Evaluation of sensitivity of apple scab pathogen to difenoconazole using the discriminatory dose technique

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Abstract
The most serious disease of the apple tree in all areas of its growth is scab. In the integrated apple tree protection system, the main method is chemical. However, the use of chemical fungicides is characterized by the risk of developing resistance to them by pathogen. The sensitivity of 118 monospore isolates of Venturia inaequalis was studied from three orchards of Jeromine, Reinette Simirenko and Gala cultivars, differing in the frequency of application of difenoconazole. Sensitivity was determined using the discriminatory dose technique (0.01 mg/l of active substance) in terms of RG, the relative growth of the mycelium. RG was expressed as the degree of change in mycelium growth in a nutrient medium with fungicide relative to the control variant in percent. All pathogen populations studied differed significantly in mean RG values. Populations treated three times per season with difenoconazole had higher RG values compared to populations treated two times. From Gala orchard, for some isolates, a stimulating effect of a discriminatory dose of difenoconazole on their growth was observed, that is, a hormesis effect was manifested. The proportion of isolates with RG values above the cutoff value, which was 84 and 100 % for the Reinette Simirenko and Gala orchards, may indicate that the pathogen populations studied are resistant, and in these orchards, there may be a decrease in the effectiveness of protection against apple scab. The discriminatory dose technique allowed us to objectively and promptly assess the sensitivity of V. inaequalis populations from orchards with varying intensity use of difenoconazole.

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