Do we have the tools to manage resistance in the future?

Pesticide resistance is a major factor affecting world food and fibre production, but that has been contained so far by the availability of diverse modes of action. Overcoming resistance by switching to a new mode of action is a concept easily grasped by growers but threatened by losses through resistance and new registration requirements. Opportunities for innovation and development of a diversity of novel modes of action exist through harnessing recent advances, fundamental to all eukaryotes and largely funded for medical rather than agricultural objectives, in understanding cell biology and development. The cystoskeleton, cell wall synthesis, signal transduction and RNAi are discussed as examples where new targets are now exposed. However, new modes of action will be delivered not only by sprayer or seed treatment but also through transgenic crops, although these still need to be transferred from experiment to practice. Improvements in modelling protein structures and target-site changes, supplemented by rapid diagnostics to detect resistance early, will improve resistance risk management and integrate chemical, biopesticide, transgenic and conventional breeding around the concept of diversity in modes of action. However, before agronomy can translate this into practical antiresistance strategies, there is a need to direct more resources to the biochemistry and cell biology of pests, diseases and weeds to translate these new discoveries into key tools needed to manage resistance in the future.     

Does agricultural use of azole fungicides contribute to resistance in the human pathogen Aspergillus fumigatus?

Azole resistance in human fungal pathogens has increased over the past twenty years, especially in immunocompromised patients. Similarities between medical and agricultural azoles, and extensive azole (14α‐demethylase inhibitor, DMI) use in crop protection, prompted speculation that resistance in patients with aspergillosis originated in the environment. Aspergillus species, and especially Aspergillus fumigatus, are the largest cause of patient deaths from fungi. Azole levels in soils following crop spraying, and differences in sensitivity between medical and agricultural azoles (DMIs), indicate weaker selection in cropping systems than in patients receiving azole therapy. Most fungi have just one CYP51 paralogue (isozyme CYP51B), but in Aspergillus sp. mutations conferring azole resistance are largely confined to a second paralogue, CYP51A. Binding within the active centre is similar for medical and agricultural azoles but differences elsewhere between the two paralogues may ensure selection depends on the DMI used on crops. Two imidazoles, imazalil and prochloraz, have been widely used since the early 1970s, yet unlike triazoles they have not been linked to resistance in patients. Evidence that DMIs are the origin, or increase the frequency, of azole resistance in human fungal pathogens is lacking. Limiting DMI use would have serious impacts on disease control in many crops, and remove key tools in anti‐resistance strategies. © 2017 Society of Chemical Industry     

Interactions in the Brassica napus–Pyrenopeziza brassicae pathosystem and sources of resistance to P. brassicae (light leaf spot)

Pyrenopeziza brassicae, cause of light leaf spot (LLS), is an important pathogen of oilseed rape and vegetable brassicas and has a wide geographic distribution. Exploitation of host resistance remains the most sustainable and economically viable solution for disease management. This study evaluated 18 oilseed rape cultivars or breeding lines for host resistance against P. brassicae in glasshouse experiments. Selected cultivars/lines were inoculated with eight single-spore isolates of the pathogen obtained from three different regions in England. Analysis of P. brassicae infection-related changes on host plants identified leaf deformation as a characteristic feature associated with P. brassicae infection, this showed poor correlation to LLS severity measured as the amount of pathogen sporulation on infected plants. Resistant host phenotypes were identified by limitation of P. brassicae sporulation, with or without the presence of a necrotic response (black flecking phenotype). Investigation of this pathosystem revealed significant differences between cultivars/lines, between isolates, and significant cultivar/line-by-isolate interactions. In total, 37 resistant and 16 moderately resistant interactions were identified from 144 cultivar/line-by-isolate interactions using statistical methods. Most of the resistant/moderately resistant interactions identified in this study appeared to be nonspecific towards the isolates tested. Our results suggested the presence of isolate-specific resistant interactions for some cultivars. Several sources of resistance have been identified that are valuable for oilseed rape breeding programmes.     

Susceptibility to neonicotinoids and risk of resistance development in the brown planthopper, Nilaparvata lugens (Stål) (Homoptera: Delphacidae)

BACKGROUND: In recent years, outbreaks of the brown planthopper, Nilaparvata lugens (Stål), have occurred more frequently in China. The objective of this study was to determine the susceptibility of N. lugens to neonicotinoids and other insecticides in major rice production areas in China. RESULTS: Results indicated that substantial variations in the susceptibility to different insecticides existed in N. lugens. Field populations had developed variable resistance levels to neonicotinoids, with a high resistance level to imidacloprid (RR: 135.3–301.3‐fold), a medium resistance level to imidaclothiz (RR: 35–41.2‐fold), a low resistance level to thiamethoxam (up to 9.9‐fold) and no resistance to dinotefuran, nitenpyram and thiacloprid (RR < 3‐fold). Further examinations indicated that a field population had developed medium resistance level to fipronil (up to 10.5‐fold), and some field populations had evolved a low resistance level to buprofezin. In addition, N. lugens had been able to develop 1424‐fold resistance to imidacloprid in the laboratory after the insect was selected with imidacloprid for 26 generations. CONCLUSION: Long‐term use of imidacloprid in a wide range of rice‐growing areas might be associated with high levels of resistance in N. lugens. Therefore, insecticide resistance management strategies must be developed to prevent further increase in resistance. Copyright © 2008 Society of Chemical Industry