Submitted to: International Conference on the Biology of the Myxobacteria
Publication Type: Proceedings
Publication Acceptance Date: July 15, 2005
Publication Date: July 15, 2005
Citation: Bull, C.T. 2005. Biological control of lettuce drop caused by Sclerotinia minor by Myxobacteria. Proceedings of the 32nd International Conference on the Biology of the Myxobacteria, July 2005, Harrison Hot Spring, British Columbia, Canada. p. 5. Technical Abstract: Myxobacteria are the third most prodigious producers of antibiotics and biocides but they have been virtually ignored by soil microbiologists and plant pathologists due to difficulty in handling non-laboratory adapted strains. Myxobacteria are ecologically very interesting because they lyse other microorganisms through the production of lytic enzymes. The myxobacteria then scavenge macromolecules that are released into the environment. We previously demonstrated that myxobacteria inhibit fungal plant pathogens including Pythium ultimum, Sclerotinia minor and Verticillium dahliae in vitro (Bull et al., 2002). These pathogens cause economically devastating disease of vegetable and strawberries. We additionally demonstrated that damping-off of cucumber caused by P. ultimum is reduced by over 30% when myxobacteria are applied to the soil 24 hours prior to planting. Myxobacteria used in these experiments include Myxococcus xanthus DK1622 and mutants derived from this strain as well as M. xanthus DZ2 and myxobacteria isolated from agricultural fields. We first screened myxobacteria for inhibition of S. minor using our previously published in vitro assay. Most of the developmental mutants tested did not differ for their ability to inhibit S. minor in culture. However, inhibition by two mutants were significantly altered. One is a mutation in a putative antibiotic gene, grenB, and the other is a mutation in relA. We altered a bioassay for evaluating resistance to this pathogen (Grube and Ryder, 2004) in order to test biological control of drop on lettuce. In short, lettuce plants (cultivar Batavia Reine de Glace) were grown for five weeks in 3 square pots in the greenhouse. Myxobacteria were grown in CTT broth for 2-4 days at 33 degrees C with shaking. Myxobacteria were concentrated and adjusted to approximately 1 x 109 CFU/ml with water spectrophotometrically. Approximately 100 ml of the solution was applied to the soil until run-off. A rye seed infested with the pathogen, S. minor, was placed 2 cm from the base of the lettuce seedling. Plants were incubated at 20 degrees C set for an 18/6 light to dark cycle in a growth chamber. At 7, 14, and 21 days after inoculation, plants were evaluated and disease incidence was recorded. Drop caused by S. minor has been consistently controlled by Myococcus xanthus DK1622 in these experiments reducing disease by >50 percent in most experiments. In all experiments to date, the grnB mutant was significantly impaired for the ability to control the disease. For example, disease incidence for the non-treated pathogen infested control and the plants grown pathogen infested soil treated with the grnB mutant, was approximately 80 percent in one experiment. Because strains that do not inhibit S. minor in vitro also do not control the disease caused by this pathogen, the in vitro assay appears to be a useful tool for screening for more aggressive strains of myxobacteria for disease control and to evaluate the role of particular genes in disease control. Using the in vitro assay to determine what strains to test in assays with lettuce plants will reduce the number of strains that are evaluated in the more labor intensive growth chamber bioassay. Additionally, these data indicate the potential for economically significant control of a major pathogen of lettuce.