Abstract
Black pod disease (BPD) is the major cocoa pathology constraint caused by an Oomycete, Phytophthora megakarya prevailing in African cocoa producing countries. The development of T. cacao planting material tolerant to BPD lies on cross-pollination of adequate parental genotypes which generate offspring with desirable traits. We assessed the susceptibility to BPD of offspring derived from three manual crosses (SNK13xUPA143, T79/501xUPA143 and UPA143xSNK64) using leaf discs test (LDT) which evaluates disease scores of hybrid genotypes. Cysteine involvement in T. cacao defence process against BPD has been studied for the potential use of this sulphur amino acid profile to identify tolerant cocoa hybrid genotypes. LDT displayed variable disease scores patterns. Within a given family, LDT revealed heterogeneity in disease scores. This heterogeneity may have been derived from polygenic character of T. cacao susceptibility to BPD. In SNK13xUPA143 family, 26.13% exhibited disease scores lower than two (tolerant hybrid genotypes). T79/501xUPA143 and UPA143xSNK64 generated 21.18 and 26.46% of tolerant hybrid genotypes respectively. This variability between families may have resulted from deferential susceptibility of parental clones; UPA143 being more susceptible than T79/501 followed by SNK13. SNK64 was the less susceptible to BPD. The percentages of tolerant hybrid genotypes in this study appeared to be consistent hence, SNK13xUPA143, T79/501xUPA143 and UPA143xSNK64 could be used to produce hybrids genotypes tolerant to BPD. Cysteine analysis was performed in none detached healthy and wounded-infected leaves of hybrid genotypes with variable disease scores. It appeared that cysteine was mobilized during the infection. Two main patterns were observed: in tolerant hybrid genotypes (disease score lower than 2), infection was associated to a significant reduction of cysteine content in young leaves while in susceptible hybrid genotypes, infection was associated to an increase (accumulation) of cysteine in young leaves. Cysteine could be involved in the synthesis of effective defence molecules against P. megakarya in tolerant hybrid genotypes. This set of finding may indicate that cysteine profile could be used to discriminate tolerant from susceptible hybrid genotypes of T. cacao to P. megakarya. Key words: Cocoa tree, black pod disease, hybrids genotypes, tolerance, cysteine.
Highlights
Chocolate tree (Theobroma cacao L.) is a tropical rainforest plant widely cultivated in Africa, America and Asia (Alverson et al, 1999; Whitlock et al, 2001; AceboGuerrero et al, 2012; Ngoh Dooh et al, 2015)
Germination of seeds from mature pods derived from ♀UPA143x♂SNK64 (AHK3), T79/501xSNK13 (AHK) and SNK13xUPA143 (KHA) families were monitored during the 12 days after seeding
KHA family germination rate appeared to be lower than AHK and AHK3 families
Summary
Chocolate tree (Theobroma cacao L.) is a tropical rainforest plant widely cultivated in Africa, America and Asia (Alverson et al, 1999; Whitlock et al, 2001; AceboGuerrero et al, 2012; Ngoh Dooh et al, 2015). There is approximately 50 - 80% of cocoa production harvest losses because of an oomycete, Phytophthora megakarya, the pathogen of black pod disease in Cameroon (Djocgoue et al, 2010; Mfegue, 2012). Pesticides are used to reduce the incidence of black pod disease (BPD). These chemicals appeared to be environmentally unfriendly, expensive, and inefficient (Sonwa et al, 2008). Breeding for tolerance to BPD appeared to be the way out for sustainable cocoa culture (Iwaro et al, 2005; Ramalho et al, 2012; Bohinc and Trdan, 2012). Generative breeding is based on the aptitude of a giving couple of parents (genotypes or clones) to generate offspring with desirable traits (Eskes and Lanaud, 1997). Many couples have not yet been assessed in this purpose, such as SNK13xUPA143, T79/501xSNK13 and UPA143xSNK64
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