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Phenolic content, germination percentage, and host preference were compared among 11 seed populations of witchweed (Striga hermonthica(Del.) Benth. # STRHE). Significant differences in germination percentage occurred among the seed populations after preconditioning by their exposure to different durations of warm-moist conditions. Greatest percentage of seed germinated after 8 days of preconditioning. Preconditioning for more than 16 days reduced percentage germination. The host specificity range of witchweed seed populations that parasitized pearl millet [Pennisetum americanum(L.) K. Schum] was broader than were those seed populations that attacked sorghum [Sorghum bicolor(L.) Moench.]. Results of thin-layer chromatography indicated large variations in phenolic content among the witchweed seed populations.

The dispersion of ethylene in the soil after appli- cation from a backpack ethylene applicator (BPEA) was determined using a bioassay of witchweed (Striga asiatica (L.) Kuntze #4 STRLU) seed germination. Witchweed seed were buried in packets in ridged seedbeds under field condi- tions. 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 in- jections 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 hori- zontally and to 75 cm vertically to cause at least 45% seed germination in a field of Lakeland soil (Typic Quartzisamn- ment). The BPEA was found to be an effective application tool in witchweed control operations. Additional index words. Germination stimulants, parasitic weed, Striga asiatica, witchweed, STRLU.

Increasing plant density of grain sorghum [Sorghum bicolor (L.) Moench.] from 57100 to 114300 plants/ha had no significant effect on grain yields, but significantly increased Striga hermonthica (Del.) ‘Benth’ # STRHE shoot yields by 49% in S. hermonthica infested soil of the Sudan. Addition of urea at 43, 86, and 172 kg N/ha increased grain yields by 14, 25, and 45%, respectively, above the control and S. hermonthica shoot yields were reduced by 14, 21, and 35%, respectively, below the control. Although average grain yields in 1981 were 77% less than in 1980, S. hermonthica shoot yields were significantly increased by about sixfold. Grain sorghum yields of pure stands were not consistent over both years. In 1980, the cultivar ‘Ariana’ outyielded ‘Tetron’, but in 1981 the reverse occurred. However, in both years, the 50:50 mixture of both cultivars gave intermediate grain yields. While pure stands of Tetron’ sustained the lowest infestations of S. hermonthica and ‘Ariana’ had the greatest, the mixture had intermediate infestations which were on average 31% less than those of ‘Ariana’. Thus, the results suggest that the cultivation of equiproportional mixtures of relatively resistant and susceptible grain sorghum varieties, coupled with the use of high rates of nitrogenous fertilizer and wider inter- and intra-row spacing, may be a better cultural practice under dense striga infestations.

Witchweed (Striga asiaticaLour. ♯4STRLU) 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.

Seed size and weight significantly affected witchweed [Striga asiatica(L.) Kuntze =S. luteaLour. ♯4STRLU] 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 maysL.) 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.

Ethylene gas stimulated seed germination of the pearl millet [Pennisetum americanum(L.) K. Schum] strain ofStriga hermonthica(Del.) Benth. # STRHE) in two soil types in the North Kordofan Region of Sudan. Soils were the noncracking clay pediplain or ‘Gardud’, in Arabic, and the stabilized sand locally called ‘Qoz’. Ethylene at 1.5 kg ai/ha induced greater seed germination (suicidal seed germination) in the Gardud soil than in the Qoz soil. However, ethylene at 3.0 kg ai/ha was better on Qoz than Gardud soil. The effect of injection depth of ethylene was dependent on soil type. Ethylene was more effective when injected at a 15-cm depth in the Gardud soil and at a 30-cm depth in the Qoz soil. The viable population ofS. hermonthicaseed was reduced by suicidal seed germination at soil depth to 60 cm and to a horizontal distance of 25 cm from the point of ethylene injection. Ethylene-induced germination reduced seed population by 67 and 34% in Gardud and Qoz soils, respectively. A single injection of ethylene diffused up to 300 cm horizontally and up to 60 cm vertically and caused 12.6 and 8.5% reduction in viable 5.hermonthicaseed in Gardud and Qoz soils, respectively. The potential of ethylene gas as an effective tool in striga control operations in a pediplain and stabilized sand soil such as is found in N. Kordofan is dependent upon the availability of the gas and the injection apparatus and the economics of its application.