Effect of triazole fungicides on growth and development of Phymatotrichum omnivorum and the relative resistance of grape rootstocks to root rot

Abstract

The effect of triazole fungicides on Phymatotrichum omnivorum (PO), grapevines and the interaction between grape rootstocks and PO were studied to have a better understanding of the control alternatives of PO in vineyards. Triazole fungicides reduced mycelial growth and formation of sclerotia of PO. Mycelial growth on PDA was highly inhibited by bromoconazole, diniconazole, flusilazole and propiconazole at less than 5 μg/L. In soil culture the rate of mycelial growth was inhibited between 40-50% by triadimenol, furconazole, triadimefon, diniconazole, penconazole and propiconazole at 10 mg a.i./kg soil. Diniconazole, hexaconazole, propiconazole and bromoconazole were the most effective fungicides to reduce sclerotial biomass. At 100 mg a.i./kg soil, triazole fungicides reduced the formation of sclerotia between 95 to 100%. Germination of sclerotia that formed in soils treated with triazole fungicides was not affected. When Cabernet Sauvignon grapevines grafted on rootstocks 3309 or SO 4 were grown in soils treated with triazole fungicides, propiconazole, myclobutanil and bromoconazole exhibited growth regulation activity. The use of triazole fungicides at 0.1, 1 and 10 mg a.i./kg soil did not affect the grapevine's growth components nor dry weight, while the fungicides at 100 mg a.i./kg soil produced significant reduction in growth components and dry weight. There was no differential response between fungicides and rootstocks 3309 and SO 4. Softwood cuttings (three nodes and one leaf) of Vitis candicans (Candicans-MH) grapevines had good development and rooting (96%) at 250 mg a.i. IBA/L. Root analysis of inoculated Candicans-MH, Dog Ridge, Champanel and Chenin blanc showed that the root systems of those plants produced new roots immediately above lesions caused by PO. Survival of grapevines may be dependent on the capability of forming new roots after infection. The selection of grape rootstocks for areas affected by PO should focus on root regeneration potential. Ergosterol extraction from roots of inoculated plants and its quantification through HPLC was a good method to determine the extent of root colonization by PO. Ergosterol content was highest in Chenin blanc, followed by Candicans-MH and Dog Ridge, and lowest in Champanel. Less colonization of root systems was found in Champanel and Dog Ridge.