A.D. Iwaro

Cacao resistance to Phytophthora: Effect of pathogen species, inoculation depths and pod maturity

Two species of Phytophthora (P. palmivora and P. capsici) and inoculations at two depths (3 mm and 9 mm) were tested each on 10 clones of Theobroma cacao to determine their effects on pod resistance. Ripe and unripe pods were also assessed to determine the influence of physiological status of the pod on the expression of resistance. The two pathogens tested (P. palmivora and P. capsici) differed significantly in their reactions on pods, with P. palmivora being more aggressive than P. capsici. However, the lack of interaction between clones and pathogen species and the similarity in the ranking of clones based on lesion size suggested that selection for resistant clones can be based on one of the two pathogens, preferably the more aggressive one. Pod reactions differed between inoculation depths (3 mm and 9 mm), and between pod maturity stages (ripe and unripe pods) with relatively larger lesions being recorded at 9 mm depth and on unripe pods as compared to those observed at 3 mm depth and on unripe pods, respectively. The magnitude of increase in lesion sizes, however, varied with genotypes, indicating that inoculation depth and pod maturity stage should be standardized in screening cacao germplasm for resistance to Phytophthora.

Sources of Resistance to Phytophthora Pod Rot at the International CocoaGenebank, Trinidad

The International Cocoa Genebank, Trinidad (ICG,T) contains about 2000 cacao (Theobroma cacao L.) accessions. The purpose of the collection is to provide a source of genes for the genetic improvement of cacao through breeding. In fulfilling this objective, it is imperative that this collection be evaluated to identify major sources of genes for the genetic improvement of important agronomic–economic traits. In this study, 816 cacao accessions were evaluated for resistance to Phytophthora pod rot (black pod disease), which causes enormous loss of yield in cocoa production throughout the world.The study identifies promising resistant genotypes that could be exploited in cacao breeding programmes. It further compares the levels of resistance to Phytophthora pod rot among two cacao types (wild and cultivated types), three major groups (Forastero, Trinitario and Refractario) and 11 accession groups (B, AM, CL, ICS, IMC, JA, LP, MOQ, NA, PA and TRD) . The distribution of scores for the 816 accessions showed skewness towards the susceptible end of the disease rating scale, indicating that a large proportion (68.9%) of the sample was susceptible (disease rating 6–8) to Phytophthora pod rot. However, 12.9% of the sample population was found to be resistant (disease rating 1–3) and 18.2% moderately resistant (disease rating 4–5). Significant differences were observed between the wild and cultivated accessions and among the Forastero, Refractario and Trinitario groups. Marked differences were also observed among 11 accession groups, each of which was represented by at least 20 genotypes. Higher proportions of resistant (17.7%) and moderately resistant (22.6%) genotypes were observed in the wild accessions than in the cultivated varieties (9.4% resistant and 14.4% moderately resistant). The Forastero group, consisting of many wild accessions, was found to contain more resistant (18.0%) and moderately resistant (23.1%) genotypes than either the Trinitario (4.8% resistant and 13.6% moderately resistant) or Refractario (11.3% resistant and 15.4% moderately resistant). Among 11 accession groups, the largest percentage of resistant (24.2%) and moderately resistant (28.8%) genotypes were found in the PA (Forastero). Different accession groups had varying proportions of resistant and moderately resistant genotypes. This is not unexpected in an out-breeding crop such as cacao with a high level of heterogeneity. This finding reinforces the idea of a pre-breeding programme (germplasm enhancement) to accumulate resistance genes over several populations as a strategy for improving the genetic base of resistance in national cacao breeding programmes. One hundred and five promising resistant genotypes were identified among the 816 accessions evaluated. These accessions are potential sources of resistance genes for the development of high-yielding resistant varieties in cacao.