Submitted to: Phytopathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/25/1996
Publication Date: N/A
Interpretive Summary: Fusarium wilt of watermelon, caused by the soil-inhabiting fungal plant pathogen Fusarium oxysporum f. sp. niveum, is a serious disease throughout the world and is often a limiting factor in watermelon production. Current control methods of this disease, as well as similar Fusarium wilt diseases on numerous other crops, are generally not effective or economically feasible and alternative methods are needed. Biological control may be possible through the use of microorganisms recovered from disease-suppressive soils. In this study, nearly 400 microorganism isolates, including bacteria, actinomycetes, and fungi, were collected from watermelon roots growing in soils both suppressive and nonsuppressive to Fusarium wilt of watermelon. These organisms were tested for their ability to control Fusarium wilt disease in microwave-treated and nontreated field soil. Several isolates of nonpathogenic F. oxysporum from suppressive soil lwere consistently effective in reducing disease (35-75% reduction of disease incidence) in both microwave-treated and nontreated field soil, and were concluded to be the primary antagonist responsible for disease suppression. The primary mechanism of action of selected isolates of nonpathogenic F. oxysporum was through the systemic induction of host plant defense responses, whereas direct competition with the pathogen was not as important. Several isolates of F. oxysporum from this suppressive soil have potential for development as biological control agents. This information is primarily of benefit to researchers working with Fusarium wilt diseases, but could lead to the development of practical biological control of these diseases, which would benefit growers and producers.
Technical Abstract: Nearly 400 microorganism isolates, including bacteria, actinomycetes, and fungi, were collected from watermelon roots growing in soils suppressive and nonsuppressive to Fusarium wilt of watermelon. These isolates were screened for their ability to restore suppressiveness to microwave-treated suppressive soil and to reduce disease incidence in conducive field soil. Specific isolates of nonpathogenic Fusarium oxysporum from suppressive soi were the only organisms consistently effective in reducing disease (35-75% reduction) in both microwave-treated and natural field soils. Thus, F. oxysporum was concluded to be the primary antagonist responsible for suppression in this suppressive soil, although other organisms may contribute to this suppressiveness. Selected isolates of F. oxysporum were effective in reducing disease when added to field soils at inoculum levels as low as 50-100 chlamydospores/g soil, which was comparable to or below pathogen inoculum levels (100-200 cfu/g soil). Root colonization data indicated that reduction of disease was not directly related to the ability of the antagonist to colonize roots extensively or to reduce colonization by the pathogen. Effective antagonists were not associated with specific vegetative compatibility groups, indicating antagonists represent diverse isolates. In split-root experiments, in which the antagonist and the pathogen were physically separated from each other, root colonization by selected isolates of F. oxysporum reduced disease incidence, verifying the mechanism of action as induced systemic resistance. Several isolates of F. oxysporum from this suppressive soil have potential for development as biological control agents.