Control of parasitic witchweeds (Striga spp.) on corn (Zea mays) resistant to acetolactate synthase inhibitors

Parasitic witchweeds inflict most of their damage while still underground and attached to crop roots. Most selective translocated herbicides are detoxified by crops such as corn and thus cannot reach the attached parasites. Corn with target site resistance to acetolactate synthase (ALS)-inhibiting herbicides was tested to ascertain whether these herbicides could control witchweeds, assuming that witchweeds do not obtain amino acids from the crop. Postemergence directed sprays of 27 g ae ha(-1) imazapyr 54 d after planting (DAP) delayed Striga asiatica emergence on corn in South Carolina from 3 wk (control) to 7 wk and to 11 wk when mixed with 45 g ae ha(-1) AC 263 222. Treatments with up to 71 g ae ha(-1) imazamox, and up to 71 g ae ha(-1) AC 263 222 only delayed Striga emergence by 1 wk, and 71 g ae ha(-1) imazethapyr was ineffective. ALS-inhibiting herbicides were far more effective when applied in 1-ml drenches above the seed at planting. Chlorsulfuron (10 g ai ha(-1)) and sulfometuron (50 g ai ha(-1)) were somewhat phytotoxic to Pioneer 3245IR. Rimsulfuron (30 g ai ha(-1)), metsulfuron (10 g ai ha(-1)), halosulfuron (120 g ai ha(-1)), and imazethapyr (140 g ae ha(-1)) were marginally active in Kenya, with some mature Striga hermonthica seed.-bearing capsules appearing at harvest (12 wk). Imazapyr at 15 g ae ha(-1) gave 70 to 95% suppression of capsule formation, whereas no capsules appeared at 30 g ae ha(-1). The use of imazapyr in Kenya increased the harvest index by 17% when corn plants in Striga-infested soils were kept insect and disease free by using insecticides and fungicides. Thus, complete control can be achieved at affordable cost by farmers in subsistence conditions.