Assessment of Resistance to Witches'-Broom Disease in Clonal and Segregating Populations of Theobroma cacao

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Abstract
Fourteen clonal genotypes of cacao (Theobroma cacao) and their open-pollinated progenies were tested for their resistance to witches'-broom disease (Crinipellis perniciosa) using an agar-droplet inoculation method in separate, replicated greenhouse experiments. Nine measures of resistance were evaluated for precision, ability to discriminate between levels for resistance, and their correlation to field resistance. Correlations among the various measures of resistance were also ascertained. Data were re-examined to determine the accuracy and precision associated with individual plant measurement as well as the optimum sample size to assess segregating populations for resistance. The agar-droplet method was able to effectively discriminate various levels of resistance. Incubation period provided a precise, sensitive, valid, and heritable measure of resistance to C. perniciosa. Furthermore, incubation period was highly correlated with broom size and moderately correlated with broom frequency, two epidemiologically important components of resistance. The study also shows that 25 to 30 open-pollinated progeny are sufficient to provide a good estimate of a parent's resistance. The implications of the findings for identifying resistance among cacao germplasm collections as well as within segregating populations for resistance breeding are discussed.