Alternaria helianthiand Imazaquin for Control of Imazaquin Susceptible and Resistant Cocklebur (Xanthium strumarium) Biotypes

The efficacy of Alternaria helianthi on imazaquin-susceptible and -resistant biotypes of common cocklebur was evaluated with and without imazaquin. A. helianthi caused severe damage to both biotypes when applied at 80,000 conidia ml −1 at the five- to six-leaf stage. Symptoms included necrotic lesions, stunting, and mortality. Mortality from imazaquin at 140 g ai ha −1 was 0 and 100% and mortality from A. helianthi at 80,000 conidia ml −1 was 33 to 75% and 67 to 100% for resistant and susceptible biotypes, respectively, and surviving plants showed reduction in height, dry weight, and number of new leaves. The combination of imazaquin at 18 g ai ha −1 and A. helianthi at 40,000 conidia ml −1 was not synergistic or antagonistic in killing common cocklebur.