Repeatability of leaf disc test for assessing resistance levels of international clone trial selections of phytophthora megakarya and phytophthora palmivora infection in Ghana

Abstract

Developing efficient and reliable screening tests to assess the level of resistance of a perennial crop such as cocoa to diseases is essential to reduce the length of the breeding cycle. The resistance of cacao (Theobroma cacao L.) leaves to two species of Phytophthora was
investigated in 25 genotypes of cacao using the leaf disc test. Repeatability of the tests was estimated by calculating the correlation (r) between resistance scores of clones in different inoculation trials of the leaf disc test. Correlations were generally significant (p<0.001), with high correlation coefficients ranging from 0.928 to 0.993 for leaf discs inoculated with Phytophthora megakarya and 0.913-0.971 for leaf discs inoculated with Phytophthora palmivora. There were significant differences (pC0.05) in disease severity scores of leaf discs among the 25 clones. Using the leaf disc test allowed the 25 genotypes to be grouped into susceptible, moderately susceptible and resistant clones. There was a significant and positive correlation for the reaction of clones to  Phytophthora palmivora and Phytophthora megakarya in three trials. This indicates that results obtained when testing resistance to one species of Phytophthora may be of value to other species. SCA 6, T85/799, LAF1, ICS 1 and GU 225V were noticed to be promising clones for breeding for black pod disease. The susceptible clones were MO 20, T79/501, VENC 4-4, PA 120 and MOCORONGO.

Keywords: Cocoa, Black pod, Phytophthora, lesions, clones, leaf disc test