Resistance test

Scab caused by Venturia inaequalis is the most significant fungal disease in apple cultivation. The scab pathogen is increasingly developing resistance to the active ingredients currently in use. Particularly at risk of resistance are specific active ingredients from the groups of sterol biosynthesis inhibitors (difenoconazole, penconazole, mefentrifluconazole, tebuconazole), the anilinopyrimidines (cyprodinil, pyrimethanil), the strobilurins (trifloxystrobin, pyraclostrobin, kresoxim-methyl), and the succinate dehydrogenase inhibitors (boscalid, fluopyram, cyflufenamid, fluxapyroxad). There is also evidence in the literature of less susceptible scab populations for the active ingredient dodine, and in recent years, scab populations with reduced susceptibility have been identified in the Lake Constance region for dithianon.

To determine the appropriate scab control strategy for your orchard, it is helpful to know the resistance status of your scab population. To test the sensitivity of the scab population in your orchard to the products you use, simply send us approximately 100 scab-infected leaves from your apple orchard (see sample submission form). If you would like to have ascospores tested, please contact us.

For the in vivo resistance test, conidia or ascospores are washed from the leaves, and hand-grafted apple trees are inoculated with the scab pathogen. Depending on the active ingredient, fungicide treatment is applied before or after inoculation at various concentrations. Approximately two to three weeks after inoculation, the scab symptoms are evaluated.

Active ingredients used for protective treatment (fluxapyroxad, trifloxystrobin, dodine, captan, dithianon) can be tested in vitro by mixing the scab pathogen with the formulation to be tested and counting the germination rate of the conidia under a microscope.

The effect of the applied concentration of the active ingredient is calculated, and the dose-response curve is compared with the baseline sensitivity. By conducting this test and, if necessary, adjusting your spraying schedule, you can optimize protection against leaf scab and fruit scab in your orchard.

Dose-response curves and the resulting ED90 values for dithianon against various scab populations from the Lake Constance region: Sensitive populations (standard) were 90% inhibited from germinating at 1.8 ppm dithianon, whereas in one population, even the recommended application concentration (RAC) of 350 ppm only prevented germination in 80% of the conidia.