Screening of segregating cocoa genotypes for resistance to vascular-streak dieback under natural conditions in Papua New Guinea

Abstract

Segregating progenies of five hybrid crosses of cocoa (Theobroma cacao) were screened in the field for resistance to Oncobasidium theobromae, the causal agent of cocoa vascular-streak dieback (VSD) disease, under natural infection conditions. Seedlings were individually assessed for VSD 11 months after planting and thereafter at monthly intervals for 10 months. Infection was evenly distributed within the plot, reaching 100% of the trees with infection symptoms at the end of the testing period. Crosses differed in the incidence of symptoms, the percentage of susceptible plants and their Susceptibility Index. Progenies of the Trinitario clone K82 were more susceptible than those of the clone KA2-106. High rainfall increased the rate of infection. Plants of the same cross ranged from highly resistant to highly susceptible. It was concluded that the conditions at the testing site were adequate for large-scale practical screening of cocoa genotypes for resistance to VSD. Guidelines for future testing are outlined. This study showed that there is great potential for further selection of cocoa clones from hybrid progenies with enhanced resistance to VSD.