Development, detection and elimination of Verticillium dahliae in mint shoot cultures and effects of bacterial cultures and bacterization on the growth and disease development Public Deposited

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  • Roots of greenhouse-grown peppermint (Mentha piperita L.) and spearmint (Mentha spicata L.) plants and in vitro-grown shoot cultures were inoculated with Verticillium dahliae conidial suspensions to study symptom development, detection, and elimination of the V. dahliae. There were significant differences in the symptom expression of control and infected shoot cultures at each conidia concentration for the four mint cultivars tested. Disease symptom ratings were proportional to the V. dahliae innoculum density. Shoot cultures were stunted when inoculated with ≥ 10³ conidia⦁ml⁻¹. V. dahliae was re-isolated from shoot cultures at all levels of innoculum, but not from any control cultures. Verticilliuw infections were easily detected by plating mint stems on potato dextrose agar. Shoot tips (0.5 to 15 mm diameter) from inoculated in vitro and greenhouse-grown plants were isolated and screened for V. dahliae. A meristem diameter of 3 to 5 mm was the most effective size for fungus elimination. Shoot tips from inoculated greenhouse plants at 10⁵ conidia⦁mL⁻¹ produced Verticillium-free cultures from 79% of 'Black Mitcham' and 90% of 'Todd's Mitcham' plants. Shoot tips isolated from in-vitro spearmint cultivars infected at 10² and 10³ conidia⦁mL⁻¹ were 100% Verticillium free, but only 42% of 'Black Mitcham' and 54% of 'Todd's Mitcham' shoot tips were free of the disease at 10² conidia⦁mL⁻¹ and none at higher inoculum concentrations. These results indicate the utility of testing for Verticillium and the utility of micropropagated mint shoots for certified planting stock programs. Endogenous bacteria were tested for inhibition of V. dahliae. Of the 34 endogenous bacterial strains screened for inhibition of V. dahliae in vitro, 10 produced a zone of inhibition around colonies of V. dahliae on PDA and one strain (Pseudomonas fluorescens (Pf-5)) produced a zone on KB medium as well. No significant effect on plant growth (height) was observed for in vitro-grown Scotch spearmint culture bacterized with Pseudomonas fluorescens (Pf-5). The bacterized plants showed no significant biological control effects when inoculated with V. dahliae. However, bacterization appeared to provide protection against Verticillium wilt during the early stages of plant growth but the effects were not lasting. Protocols for certification of Verticillium-free mint in-vitro cultures were developed based on this study.
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