A reassessment of the risk of rust fungi developing resistance to fungicides

Abstract

Rust fungi are major pathogens of many annual and perennial crops. Crop protection is largely based on genetic and chemical control. Fungicide resistance is a significant issue that has affected many crop pathogens. Some pathogens have rapidly developed resistance and hence are regarded as high-risk species. Rust fungi have been classified as being low risk, in spite of sharing many relevant features with high-risk pathogens. An examination of the evidence suggests that rust fungi may be wrongly classified as low risk. Of the nine classes of fungicide to which resistance has developed, six are inactive against rusts. The three remaining classes are quinone outside inhibitors (QoIs), demethylation inhibitors (DMIs) and succinate dehydrogenase inhibitors (SDHIs). QoIs have been protected by a recently discovered intron that renders resistant mutants unviable. Low levels of resistance have developed to DMIs, but with limited field significance. Older SDHI fungicides were inactive against rusts. Some of the SDHIs introduced since 2003 are active against rusts, so it may be that insufficient time has elapsed for resistance to develop, especially as SDHIs are generally sold in mixtures with other actives. It would therefore seem prudent to increase the level of vigilance for possible cases of resistance to established and new fungicides in rusts.

Homozygous and heterozygous point mutations in succinate dehydrogenase subunits b, c and d of Rhizoctonia cerealis conferring resistance to thifluzamide

“…The concept that non-haploid organisms would present limited ability for evolving resistance to pesticides is based on the assumption that the resistant allele of a resistance gene would be recessive. 41 R. cerealis and R. solani belong to binucleate and multinucleate Rhizoctonia species respectively. 30 In this context, TR mutants are readily available by UV mutagenesis in R. cerealis, and coexistence of homozygous and heterozygous TR mutants was observed.…”

A review of wheat diseases—a field perspective

“…In Australia, during the period 2003-2005, approximately AU$ 40-90 million per year was spent on chemical control to prevent stripe rust epidemics (Wellings, 2007). These regular chemical applications pose a potential risk of the reduction or loss of fungicide sensitivity (Arduim et al, 2012;Oliver, 2014).…”

The past, present and future of breeding rust resistant wheat

“…There are two ways to control rust in cereals, chemical control and genetic resistance. Genetic control has advantages for environmental and economic reasons, particularly for farmers in the developing world, and because of the possibility that rust pathogens develop resistance to fungicides (Oliver, 2014). When it comes to genetic resistance used by wheat breeders there are two general classes of genes based on their phenotypic effects, pathogen race- or strain-specific resistance (R genes) and adult plant resistance (APR) genes.…”