Inhibition of the apple scab pathogen Venturia inaequalis and the grapewine downy mildew pathogen Plasmopara viticola by extracts of green waste compost

Summary

Extracts of green waste compost have been shown to inhibit plant diseases. In this study, the factors influencing the mechanism of inhibition of apple scab (Venturia inaequalis) and grapevine downy mildew (Plasmopara viticola) were studied. Extracts were prepared from samples of 30 composts from commercial composting plants. Composts were extracted with 1:2 or 1:5 water for 2 or 7 days. Extracts were applied to seedling of apple and grapevine. The seedlings were artificially inoculated with V. inaequalis or P. viticola, respectively and incubated under controlled conditions. After inoculation, severity of diseases and lesion diameter were measured. The incubation time and the compost/water ratio did not influence the capacity of the extracts to protect the apple plants. All treatments with compost extracts reduced disease severity in both host pathogen systems, and there was no difference in efficacy between autoclaved, sterile filtrated (0. 2 µm) and untreated extracts. From this, we conclude that the inhibition by compost extracts is not linked to their microbial activity. Rinsing apple seedling leaves 1 and 48 hours after application of the compost extracts did not diminish the protective effect against V. inaequalis. On the other hand, the severity of P. viticola increased, when the seedlings were rinsed after the application of compost extracts, and was similar to the untreated control. Compost extracts enhanced in vitro germination of conidia of V. inaequalis and showed no fungicidal effect. Thus, inhibition apparently acts indirectly in this host-pathogen system. On the other hand, the activity of zoospores of P. viticola was inhibited by 70 % compared to the control. The salt content of the extracts and their effect on the zoospores were positively correlated. For this host-pathogen system, there is thus evidence for a direct inhibition by compost extracts. We conclude that the active principle against V. inaequalis and P. viticola must be a water soluble, heat-stable metabolite produced in the compost before its extraction. The mechanism of inhibition in both plant-pathogen systems is different.