General combining ability of selected black pod disease resistant cocoa(Theobroma cacao L.)hybrids

Abstract

Cocoa originated in the Amazon riverbasin of South America. The cultivation of cocoa had extended from Mexico to Costa Rica and over the time it had spread to Caribbean and other parts of South America (Wood and Lass» 1985). It was introduced to India during 1979. The generic name Theobronia was derived from two greek words 'Theo' means god and 'hroma' means food, hence known as '"food of god". The beans are the only source for chocolate. Cocoa is affected by many biotic and abiotic stresses. Among the biotic stresses Phytophthora pod rot is the most serious one affecting cocoa, leading to a total of 64% yield loss (Adomako, 2007). Therefore the control of black pod is a major challenge for cultivation of cocoa. The farmers adopt several measures to control the disease of which the use of copper based fungicides is the most predominant one (Tan and Tan, 1990). Although it is reasonably effective, their indiscriminate use poses serious environmental issues. To overcome this situation breeding for resistance to black pod is the most economical, environmental friendly and effective control method (Iwaro et al., 2004). Hence an organized breeding progianmiewas initiated at Cocoa Research Center (CRC), KAU, Vellanikkara during 2005 for the development of varieties resistance to black pod disease and high yield. Twenty five hybrids showing considerable level of resistance after 6 years of screening were selected for the present study. The morphological evaluation ot these hybrids were earned out based on eight qualitative and eight quantitative characters.Variability was observed among the hybrids for all the qualitative characters and quantitative characters. Among the hybrids, hybrid H22 recorded the highest pod weight (724g) and total wet bean weight per pod (176.98g). Tlie husk thickness and number of ilat beans were lowest for hybrid H16 (0.86 cm) and HII (1.40). Self incompatibility was assessed by self pollinating 100 flowers per hybrids. Twenty two hybrids turned to be self incompatible, one self compatible and rest with insufficient number of flowers. Out of 22 self incompatible hybrids eleven were selected for further study based on their yield (No. ol pods/ tree/ year). They were crossed with tester (G.I 5.9) in top cross model and only four yielded fruits. Seedlings were raised in the nursery and observations were recorded in the third month. The analysis of variance of top cross showed significant variation for two characters. The characters like height, diameter, chlorophyll and number of leaves did not express any significant difference. HD2is considered as an indicator of initial vigour and it is found to be directly correlated to the final vigour.The significant value for variance (p Vs c) for HD2 indicated substantial difference between the parents as agroup and their hybrid progenies as another group. This shows that the average hetero.sis is significantly high. Seven hybrids out of selected eleven did not yield any pod on cross pollination, progenies developed from open pollinated pods of these hybrid.s were evaluated to predict the performance of the parent. Progenies expressed high HD2 value. Based on this, these parents can be considered as superior ones. However this has to be confirmed further by crossing them with another tester. Nursery screening for Phytophthora resistance of top cross progeny and open pollinated progeny of all eleven selected hybrids were done at two weeks interval for three months. Lab screening for disease resistance was carried out in semi translucent leaves from all the progenies raised through top cross and open pollinated method along with control for eight days by providing artificial inoculation and percentage of infection was calculated. No disease incidence was noticed in any progenies indicating high GCA for this character. H 17 found to be superior combiner can be evaluated in comparative yield trial. H4, HIS and H8 were selected as superior ones based on the performance of open pollinated progeny which has to be further evaluated with another tester.