Sensitivity to DMI fungicides and relative fitness of apple black spot isolates

Abstract

Methods for detection of sensitivity of Venturia inaequalis to demethyltion inhibitor (DMI) fungicides were assessed. Three isolates differing in EC50 were tested with myclobutanil. A hyphal growth test was more reliable than either a germ tube growth test or spore germination test. Image processing and analysis could provide rapid and more accurate measurements of the lengths of germ tubes and hyphal growth especially for irregular shapes. However, it was considered relatively time consuming in image processing stage.

A hyphal growth test was used to assess sensitivity of 10 isolates of V. inaequalis from main apple growing districts of New Zealand to myclobutanil. The EC₅₀ of isolates ranged from 0.035-0.679 mg/litre. The EC₅₀ value of the most resistant isolate was about 20 times the most sensitive one. Cross resistance was assessed using a colony growth test with 9 isolates and myclobutanil, bitertanol, triforine, flusilazole, penconazole, pyrifenox and fenarimol. All isolates were cross resistance to both trlazole and non-trlazole DMI fungicides. Isolate 5 was the most resistant isolate and had high cross resistance to most products.

A leaf disc system was used to estimate the duration of the incubation period, percentage of diseased area and conidial production. Six isolates classed as medium-resistant type (MR), low-resistant type (LR) and sensitive type (S) were tested with 2 levels of inoculum density (5x10³ and 5x10⁴ spores per ml). The higher the inoculums density, the greater was the degree of host tissues colonization and consequently higher conidia produced. By contrast, the higher the inoculum density, the shorter was the duration of the incubation periods. However, none of the fitness parameters were significantly different between resistant and sensitive isolates of V. inaequalis.