Baseline Sensitivity of Natural Population and Resistance of Mutants in Phytophthora capsici to Zoxamide

Abstract

Laboratory experiments were conducted to determine the baseline sensitivity of Phytophthora capsici and its risk for developing resistance to zoxamide. In total, 158 P. capsici isolates were collected from China. All 158 isolates were sensitive to zoxamide, with effective concentrations for 50% inhibition of mycelial growth of 0.023 to 0.383 μg/ml and a mean of 0.114 μg/ml, which showed a skewed unimodal distribution. Zoxamide-resistant mutants of P. capsici were obtained by either treating mycelial culture and zoospores with ultraviolet irradiation or adapting a culture on zoxamide-amended plates. The frequency of resistance selection averaged 1.8 × 10(-7). Resistant isolates were also derived by selfing or crossing two sexually compatible isolates, resulting in a mean selection frequency of 0.47. The resistance factor (RF) for zoxamide was 25 to 100 in P. capsici mutants. Through 10 culture transfers, the mutants maintained high levels of RF (between 14 and 134) and had almost equal fitness as their wild-type parents in mycelial growth, sporulation, and virulence. There was no cross resistance between zoxamide and either flumorph, metalaxyl, azoxystrobin, or etridiazole. Based on the results above, P. capsici can develop resistance to zoxamide, and the risk is predicted to be moderate in nature.

Proteomic analysis on zoxamide-induced sensitivity changes in Phytophthora cactorum

“…Similar results for zoxamide sensitivity were observed in P. capsici isolates collected from South Carolina and China [3,27]. Because no zoxamide had been applied in the field before P. cactorum was collected, all 73 field isolates of P. cactorum were sensitive to zoxamide, and no spontaneous mutants were detected.…”

“…Currently, zoxamide-resistant mutants of Phytophthora pathogens have not been reported in natural populations [6,7]. However, Bi et al's studies showed that zoxamide-resistant mutants of P. capsici were obtained by treating either mycelial cultures or zoospores with UV irradiation or by selecting with zoxamide [3]. The stable resistant mutants in the present study were not obtained using these methods; however, the isolates showing induced decreases in zoxamide sensitivity were obtained easily under increasing zoxamide concentrations.…”

“…Ultraviolet irradiation and fungicide taming methods were used to generate isolates that were less sensitive to zoxamide according to Bi et al and Young et al's methods [3,7]. Four isolates of P. cactorum were randomly selected from natural populations to generate the less sensitive isolates.…”

“…This fungicide disrupts the microtubule cytoskeleton by binding to β-tubulin [1,2]. Currently, this fungicide is widely used on potatoes, vine crops, and other vegetables [3]. Many benzimidazole and N-phenylcarbamate fungicides, which share a similar mode of action with zoxamide, have faced serious resistance problems with many fungal pathogens [1,4,5].…”

“…resistance to zoxamide was previously considered low because zoxamide resistance in Phytophthora spp. has not been reported anywhere in the field until recently [1,3,6] and because attempts to obtain highly resistant isolates to zoxamide in Phytophthora infestans and Phytophthora capsici have been unsuccessful [6,7]. Recent studies have reported that stable zoxamideresistant isolates of P. capsici were obtained by treating either mycelial cultures or zoospores with UV irradiation and subsequently selecting on zoxamide amended agar media [3].…”