Site of action
Even after almost 30 years of research the mode of action of dicarboximides is still not certain. Recent evidence (for review see Yamaguchi & Fujimura 2005) suggests that they interfere with the osmotic signal transduction pathway consisting of histidine kinase and MAP kinase cascades.
Mechanism of Resistance
In B. cinerea and A. alternata , resistance has been attributed to mutations in the histidine kinase genes (Yamaguchi & Fujimura 2005; Dry et al. 2004; Oshima et al. 2002). Similar evidence that the osmoregulatory system is involved in resistance in Cochliobolus heterostrophus is given by Yoshimi et al. (2003).
Resistant isolates of B cinerea typically show either low, moderate or high levels of resistance according to laboratory assay. Low and moderate levels are normally associated with field isolates, but are still capable of causing disease control failure. Isolates rated as highly resistant in laboratory assay are rarely found in the field.
Cross-resistance exists between all dicarboximides although due to differences in activity spectra it may not appear as a perfect relationship with some fungi. In addition, fungi resistant to dicarboximides were shown to have various levels of resistance to aromatic hydrocarbon fungicides such as chloroneb and tolclofos-methyl, although such cross-resistance is not always present. In laboratory conditions some cross resistance to phenylpyrroles has been noted but this phenomenon has not been found in field isolates resistant to dicarboximides.
B. cinerea is a pathogen famous for its ability to become resistant to fungicides. Resistance to benzimidazoles (MBC fungicides) is common and can be easily accompanied by resistance to dicarboximides and to phenylcarbamates (e.g. diethofencarb). Double or triple multiple resistance is thus quite possible and must be considered when choosing disease control programs that include such chemistry. Please note that resistance to dicarboximides does not make fungi resistant to benzimidazoles or phenylcarbamates i.e. there is no cross-resistance between these chemical groups.
Persistence of Resistant Isolates
As a general guide it is accepted that the more resistant an isolate becomes to a dicarboximide fungicide, the less able it is to compete with non-resistant isolates in the wild. However, the picture may not be as definite as suggested. In a study published by Fourie and Holz in 2003 using isolates of B. cinerea and grapes, both sensitive and resistant germlings colonised fungicide free berries equally well, although sensitive strains penetrated the surface more often. Fitness decreased as resistance increased.
When using laboratory selected strains of Sclerotinia minor isolated from lettuce, Hubbard et al. (1997) found that the virulence of resistant strains generated from resistant sclerotia declined with age, occasionally resulting in complete loss of virulence. Similar results were found by Raposo et al. (2000) when looking at B. cinerea from tomatoes grown in glasshouses. Sclerotia of resistant isolates survived less well than sensitive ones. There was no relationship found between survival ability and resistance when mycelial survival was considered.
In contrast LaMondia & Douglas (1997) and Moorman & Lease (1992) report no change in fitness levels of B cinerea isolated from glasshouse crops in the USA. It is thus wise to assume that reduced fitness is not a phenomenon that can be relied upon as a component of a resistance management program.
Dry, I. B., Yuan, K. H., Hutton, D. G. (2004) Dicarboximide resistance in field isolates of Alternaria alternata is mediated by a mutation in a two component histidine kinase gene. Fungal Genet Biol 41 , 102 – 108.
Fourie, P. H., Holz, G. (2003) Fitness on grape berries of Botrytis cinerea isolates belonging to different dicarboximide sensitivity classes. South African Journal of ecology and viticulture, 24, 1 - 10.
Hubbard, J. C., Subbarao, K. V., Koike, S. T., (1997) Development and significance of dicarboximide resistance in Sclerotinia minor isolates from commercial lettuce fields in California . Plant Disease 81, 148 - 153.
LaMondia, J. A., Douglas, S. M., (1997) Sensitivity of Botrytis cinerea from Connecticut greenhouses to benzimidazole and dicarboximide fungicides. Plant Disease 81, 729 – 732.
Moorman, G. W., Lease, R. J., (1992) Benzimidazole and dicarboximide resistant Botrytis cinerea from Pennsylvania greenhouses. Plant Disease 76, 477 – 480.
Oshima, M., Fujimura, M. Banno, S., Hashimoto, C., Motoyama, T., Ichiishi, A., Yamaguchi, I. (2002) A point mutation in the two component histidine kinase BcOS-1 gene confers dicarboximide resistance in field isolates of Botrytis cinerea Phytopathology 92, 75 - 80.
Raposo, R., Gomez, V., Urrutia, T., Melgarejo, P. (2000) Fitness of Botrytis cinerea associated with dicarboximide resistance. Phytopathology 90, 1246 - 1249.
Yamaguchi, I., Fujimura, M. (2005) Recent topics on action mechanisms of fungicides. Journal of Pesticide Science 30, 67 – 74.
Yoshimi, A., Imanshi, J., Gafur, A., Tanaka, C., Tsuda, M. (2003) Characterisation and genetic analysis of laboratory mutants of Cochliobolus heterostrophus resistant to dicarboximide and phenylpyrrole fungicides. Journal of General Plant Pathology 69 , 101 – 108.