Management of Chinese hickory (Carya cathayensis) trunk canker through effective fungicide application programs and baseline sensitivity of Botryosphaeria dothidea to trifloxystrobin

Trunk canker of Chinese hickory (Carya cathayensis), caused by Botryosphaeria dothidea, is the most important disease threatening production. Although fungicides are applied to control the disease, efficacy is very poor. In this study, fungicides were studied and selected for effective control of trunk canker through assessments of inhibitory activity and application programs. Seventeen fungicides from eight different FRAC groups were tested for their in vitro activity against B. dothidea. Difenoconazole, tebuconazole, and prochloraz (FRAC 3) were the most effective in inhibiting mycelial growth. Trifloxystrobin (FRAC 11) had the most significant inhibitory activity against spore germination with an EC50 of 1.78 +/- 0.61 mg/ L. Seven different application programs with three sprays of trifloxystrobin and/or difenoconazole for each program were evaluated at two forest sites over 3 years. Results indicated that the date of the first spray is crucial. If the first spray was applied on March 10, significantly better efficacy was achieved compared to an application on March 20. In 2016, a real-time PCR technique was adopted to help quantify the pathogen and thereby assess canker control. The results suggested a good relationship between gDNA content of B. dothidea in trunk tissues and the development of the canker lesions at both sites. Moreover, a total of 107 single-conidium isolates of B. dothidea were further tested for their sensitivities to trifloxystrobin in conidial germination experiments. In the presence of 100 mg/L SHAM, the EC50 values ranged from 0.29 mg/L to 8.38 mg/L, with an average of 3.16 +/- 0.85 mg/L. The range of variation was 28.9-fold. These sensitivity data can be used as a baseline for monitoring putative sensitivity shifts in B. dothidea to trifloxystrobin, a Qo-inhibiting (QoI) fungicide, members of which are commonly considered to be at a high risk for resistance selection.