Studies on the repeatability and reliability of Phytophthora resistance screening tests carried out in Cameroon

Phytophthora pod rot (Ppr) disease caused by P. megakarya is the major constraint to cocoa production in Cameroon. The development of resistant varieties requires the use of effective resistance testing methods. The repeatability and reliability of the leaf disc and detached pod tests, as applied in selection activities in Cameroon, were studied. Repeatability of the tests was estimated by calculating the correlation (r) between resistance scores of clones, progenies and individuals within progenies in different inoculations rounds of the leaf disc and detached pod tests. Such correlations were generally significant for both tests. For the leaf disc test, as expected, correlations were lower for individuals within seedling progenies than for the average of seedling progenies or of clones. This suggests that a higher number of replicate observations are required for correct evaluation of individual seedlings than for evaluation of the average level of resistance of progenies. Observations carried out 5 or 7 days after inoculation were highly correlated, suggesting that scoring in the leaf disc test may be done only once at 5, 6 or 7 days after inoculation. In one experiment the ranking of leaf disc and detached pod inoculation test results could be compared statistically, with data being significantly correlated (r=0.78). The reliability of the tests was evaluated by correlating results of the inoculation tests with the level of field attack. These correlations were generally positive and significant, both for the leaf disc and detached pod test. A variation between mean scores of 5 and 8 in the detached pod test was related to a 35% difference in field infection with P. megakarya in years with medium disease pressure (29-47% average pod infection). However, correlations with field resistance were not always significant, suggesting the influence of uncontrolled environmental factors affecting field observations or the results in the screening tests. It is concluded that the leaf disc and detached pod tests, if applied under standardized conditions, can be of great value to speed up selection for Ppr-resistant varieties. (Résumé d'auteur