Response of different forms of propagules of Rhizoctonia solani AG2-1 (ZG5) exposed to the volatiles produced in soil amended with green manures

The sensitivity of different forms of propagules of Rhizoctonia solani anastomosis group (AG) 2-1/zymogram group (ZG)5 to volatile compounds produced in soil amended with green manure will influence the efficacy of green manures used to manage the disease. In laboratory experiments, we determined the impact of volatiles arising from residues of five species of Brassicaceae, and Avena sativa (oat), a non-Brassicaceae species, and volatiles of pure allyl isothiocyanate (AITC) or 2-phenylethyl isothiocyanate (2-PEITC) in either their soluble or vapour phase on the hyphal growth of R. solani arising from different propagules. The brassicaceous species were Brassica napus var. Karoo, B. napus B1, B. napus B2, B. nigra and Diplotaxis tenuifolia (a brassicaceous weed). Colony growth and hyphal density on water agar were measured up to 7 days. The amendment of a sandy soil with green manures at a high (100 g kg(-1), 10%) concentration generally suppressed the growth of the pathogen, but at a low (10 g kg(-1), 1%) concentration, the amendment had little effect on the radial fungal growth of the pathogen but increased the density of hyphae through more branching. The inhibition by volatiles from the residues of Brassicaceale species at 10% concentration was stronger (82-86%) than that by volatiles from oat (64%) amendment at the same rate. Hyphale arising from sclerotia and precolonised ryegrass seed were less sensitive than hyphae growing out of potato dextrose agar plugs. This indicates that thick-pigmented cell walls may have protected the fungus from these unfavourable conditions. Pure AITC and 2-PEITC in the range of 0.5-2.0 mM inhibited the growth of R. solani from all forms of propagules, but hyphae originating from agar plugs were the most vulnerable compared with the two others. Thus, hyphae arising from the medulla of the sclerotia may be relatively tolerant to volatile compounds emanating from decomposing Brassica green manure amendments in the field and in vitro inhibition of the vegetative growth of the pathogen may not reflect its response to the amendments in the field.