D3272: Evaluation of long bean (Vigna unguiculata ssp. sesquipedalis) breeding lines for aphid resistance

Long bean (Vigna unguiculata subsp. sesquipedalis) is grown on small-scale, diversified farms by Southeast Asian refugee farmers in central California. Farmer surveys and focus groups have identified the cowpea aphid (Aphis craccivora Koch) as a pest of importance that is managed with pesticide applications. To develop long bean varieties with resistance to aphids, marker-assisted backcrossing was performed to introduce two aphid-resistance quantitative trait loci (QTL) from California Blackeye 77, a new cultivar developed by UC Riverside, into three local long bean varieties as recurrent parents with market-preferred pod colors (light green, dark green, and purple). A field trial with early backcross generations was conducted in Parlier, CA to evaluate selected F2 progenies of the second backcross cycle (BC₂F₂) that carried or two resistance QTL haplotypes and had the highest recovery of recurrent-parent backgrounds. Fifteen BC₂F₂ lines were planted in a location known for high aphid pressure and rated for severity of aphid damage on leaves in comparison to susceptible long bean varieties, with a highly susceptible cowpea cultivar planted in spreader rows to attract aphids and encourage uniform infestation. Aphid damage symptoms were reduced by 41-47 % in the BC₂F₂ lines fixed for both QTLs compared to the susceptible local varieties. These results provide preliminary information to confirm the effect of resistance QTLs in long bean genetic backgrounds in early generations and will inform continuing efforts to develop advanced lines with fixed QTLs for aphid resistance as well as desirable agronomic and marketing traits.