Resistance of Pseudoperonospora cubensis to flumorph on cucumber in plastic houses

Abstract

Plastic-house experiments were conducted over a 2-year period to estimate the effects of successive applications of flumorph or a mixture of flumorph with mancozeb to cucumber plants on selection for flumorph resistance in the downy mildew oomycete, Pseudoperonospora cubensis . Application of flumorph alone favoured the selection of resistant isolates of Ps. cubensis . Resistant populations were detected at a frequency of 2·5% after six successive applications of flumorph alone in a plastic house in 2004. Resistant isolates were also detected (4·8%) after eight successive applications of the mixture of flumorph and mancozeb in 2004, although the mixture gave significantly better disease control than flumorph alone and produced a slight delay in the development of resistance. In a second cucumber crop in the same plastic houses in 2004, the frequency of resistant isolates increased to 100% after three successive applications of flumorph or four of flumorph + mancozeb. Under laboratory conditions, most flumorph-resistant isolates showed high levels of resistance and their levels of pathogenicity and sporulation were as high as that of wild-type isolates. Flumorph showed cross-resistance with dimethomorph and iprovalicarb, but not with azoxystrobin, cyazofamid, cymoxanil or metalaxyl. These studies suggest a high risk for the occurrence of resistance to flumorph in Ps. cubensis in cucumber crops under plastic-house conditions.

Assessing the risk of resistance in Pseudoperonospora cubensis to the fungicide flumorph in vitro

“…This result was consistent with previous reports and supported the hypothesis that flumorph and dimethomorph have the same mode of action 4. In addition, flumorph‐resistant isolates also show decreased sensitivity to another CAA fungicide, iprovalicarb 24. Previous reports showed that populations of P. viticola can be found in certain regions that are simultaneously resistant to dimethomorph, benthiavalicarb, iprovalicarb and mandipropamid 13.…”

“…Under field conditions, however, the resistance risk of P. cubensis to flumorph is high. After 6–8 successive applications of flumorph alone, resistant isolates with a high level of resistance and good fitness were easily detected 24. The difference in resistance risk of P. cubensis to flumorph between laboratory and field conditions may be due to the diploid nature of oomycetes.…”

Fungicide modes of action and resistance in downy mildews

“…Mean resistance factors are often >300, and resistant isolates are stable when transferred onto untreated grape leaves (Gisi et al 2007b). Also, in P. cubensis, resistant isolates have recently been detected in a few trial site locations, one each in South Korea, Israel and USA (FRAC CAA working group reports, www.frac.info) and in China (Zhu et al 2007). When a CAAsensitive and CAA-resistant single sporangiophore isolate of P. viticola were crossed (method see above), resistance segregated in the F1 in a 0:1 (entire F1 progeny sensitive) and in the F2 in a 1:9 pattern, suggesting that two recessive nuclear genes are involved in CAA resistance (Gisi et al 2007b).…”

Generation and Characterization of Isolates of Peronophythora litchii Resistant to Carboxylic Acid Amide Fungicides

“…The CAA fungicide dimethomorph is used extensively in the world against different plant-pathogenic oomycetes, such as Pseudoperonospora cubensis, Plasmopara viticola, and Phytophthora infestans. After intensive use of CAA fungicides for more than 15 years against oomycete pathogens, no resistant field isolates have been detected in Phytophthora infestans populations (FRAC CAA Working Group, www.frac.info) (8), but CAAresistant isolates have been detected in populations of Plasmopara viticola in some European regions and there are also reports of CAA resistance in Pseudoperonospora cubensis from a few trial site locations in South Korea, Israel, the United States (FRAC CAA Working Group), and China (28). In recent moni-toring studies no resistant isolates have been reported in Peronophythora litchii populations (FRAC CAA Working Group-2008 Minutes).…”