Variability in control of onion white rot by Trichoderma atroviride under different disease pressures

Abstract

Pellet and solid substrate formulations combined with a soil drench formulation of Trichoderma atroviride were evaluated under varying disease pressures in field trials to determine which formulation(s) provided the best control of the soil-borne disease onion white rot. Under low (11.8%) (average Area Under the Disease Progress Curve (AUDPC): 5.94%), moderate (37.7%) (average AUDPC: 11.26%) and high (64.4%) (average AUDPC: 24.76%) disease pressure, the standard fungicide programme (procymidone coated seed plus tebuconazole foliar spray) gave statistically significant disease control (76.3%, 91.3% and 58.8%, respectively) compared with the untreated control. Under low and high disease pressure, significant control, equal to the fungicide treatment, was achieved with the T. atroviride pellet plus soil drench treatment (72.0 and 39.6%, respectively). However, no Trichoderma formulation gave significant control of onion white rot under moderate disease pressure (20.7–34.2% control). When disease progress was examined over time for each trial, it was evident that the onset of disease occurred later in the growing season at the moderate disease pressure site (3 months after sowing) compared with Trial 3 at the high disease pressure site (2 months after sowing). At the low disease pressure site, the low level of inoculum and disease incidence enabled T. atroviride to bring about disease control. These results indicated that the performance of T. atroviride may be influenced as much by the timing of disease onset as the absolute disease pressure.