antagonists frequently were more effective when produced on 1/5 tryptic soy broth agar (TSBA/5) (43% disease decrease). In separate experiments to compare antagonists produced on nutritionally identical media in liquid and solid form, antagonists were highly effective against all pathogen strains, though antagonists produced in TSB/5 were less efficacious than those produced on TSBA/5 (87 against 92% disease decrease, respectively). Rankings of antagonists' effectiveness changed with the production media and the pathogen strain. Selecting antagonists using the criteria of efficacy against a range of pathogen strains and antagonist amenability to production in liquid culture increases the likelihood of choosing strains with commercial development potential.

References
1. Slininger PJ, Schisler DA, Bothast RJ, 1994. Improving Plant Productivity with Rhizosphere Bacteria, 3rd International Workshop on PGPR, Adelaide, South Australia, pp. 29-32
2. Schisler DA, Slininger PJ, Bothast RJ, 1997. Phytopathology 87, 177-183.
3. Burkhead KD, Schisler DA, Slininger PJ, 1995. Soil Biology and Biochemistry 27, 1611-1616.