Rhizoctonia como patógeno em batata: influência de grupos de anastomose na adaptabilidade e controle com óleos essenciais

Abstract

Rhizoctoniosis is an important potato disease in Brazil, caused mainly by the AG-3 PT, AG-4 HGI and AG-R anastomosis groups of Rhizoctonia. The objective of this study was to compare the adaptability of 15 isolates of each AG in relation to temperature, pH, water potential, salinity, fungicides and aggressiveness in several plant species, as well as to select essential oils with antifungal activity in AG-3 isolates PT and AG-R. Isolates belonging to the same AG did not differ in relation to mycelial growth in the different experiments. The optimum temperature for AGR-R (29.5° C) growth was higher than the others and AG-4 HGI had the highest mycelial growth (96.6 mm). The pH levels tested (5, 6 and 7) did not influence the mycelial growth of the three AGs. AG-4 HGI demonstrating less sensitivity to the water deficit, while AG-3 PT presented the highest sensitivity to the salinity. The AGs differed only in relation to the fungicide fluazinam, with AG-3 PT showing the greatest reduction in growth. All isolates induced symptoms in 14 inoculated plant species and AG-4 HGI was the most aggressive. In all situations, AG-4 HGI isolates showed greater saprophytic and pathogenic adaptive potential than AG-3 PT and AG-R isolates. The antifungal activity of 30 essential oils from different plants was evaluated in the inhibition of mycelial growth of two Rhizoctonia isolates belonging to AG-3 PT and AG-R, respectively, by disc-diffusion method in agar. Seven essential oils completely inhibited the mycelial growth of the fungus, of which the oils of Mentha rotundifolia, Thymus zygis and Satureja montana were selected. GC-MS identified 26 chemical compounds in the essential oil of M. rotundifolia and T. zygis and 14 chemical compounds in S. montana essential oil. Piperitone oxide (36.97%) and piperitone (20.77%) were the major components in the oil of M. rotundifolia, thymol (60.27%), piperitone oxide (6.56%) and piperitone 18%) were the main components in the oil of T. zygis and carvacrol (49,38), p-cymene (28,08) and thymol (8,30) were the main components of S. montana oil. TBARS showed higher values of ROS (127.4 nmol g-1) and MDA (5.9 nmol g-1) for AGR-R with S. montana oil treatment at 5 mg mL-1 and higher ROS values (131.9 nmol g-1) and MDA (5.4 nmol g-1) for AG-3 PT in the treatment with M. rotundifolia oil at 5 mg mL-1. The data demonstrate that the essential oils are promising in the management of Rhizoctonia.