Isolation and Bioassay of Striga Hermonthica Seed Germination Stimulants From Non-host Crops and Field Testing for Control Efficacy
Striga hermonthica (Del.) Benth. is a severe parasite on maize and a limiting factor in achieving optimum maize yields in infested areas in Africa. The parasite seeds are stimulated to germinate by root exudates from host and non-host plants as well as by synthetic germination stimulants. Objectives of these experiments were to determine laboratory, screenhouse and field conditions for inducing maximum germination of S. hermonthica seeds with extracts from cultivars of cowpea (Vigna unguiculata (L) Walp.), cotton (Gossypium spp.) and soybean (Glycine max. L.) plants, their residues and a synthetic germination stimulant, GR 24. Extracts of cowpea roots and S~lOOtS stimulated about 30 to 40% as much germination as 10-2 mgL-1 GR 24. No differences were observed among cowpea cuitivars in germination percent of S. hermonthica seeds induced by the aqueous extracts. The population from Mokwa responded most to the extracts. Increasing concentrations of both whole cowpea plant extracts and GR 24 resul ted in increased S. hermonthica seed germination upto 6.3mg plant tissue mL-1 of water and 10-2 mgL -1 respectively, beyond which germination decreased. Higher germination was recorded when incubation time of S. hermonthica seeds with GR 24 at 30°C was increased from 24 to 48 hours in microtiter wells. Storing plant extracts at 7°C for 20 days did not reduce potency. Germination of S. hermonthica seeds by cotton and cowpea plant parts were dependent on the source of the stimulant, weight of plant part and the distance of the parasite seeds from the source of the stimulant. Germination of S. hermonthica seeds decreased with increase in weight of plant part and period of time each cultivar was grown. Cotton roots contained the most effective S. hermonthica seed germination stimulant among the plant materials tested. With both S. hermonthica and S. aspera, mixtures of cotton extracts from different plant parts depressed germination of the seeds below the average induced by individual extracts in the mixture. Some extracts and mixtures were as effective as 10-2mgL-l of GR 24. DCM soluble extracts of cottoq and cowpea plant parts of different ages separated by thin layer chromatography showed that the extracts contained several similar compounds as shown by position of relative fronts. DCM extracts of S. hermonthica seeds contained similar compounds. Germination of S. hermonthica seeds by DCM soluble extracts of cotton and cowpea roots increased with increasing concentration from 0.1 to 0.5 % and 0.1 to 1.0 % for cotton and cowpea respectively and then declined with further increase in concentration. Soybean extracts from TGX-1674-1F was as effective as 10-3 mgL'l GR 24 in inducing germination of S. hernJonthica seeds collected from sorghum in Zaria, Mokwa and Abuja in 1991. Other very effective cultivars were TGX-1660-18F and TGX-1674-8F from Zaria and Mokwa respectively. Screenhouse experiments indicated that growing cotton (Abuja Local) or cowpea (Tvx 3236) in S. hermonthica infested soil the previous season significantly reduced parasitism on maize planted the following season and increased yields. Unemerged attached, emerged, total attached S. hermonthica and root dry weight were positively correlated with symptom severity on maize. All parameters were negatively correlated with grain weight, and all, except stem dry weight were negatively correlated with harvest index. Unemerged attached, emerged and total attached parasites on each maize plant decreased with increasing length of cotton or cowpea growth the previous season. No significant advantage was obtained by addition of fresh cotton or cowpea mulch. Pot experiments with crop residues indicated that at all incubation periods of cowpea and soybean residues increasing weight of residue delayed S. hermonthica emergence, reduced total number of parasites attached per maize plant and increased plant height and total dry matter yield of maize. At all residue levels, incubation period of the parasite seeds and the residues for at least 7 days was required to significantly reduce parasitism of S. hennonthica on maize. Field experiments showed that all rotations significantly reduced the number of attached parasites on maize plants and increased grain yields the following season. Addition of residues from Tvx 3236 reduced parasitism of the parasite on maize and increased yields.