Inheritance of downy mildew resistance at cotyledon and adult-plant stages in ‘Couve Algarvia’ (Brassica oleracea var. tronchuda)

Abstract

The progeny of several crosses between the resistant ‘Couve Algarvia’ of Brassica oleracea var. tronchuda Bailey and susceptible hosts was evaluated with the objective of determining the mode of inheritance of resistance to downy mildew (DM) (Peronospora parasitica (Pers. ex Fr.) Fr.) at cotyledon and adult-plant stages and to find out whether the resistance observed at the two plant developmental stages was under the same genetic control. In an assay, conducted under controlled environment with artificial inoculation, the F2 of a cross between resistant ‘Couve Algarvia’ and susceptible ‘Penca de Chaves’ segregated 15 resistant:1 susceptible, indicating that ‘Couve Algarvia’ cotyledon resistance was controlled by two duplicate dominant genes. In a field assay under natural DM infection, the F2 of a cross between resistant ‘Couve Algarvia’ and susceptible ‘Couve de Corte’ segregated 3 resistant:1 susceptible, indicating that ‘Couve Algarvia’ adult-plant resistance was under the control of a single dominant gene. The 15:1 segregation at cotyledon stage and the 3:1 segregation at adult-plant stage were confirmed in the progeny of a cross between a susceptible doubled-haploid rapid-cycling B. oleracea line and the resistant ‘Couve Algarvia’. Besides, the inoculation of the same plants at cotyledon and adult-plant stages showed that the F2 segregation for resistance at cotyledon was independent from the F2 segregation for resistance at adult-plant stage, evidencing that ‘Couve Algarvia’ DM resistance at cotyledon and adult-plant stage was under the control of two different genetic systems. Therefore, field resistance cannot be predicted from cotyledon resistance.