Submitted to: Phytopathology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 10/9/2006
Publication Date: 4/1/2007
Citation: Mazzola, M., Brown, J., Izzo, A., Cohen, M.F. 2007. Mechanism of action and efficacy of seed meal-induced suppression of pathogens inciting apple replant disease differ in a brassicaceae species and time-dependent manner. Phytopathology. 97:454-460 Interpretive Summary: Organic soil amendments often have been promoted as a 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 concerning the means by which amendments provide disease control. Tissues from plants belonging to the Brassicaceae have been promoted as a soil amendment for the control of soilborne plant diseases due to their production of glucosinolates, which yield anti-microbial compounds upon hydrolysis. In this study, brassicaceae seed extraction were evaluated for the ability to control multiple soil inhabiting pathogens that contribute to development of apple replant disease. All brassicaceae seed meals examined were found to require an active soil microbial community to provide long term control of the fungal pathogen Rhizoctonia solani. However, in the case of Brassica juncea seed meal, initial disease control was directly related to the release of the chemical allyl isothiocyanate, a product resulting from the hydrolysis of plant glucosinolates. B. juncea seed meal was also superior to Brassica napus or Sinapis alba seed meals for the control of root infection by Pythium spp. and long term suppression of the plant parasitic nematode Pratylenchus pentrans. However, in the field B. juncea seed meal amendment was associated with increased infection of apple roots by the pathogen Phytophthora cambivora. A composite amendment containing seed meal of B. napus and B. juncea may be a superior product for control of apple replant disease as this mixture appears capable of controlling all components of the pathogen complex identified to date that are responsible for causing apple replant disease in Washington state.
Technical Abstract: Tissues of plant in the family Brassicaceae contain glusinolates, compounds whose dyrolysis results in the release of various bioactive products including isothiocyanates. The broad spectrum of biological activity of these glucosinolate hydrolysis products has led to the promotion of brassicaceae plant residues for use as an amendment for the control of soilborne plant diseaes. Studies demonstrated that mechanisms operating in the control of Rhizoctonia root rot of apple in response to Brassicaceae seed meal amendments varied in both a species and temporal manner. Seed meal of Brassica juncea, B. napus and Sinapis alba all required an active soil microbial community for the optimal induction of disease control. However, for B. juncea, disease control elicited in the first 24 h post-amendment was directly related to the emission of allyl-isothiocyanate (AITC). Generation of AITC peaked within the first 3 hours post seed meal amendment, declining rapidly thereafter and by 24 h was near zero. Likewise, when introduction of Rhizoctonia solani was delayed until 24 h post-amendment disease control was abolished. Disease suppressiveness was re-established when B. juncea seed meal amended soils were incubated for 4-8 weeks, and this restoration was associated with a proliferation of resident Streptomyces spp. Similarly, suppression of R. solani in B. napus and S. alba seed meal amended soils required an active soil microbial community and suppressiveness was associated with elevated Streptomyces numbers in treated soils. B. juncea seed meal was superior in the capacity to control lesion nematode populations and was the only seed meal that did not induce a proliferation of Pythium spp. and subsequent apple root infection. However, B. napus and S. alba seed meal amendments exhibited a greater capacity to control root infection by R. solani. When applied in a composite amendment, B. juncea seed meal abolished the B. napus-induced proliferation of resident Pythium spp. populations.