Nondestructive Sampling to Monitor Macrophomina phaseolina Root Colonization in Overwintering Stevia

Abstract

Macrophomina phaseolina is a soilborne fungal pathogen in the family Botryosphaeriaceae. Microsclerotia of M. phaseolina were first observed at the base of overwintering stevia stems in North Carolina in spring 2016. Previous studies utilized destructive sampling methods to monitor M. phaseolina in stevia fields; however, these methods are not feasible for long-term monitoring of disease in a perennial system. In the current study, nondestructive root soil-core sampling was conducted during overwintering months, from October 2018 to January 2020, to monitor M. phaseolina root colonization in stevia in Rocky Mount, NC. Two-inch-diameter soil cores were collected through the root zone, and fresh weight of roots was recorded for each soil core. M. phaseolina recovery was evaluated by examining mycelial growth from roots plated onto potato dextrose agar. There was no significant effect of sample weight on M. phaseolina across all dates, but there was one date for which sample weight had a significant effect on recovery (P = 0.01; α = 0.05). For both recovery and sample weight, sampling date was a significant predictor (P = 1.68e-5 and P = 0.0389, respectively; α = 0.05). Weather and climate data revealed that dates with no M. phaseolina recovery had lowest mean air and soil temperatures and the greatest number of days below freezing in the month prior to sampling. In separate sampling years, October sampling dates had the highest recovery of M. phaseolina. Future field trials should determine if October samplings can predict survival and vigor of reemerging stevia plants.