The dispersion of ethylene in the soil after application from a backpack ethylene applicator (BPEA) was determined using a bioassay of witchweed [Striga asiatica (L.) Kuntze # STRLU] seed germination. Witchweed seed were buried in packets in ridged seedbeds under field conditions. Ethylene dispersion was better when injected into the furrow than into the ridge top. Injections 30 cm deep were more effective in causing seed germination than injections 15 cm deep. Seed germination induced by ethylene injection into the furrow 30 cm deep was 13% more than that obtained from the ridge top injection 15 cm deep. Seed germination as high as 90% was obtained at soil depths to 75 cm at a horizontal distance of 100 cm from the point of ethylene injection. Ethylene diffused up to 300 cm horizontally and to 75 cm vertically to cause at least 45% seed germination in a field of Lakeland soil (Typic Quartzisamment). The BPEA was found to be an effective application tool in witchweed control operations.

Witchweed (Striga asiatica Lour. ♯4 STRLU) seed remained viable for 6 yr under open-shelf laboratory conditions and for 14 yr when deep buried under field conditions. Seed kept in normal atmosphere of the laboratory for < 1 through 6 yr had the following germination percentages: 2, 95, 95, 94, 83, 38, and 7. Newly harvested seed will not germinate because of postmaturity ripening requirements. There were no viable seed after 7 yr of shelf storage in the laboratory. Seed buried in the field gave 60 to 90% germination the first season after production. Annual recovery and testing showed a gradual reduction in viability to 10% at year 14 for the deepest buried seed; shallower buried seed expired sooner. No germination occurred thereafter.

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.

Twelve analogs of the strigol D-ring (butenolides) were evaluated at 0.1 mM as germination regulators of 26 weed and crop seeds. In general, the butenolides either inhibited or did not affect monocot germination, although the hydroxybutenolide stimulated germination of dormant cheat and nondormant perennial ryegrass and sorghum seeds. The response rate for dicots was greater but still primarily inhibitory. Dormant lettuce seeds (both light-sensitive and light-insensitive), Palmer amaranth, and redroot pigweed showed several significant responses, both inhibitory and stimulatory. Some of the butenolides that had not been previously assayed against witchweed were tested as 10-4 to 10-11 M dilution series. The allyloxybutenolide demonstrated activity comparable to strigol (30 to 60% germination at 10-6 to 10-11 M); the sec-butyloxybutenolide was somewhat less active (20 to 40% germination) but still effective at 10-9 M. In general, the butenolide analogs of strigol were less effective in stimulating seed germination than the multiring analogs.

Leaching of dl-strigol at six rates between 0.3 and 4.5 kg/ha through columns of sandy loam soil was estimated using a witchweed [Striga asiatica (L.) Kuntze # STRLU] germination bioassay. After 21 days of leaching daily with 1.27 cm of simulated rainfall, about 86% of the applied chemical remained in the top 2.5 cm of the soil, about 6% in the zone between 2.5 and 7.5 cm, and less than 1% in the soil at depths between 7.5 and 30 cm. However, even with the lowest rate of the chemical application, sufficient dl-strigol was leached to a soil depth between 22.5 and 30 cm to cause most, if not all, of the witchweed seeds to germinate. These results suggest that dl-strigol has a potential for use as an effective tool for a witchweed control or eradication program. No significant degradation of the chemical occurred in moist soil during the 21-day period.

Witchweed, [Striga asiatica (L.) Kuntze5 STRLU], an important parasitic weed throughout much of the old world, occurs as a recent (30 yr) introduction in a limited region in North and South Carolina. Reproduction in this population has been shown to occur primarily through self-pollination of flowers. Genetic diversity in two populations from this region was assessed, utilizing starch gel electrophoresis and staining of enzymes. An average of 64 individuals was observed to be monomorphic at an estimated minimum of 32 genetic loci coding 18 enzyme systems. This lack of polymorphism is attributable to the probable founding of the population by one or a few individuals coupled with the predominantly autogamous breeding system.

Seed size and weight significantly affected witchweed [Striga asiatica (L.) Kuntze = S. lutea Lour. ♯4 STRLU] seed germination, witchweed emergence, and host-parasitization. Witchweed seed were divided into four groups for testing; large (> 149 microns in diameter) and heavy (specific gravity greater than one) (LH), large and light (specific gravity less than one) (LL), small (< 125 microns in diameter) and heavy (SH), and small and light (SL). Germination percentages were 87, 84, 58, and 26 for LH, LL, SH, and SL, respectively. Corn (Zea mays L.) was planted in pots containing soil infested with these four seed categories. The total number of witchweed plants (submerged plus emerged) per pot containing LH, LL, SH, and SL witchweed seed was 65, 12, 0, and 0, respectively, and the number of witchweed plants that emerged was 19, 4, 0, and 0 per pot. There were no significant differences in shoot weights of corn grown in the pots either between kinds of witchweed seed infesting the soil or between plants grown in infested soil and plants grown in soil that did not contain witchweed seed. Roots of corn plants grown in soil infested with LH seed were significantly heavier than those grown in soil infested with any other seed class and heavier than those from soil that did not contain witchweed seed. No other seed class significantly affected root weight.

Hydrophobic exudate from roots of sorghum [Sorghum bicolor (L.) Moench. #3 SORVU ‘IS 8768’] contain four p-benzoquinones which in the dihydroquinone form are active as germination stimulants of witchweed [Striga asiatica (L.) Kuntz. # STRLU]. The three minor p-benzoquinones were partially characterized and found to be structurally similar to sorgoleone, the major p-benzoquinone of this exudate. Herbicidal activity of the hydrophobic exudate was due to concentration- and pH-dependent inhibition of root elongation in some but not all weeds tested. Witchweed has apparently adapted these “defense” compounds of sorghum as host-specific germination stimulants.