Submitted to: Proceedings of Management of Soilborne Plant Pathogens
Publication Type: Proceedings
Publication Acceptance Date: 9/14/2005
Publication Date: 9/21/2005
Citation: Mazzola, M. 2005. Consideration of microbial ecology in the purported "biofumigatin" response to brassica soil amendments. 15th Annual Southern Africa Symposium on Soilborne Plant Diseases. pp. 36-42. Interpretive Summary: Organic soil amendments often have been promoted as means to control soilborne plant diseases. However, the effective use of such an environmentally sensitive method of disease control has been impeded by a lack of understanding of how such amendments provide disease control. Tissues from plants belonging to the Brassicacea have been promoted as a soil amendment for the control of soilborne plant diseases due to their production of glucosinolates, which yield anti-biological compounds upon hydrolysis. In ths study, differential control of the fungal pathogen Rhizoctonia solani, the parasitic nematode Pratylenchuse spp., and the weed Amaranthus refroflexus (pigweed) was achieved by using brassicaceous seed meals as a soil amendment. However, in most instances, pest control was not associated with glusosinolate content of the plant residue. Nematode control was achieved irrespective of glucosinolate content, but greater suppression was obtained with a seed meal that yielded an active glucosinolate hydrolysis product. Control of the fungal plant pathogen Rhizoctonia solani through application of these plant residues was found to operate through the activity of microorganisms native to soils, and which are stimulated by seed meal amendment. Specifically, these microorganisms were shown to stimulate plant defense responses which limited infection by the fungus. The seed meals also suppressed emergence of pigweed in orchard soils, but weed control was only observed for those residues that induced an increase in populations of Pythium. Likewise, elimination of Pythium by chemical applications abolished the herbicidal activity of the seed meal. As Pythium spp. are known to infect seeds and young seedlings resulting in plant death, these findings strongly implicate these microorganisms as agents contributing to the observed weed control.
Technical Abstract: Brassica tissues are promoted as a soil amendment for control of soilborne plant diseases due to their production of glucosinolates, which yield anti-microbial compounds upon hydrolysis. Application of brassicaceous seed meals were shown to control various pests and pathogens in orchard ecosystems. The mechanism responsible for pest control varied with the target of concern, but typically did not function through plant glucosinolates or its hydrolysis products. Initial soil and root nematode suppression was observed irrespective of seed meal glucosinolate content, and likely resulted from the known nematistatic nature of high nitrogen content residues. However, long-term suppression of soil nematode numbers was mainained only in response to Brassica juncea seed meal, a residue that yields a nematicidal isothiocyanate. Brassicaceous soil amendments provided control of apple root rot incited by native or introduced populations of Rhizoctonia solani, but this response required the activity of resident soil microbial populations. Preliminary evidence suggests that disease control function through the induction of plant host defense responses, which are stimulated via bacterial sources of nitric oxide (NO). The source bacterial communities may vary in a temporal manner post residue amendment. Weed control in reponse to seed meal amendment also was observed in a differential manner, but again was not associated with residue glucosinolate content. Suppression of Amaranthus retroflexus (pigweed) emergence was detected only in solids amended with seed meal amendments that induced a proliferation of resident Pythium spp. populations. Likewise, specific elimination of resident Pythium spp. populations via application of mefenoxam, abolished the herbicidal nature of the active brassicaceous seed meals.