Intra- and Inter-Season Stability of Cassava Plant Morphological Traits Associated with Host-Plant Resistance Against Cassava Green Mite in Zambia

Cassava genotypes that combine earliness with prolonged underground storability are most preferred for food security under subsistence farming. However, the long growth cycle of cassava coupled with the delayed harvesting by local farmers in Zambia exposes the crop to cassava green mite (CGM) attack which contributes to instability in yield performances of cassava. Various plant morphological traits have been recognized as direct or indirect defense mechanisms that enhance host plant resistance (HPR) to CGM. However, little research has been done to understand the stability of such traits despite their potential impact on the durability of HPR. With this background, field trials, involving sequential harvesting of cassava at 9, 12, and 15 months after planting (MAP) were conducted for two seasons. The objective of the study was to understand the variability of the indirect plant defense mechanisms, and how the interactions of genetic factors with crop age and season influence the expression of these vital traits. The genotype stability index was computed for each genotype for CGM population density and leaf damage, leaf retention, stay green, and apical leaf pubescence. There were highly significant differences among genotypes at different sampling dates for all the traits studied. Genotypes TMS 4 (2) 1425, L9.304/175, and L9.304/147 exhibited high intra-season and inter-season stability for low CGM-induced leaf damage. Genotypes Kapeza, Bangweulu and I60/42 exhibited a tolerance mechanism towards CGM. Two of these genotypes L9.304/175, and TMS 4 (2) 1425 also combined high intra-season and inter-season stability for increased leaf retention and apical leaf pubescence, while Kapeza and Bangweulu combined high inter-season stability for increased stay green and leaf retention. Genotypes that combined intra- and inter-season stability for both Low CGM population density and low CGM-induced leaf damage were also identified.