Inbreeding depression for severity caused by leaf diseases in cassava

Abstract

The use of inbred populations in cassava has been little exploited by breeding programs. Therefore, the objective of this study was to evaluate the effects of inbreeding depression (ID) in S1 families for resistance to leaf diseases and to select transgressive individuals. Five cassava varieties (Cascuda, BRS Formosa, Fecula Branca, Mani-Branca and BRS Mulatinha) were selfed, and the families S0 and S1 were evaluated for severity caused by cassava white leaf spot (CWLS), cassava brown leaf spot (CBRLS) and cassava blight leaf spot (CBLS) under field conditions. Differences among S1 families and between generations (S0 × S1) were identified for all diseases. The wide variation of CWLS (2.16–93.38%), CBRLS (15.31–69.39%) and CBLS (15.70–53.64%) severity highlighted the occurrence of ID in S1 families, in which the average was 12.61% (CWLS), 13.72% (CBRLS) and 15.92% (CBLS). Mean estimates of homozygous loci contribution (µ + α) were of higher magnitude compared with heterozygous loci contribution (δ) for all diseases and families. Even in the occurrence of ID, it was possible to select resistant S1 individuals, in a way that the reduction in the severity of diseases based on the selection of the five most resistant individuals was 75.80% (S1-BRS Formosa) for CWLS, 38.64% (S1-Cascuda) for CBRLS and 33.51% (S1-Cascuda) for CBLS. These results demonstrate the predominance of additive genetic effects and the possibility to select transgressive individuals in S1 families for increasing the resistance to cassava leaf diseases.