Effect of Soil Amendments on Allelopathy and Tolerance to Violet Root Rot in Mycorrhizal Asparagus Plants

Allelopathy tolerance of arbuscular mycorrhizal (AM) fungi [Glomus intraradices (Gi) and Gigaspora margarita (Gm)] - inoculated asparagus seedlings (Asparagus officinalis L., 'Welcome') were investigated in the presence of different soil amendments: activated charcoal (AC) and coffee residue (CR). The growth-inhibitory effect of allelopathy in asparagus decline field soil was confirmed by bioassay. Ten weeks after AM fungal inoculation growth of control asparagus seedlings was enhanced in CR and AC-added decline soil plots compared with healthy and non-added decline soil. Gi colonization level in the root system was comparatively higher than Gm. Moreover, maximum dry weight of shoots and roots were observed, in addition of CR with Gi-inoculated commercial soil plot, which was also more effective than AC with Gi-inoculated decline and commercial soil plots. These results reveal that allelopathy tolerance appeared in AM fungal inoculated asparagus plants by addition of coffee residue or activated charcoal, though the effect differed between the soil amendments. In violet root rot study, symptom of violet root rot appeared in all the treatments 12 weeks after H. mompa inoculation and the severity of the incidence was less in AM fungal inoculated asparagus plants than non-inoculated plants. Cultivar difference ('Welcome' and 'Mary Washington 500W') was also observed and the disease indices revealed that the incidence was remarkably low in Gi-inoculated 'Welcome' than 'Mary Washington 500W'. These results suggest that tolerance to violet root rot appeared in mycorrhizal plants, though the tolerance level differed with the cultivars.