Development of a fungicide sensitivity monitoring service for SA pome fruit pathogens

The objective was to verify protocols for monitoring of fungicide sensitivity of apple scab, caused by Venturia inaequalis, and pear gray mould, caused by Botrytis cinerea, from apple and pear growing regions in SA. The project aimed develop a more rapid and cost effective fungicide sensitivity monitoring service for the pome fruit industry.

Pear gray mould and apple scab isolates were collected from unexposed fruit (50 each). Botrytis cinerea fungicide baseline sensitivity was evaluated using mycelial growth tests for anilinopyrimidine (AP), benzimidazole, dicarboximide, phenylpyrrole class fungicides. Another set of 165 B. cinerea isolates collected from packhouses was tested at discriminatory doses of the fungicides.

For V. inaequalis, the active ingredients of four fungicide classes, namely anilinopyrimidine, guanidine, sterol demethylation inhibitor (DMI), and ethylene-bis dithiocarbamates (EBDC) were tested. The concentration to inhibit 50% mycelial growth (EC₅₀) was determined for each isolate from the baseline populations and sanitation trial orchards and the means calculated.

Botrytis cinerea fungicide sensitivity was also tested using a microtiter plate reader for the fungicides fludioxonil, iprodione and pyrimethanil. The microtiter method was also optimised for V. inaequalis isolates and tested using mancozeb and azoxystrobin (a QoI fungicide). Possible molecular mechanism for AP and DMI fungicide resistance were investigated in V. inaequalis isolates

Mean baseline sensitivities to benzimidazole, phenyl pyrrole, dicarboximide and anilinopyrimidine group fungicides were evaluated for a population of previously unexposed isolates from the Ceres region in the Western Cape (WC). The concentration that inhibited 50% mycelial growth in vitro was EC₅₀(benomyl)= 0.08 mg/L, EC₅₀(fludioxonil)= 0.005 mg/L, EC₅₀(iprodione)= 0.24 mg/L, and EC₅₀(pyrimethanil)= 0.21 mg/L. The mean EC₅₀ of the V. inaequalis baseline population from Greyton for sensitivity towards the DMI group fungicide flusilazole was 0.22 mg/L, and 87% of the tested isolates were categorised as sensitive. Baseline sensitivity to cyprodinil (an AP fungicide) was 0.88 mg/L and 0.08 mg/L for the guanidine group fungicide dodine. The EBDC fungicide mancozeb was highly effective when tested in a glasshouse inoculation experiment. Even at mancozeb concentrations as low as 0.00015 mg/L over 70% of leaf infections could be prevented. Using the microtiter method, an EC₅₀ of approximately 1 mg/L was estimated for mancozeb. Resistance frequencies of B. cinerea isolates from WC packhouses were 29.3% for pyrimethanil, 6.7 % for benomyl, 2.4% for iprodione, and 0.6% were fludioxonil resistant. A large proportion (>10%) of 40 tested V. inaequalis isolates contained an insertion in the promoter region of the CYP51A1 gene, potentially contributing to DMI resistance development. The point mutations detected in the cystathione lyase gene of V. inaequalis isolates from cyprodinil exposed populations, could be partially responsible for AP resistance. Strobilurin resistance conferring mutations were identified in the tested population from Elgin.

Baseline sensitivity of the tested B. cinerea and V. inaequalis populations show similar ranges as previously reported from the USA, although some fungicide groups such as the AP (cyprodinil, pyrimethanil) appeared to be slightly less effective with mean EC₅₀ values of approximately 3-times higher than the ones from literature. This has also been observed in a parallel study on another apple pathogen, thus cross-resistance to other fungicides is expected from this fungicide group.