Inheritance of resistance to Pythium dieback in the medicinal plant periwinkle

Inheritance of resistance to dieback, caused by Pythium aphanider‐matum, in periwinkle (Catharanthus roseus) was studied using a dieback‐resistant dwarf mutant, Dw, with a green stem, and a susceptible accession, OR, with a purple stem. Plants of parental F₁, F₂ and backcross generations were evaluated for resistance to dieback in a randomized complete block design with four replications, in a field plot that had been continuously planted with periwinkle in previous years and where dieback severity was high. Resistance to dieback was evaluated on two dates, i.e. when distinct differences between plants of the parental strains were noticed. From both quantitative and qualitative analyses of data, resistance to dieback appeared to be governed by a single gene (with a broad‐sense heritability of 0.85 and 0.79, depending on the date of evaluation), and was inherited independently of genes governing dwarfness and stem pigmentation.