Four bioassays were developed under greenhouse and laboratory conditions to identify Euphorbia heterophylla (EPHHL) biotypes with multiple resistance to PROTOX and ALS-inhibiting herbicides. In the greenhouse bioassays, plants of a susceptible (S) biotype and of two biotypes suspected of multiple resistance (#4 and #23) were sprayed using different levels of the herbicides imazethapyr and fomesafen. The laboratory bioassays consisted of a comparative test between biotypes S and #4, exposed to germination in imazethapyr and fomesafen solutions. Dose-response relationships were adjusted using logistic and polynomial models, respectively, for data obtained from greenhouse and laboratory bioassays. For biotype #4, the resistance factor (RF) to imazethapyr was above 24, while for biotype #23 it was 15. FR to fomesafen were, respectively, 62 and 39, for the same biotypes. In the laboratory bioassay, a period of 144 hours was sufficient to discriminate differencial growth response to imazethapyr and fomesafen levels between R and S biotypes. The results confirm that both greenhouse and laboratory tests with Petri dishes are appropriate methodologies to discriminate S and multiple R EPHHL biotypes.
methodology; weed; resistance to herbicides
