Earliness and Crop Morphological Traits Modulate Field Pest Infestation in Green Gram

Breeding has significantly improved drought tolerance in green gram but marked yield losses continue due to damage by insect pests. Important pests of green gram are pod borers, Maruca vitrata (F.), aphids, Aphis gossypii Glover and whiteflies, Trialeurodes vaporariorum (Westwood). Management of these pests has partly been constrained by the limited understanding of crop traits that modulate their infestation. Field experiments were conducted in southeastern Kenya to evaluate a collection of old and new green gram varieties for tolerance to field pests, and to identify traits that confer resistance. The old varieties were KS20 and N26, both released in 1990s whereas the modern counterparts were Biashara, Karembo, and Ndengu-Tosha. Results showed significant differences among the varieties in maturity, leaf area, leaf hair density, leaf moisture content, and pod wall thickness. Earliness significantly reduced pest infestation, whereby KS20 matured early while N26 was late, and the new varieties were intermediate. On average, across the three environments, leaf area (R2 ≥ 0.32) and leaf moisture content (R2 ≥ 0.18) positively correlated with pest infestation while leaf hair density (R2 ≥ 0.30), and pod wall thickness (R2 ≥ 0.54) showed a negative association with pod borer and aphid counts. However, results did not reveal any particular traits that associated with the varieties, which implied that breeding of green gram in Kenya has not selected for tolerance to field pests. Nonetheless, green gram field pest management practices could select for varieties with early maturity, open plant canopy, pubescent leaves, and thicker pod walls.