A soil inoculant inhibits Armillaria mellea in vitro and improves productivity of grapevines with root disease

A soil inoculant, Vesta (Biologically Integrated Organics, Inc., Sonoma, CA), was tested for its ability to inhibit Armillaria mellea, causal agent of Armillaria root disease of grapevine (Vitis vinifera). Colony diameter of A. mellea was significantly inhibited by undiluted inoculant (P < 0.0001) and by bacterial isolates cultured from the inoculant (Bacillus subtilis, B. lentimorbus, Comamonas testosteroni, Pseudomonas aeruginosa, R mendocina; P < 0.0001) relative to diameter of the nontreated control. Efficacy of the inoculant for postinfection control of Armillaria root disease of grapevine was examined in an A. mellea-infested vineyard in northern California. Inoculant was applied via drip-irrigation to vine rows in replicate blocks in 2003 and 2004. Yield, growth, mineral nutrition, and juice quality parameters of healthy and symptomatic vines were measured in treated and nontreated vine rows. Significantly decreased petiole P and K concentrations and significantly lower soluble solids content in fruit from symptomatic vines demonstrated that Armillaria root disease negatively affects vine mineral nutritional status and fruit quality, findings that have not been previously reported for an agronomic host of A. mellea. The inoculant significantly increased cluster weights of symptomatic vines (109.63 g/cluster), relative to those of symptomatic-nontreated vines (92.05 g/cluster), to levels comparable to those of healthy vines (122.09 g/cluster). However, the inoculant did not decrease the rate of symptom development or mortality of treated vines from 2002 to 2004. The results of our field experiment suggest that the inoculant may not prevent Armillaria root disease, but can provide therapeutic benefit by improving productivity of infected vines.