ABSTRACT

Field and green house trials were conducted to study the effects of fertilizers and herbicides on the reaction of cowpea varieties to Alectra voqelii (Benth.) at Samaru in the northern Guinea savanna ecological zone of Nigeria in 1991 and 1992 wet seasons and the reaction of cowpea varieties to Alectra seed concentration in the green house in 1992 and 1993 dry seasons. Cowpea variety B 301 and the derivatives of its crosses with IT84 S-2246-4 (IT 90 K-59 and IT 90 K-76) did not support Alectra emergence. The varieties also produced cowpea grain yields that were comparable to the maximum in the 1991 trial and significantly higher than the least in the two years. The nodulation of these varieties were however reduced in the pot by 21.57%, 43.9 and 76.9%. for B 301, IT 90K-59 and IT90K-76, respectively. However, IT 89 KD-245 and IT 89KD-245-1, both of which are derivatives of B 301 and local variety Kananado supported low emergence of Alectra. Inspite of high reduction of nodulation in the pot by Alec tra and support for low Alectra infection in all the trials, IT86D-534 did not suffer any reduction in cowpea grain yield in two field trials. Inspite of high infestation by Alectra and very low pod number Kano 1696 still produced acceptable grain yield. Cowpea varieties IT82D-849 and SUVITA-2 known to be resistant to Striqa qesnerioides, exhibited susceptibility to Alectra. Both TVx 3236 and SAMPEA 7, supported high Alectra infection with consequent high susceptibi1ty to the parasite in the field. In the green house V TVX 3236 initiated early emergence of Alectra and had more number of emerged Alectra compared with all the other varieties. Under artificial inoculation, all the varieties tested, including B 301, produced lower grain yield than VITA-3, the only variety in which Alectra infection did not cause cowpea grain yield reduction in 1992. In the evaluation of nitrogen and phosphorus combinations on the reaction of cowpea varieties to Alectra voqeli i , cowpea varieties TN93-S0 and IT89KD-245-1 significantly delayed Alectra emergence compared with VITA-3 and SAMPEA-7 in 1991 and 1992, respectively. Inspite of higher number of plants infected and Alectra shoot count, VITA-3 still produced cowpea grain yield comparable to the maximum tined by IT 89 KD-245-1 in 1992. Phosphorus application at 20 kg P/ha significantly increased cowpea grain yield without any significant effect on Alectra incidence. Nitrogen application at 60 kg M/ha consistently reduced the number of plants infected and Alectra shoot count, but resulted in cowpea grain yield depression compared with 0 and 30 kg N/ha. It was apparent from the 2-order interaction, that varieties which exhibited moderate (TN 93-80 in 1991) to high (IT89KD-245-1 in 1992) resistance responded to high doses of N (60 kg/ha) and P(40 Kg P/ha) by delaying the emergence of Alectra. In the two years, pre-emergence herbicide applications the mixtures containing Pree (metazachlor + antidote) followed by imazaquin at 0.18 kg a.i./ha resulted in significantly lower number of plants infected. Similarly, in 1991, Pendimetha1 in plus imazaquin at 0.87 + 0.15 and 1.09 + 0.19 kg a.i/ha and vi pendimethalin plus imazethapyr at 1.58 + 0.12 kg a.i/ha, all followed by imazaquin at 0.18 kg a.i/ha, resulted in lower number of cowpea plants infected by Alectra compared with the weedy check. In 1992, post-emergence application of Pree plus imazethapyr at 1.0 + 0.075 kg a.i/ha resulted in lower number of infected plants compared with the maximum obtained with pendimenthalin plus imazaquin at 1.09 + 0.19 and pendimethalin plus imazethapyr at 1.58 + 0.12, both followed by SHW which also resulted in significantly higher cowpea grain yield than the weedy check. In the green house, the reaction of cowpea varieties to preconditioning and concentration of seed inoculant of Alectra voqelii indicated that Alectra emergence increased wih Alectra seed concentrations in the first trial, while lOc.c/pot Alectra seed gave the highest number of emerged Alectra with compared to other Alectra seed concentrations in the second trial. Preconditioning period had no significant effect on number of days to Alectra emergence but Alectra emergence increased with preconditioning period of 7 and 14 days.