Detection of herbicide resistance in black-grass, Italian rye-grass and wild-oat at all growth stages

Abstract

Herbicide resistant black-grass was first detected in 1982, since then herbicide resistance has also been confirmed in Italian rye-grass and wild-oat. A need exists for a rapid, reliable and inexpensive method for the detection of herbicide resistance during the development of the crop. The aim of this HGCA-funded project was to develop a rapid bioassay for the detection of herbicide resistance in black-grass, Italian rye-grass and wild-oat at various growth stages. Confirmed susceptible (S) and resistant (R) biotypes of each species were used to define suitable dose-ranges to discriminate between R and S glasshouse grown propagules of each species. Soil was removed from the root systems of seedlings or rooted tillers, these were subsequently trimmed to 5 cm shoot and root prior to immersion in a range of herbicide doses. Dose-response curves were fitted and discrimination between biotypes was based on I50 values of length and fresh weight of new shoot growth.

Assays conducted on seedlings and tillers of all species were able to discriminate between R and S biotypes over a range of herbicide doses. Clear discrimination between Peldon (R) and Rothamsted (S) biotypes of black-grass were achieved with fenoxaprop-P-ethyl, isoproturon and chlorotoluron. Differences between two Italian rye-grass biotypes, Clev (R) and Halja (S) were achieved over a range of doses of fenoxaprop-P-ethyl and isoproturon. Fenoxaprop-Pethyl was also used to discriminate between R (T/11 and T/41) and S (LLUD) biotypes of wild-oat. Fresh weight was found to be the most accurate indicator of propagule response to herbicide dosage, especially for those assays conducted with Photsystem II inhibitor herbicides. Resistance to fenoxaprop-P-ethyl was detected in all species, even where resistance was partial due to enhanced metabolism.

The assays are suitable for detecting herbicide resistance until nodes are detectable on the weed species. Results obtained for resistance detection assays at later growth stages utilising cut stem node segments proved unsuccessful and it was concluded that tests using collected seeds would give more reliable results.

The robustness of the assays under contrasting temperature, humidity and light were examined using seedlings of S and R biotypes. Distinction between R and S biotypes was achieved under all conditions the assays were subjected to. The study also examined the effect of storage on propagules. Seedlings and tillers of all biotypes were packaged and stored at 1^oC for 31 and 29 days respectively and were found to still be viable after these time periods.