Application method and rate of Trichoderma species as a biological control against Pythium aphanidermatum (Edson) Fitzp. in the production of microgreen table beets (Beta vulgaris L.)

Seed balls of ‘Early Wonder Tall Top’ table beet (Beta vulgaris L.) were incubated for 2 days at 21 °C in moist exfoliated grade 3 vermiculite (3 g seed balls 168 seed balls] and 1.1 g vermiculite) containing equal weights of Trichoderma harzianum Rifai strain KRL-AG2 G41 and T. virens strain G-41 (ThTv) at 0, 0.25, 0.50, 0.75 or 1.00 mg ThTv per seed ball. ThTv also was applied to the peat-lite growth medium 14 days before planting, at the same rates per seed ball as the seed ball treatments. Four days before planting, the peat-lite was inoculated with Pythium aphanidermatum (Edson) Fitzp. (Pa) at 0, 0.5 and 1.0× the rate that resulted in 96% damping-off when non-ThTv-treated dry seed balls were sown in peat-lite containing 1.0 Pa. Increasing ThTv level per seed ball decreased percentage damping-off, with 1.0 Pa giving greater percentage damping-off than 0.5 Pa. At 1 mg ThTv per seed ball, damping-off was 5% and 19% at 0.5 and 1.0 Pa, respectively. Including Agro-Lig UF (mostly humic acids) in the incubating seed-ball-ThTv mixture further decreased damping-off by an average 13 and 10 percentage points in 0.5 and 1.0 Pa, respectively. Increasing ThTv per seed ball in growth media decreased percentage damping-off, but not to the extent achieved with seed ball treatment, with 1 mg ThTv per seed ball giving 20% and 55% damping-off in 0.5 and 1.0 Pa, respectively. Decreasing incidence of damping-off with increasing ThTv application to seed balls or growth media was associated with increasing shoot fresh weight m⁻² at 14 days after planting, a response attributable to increased percentage plant survival and not to a ThTv growth-promoting effect.