EFFECT OF METHOD OF INOCULATION, INOCULUM DENSITY AND SEEDLING AGE ATINOCULATION ON THE EXPRESSION OF RESISTANCE OF COCOA (THEOBROMA-CACAOL) TO VERTICILLIUM-DAHLIAE KLEB

Soil drench and stem puncture inoculation were compared as methods for selecting cocoa cultivars with resistance to Verticillium dahliae. Di sease progress was more rapid and induced symptoms were more severe fo llowing stem puncture and, under glasshouse conditions, differences be tween cultivars were detected 15 days after inoculation. Moreover, usi ng stem puncture, inoculum densities of 10(4) conidia/ml mi were suffi cient to differentiate resistant and susceptible cultivars, whereas wi th the soil drench method, inoculum densities of 10(7) conidia/ml were necessary. Although a substantially higher proportion of plants were affected by stem puncture inoculation, the resistance of cultivar Poun d-7 remained effective at high inoculum densities of 10(8) conidia/ml. With either method, older seedlings were more susceptible to V. dahli ae than younger ones. However, with stem puncture, 15-day-old seedling s were sufficiently susceptible for a valid disease assessment. In con trast, with soil inoculation, 60-day-old plants were required. In a nu rsery trial with 15-day-old seedlings, seven cocoa genotypes previousl y selected as resistant, moderately resistant or susceptible to Vertic illium dahliae, on the basis of root inoculation, were ranked in the s ame order when stem punctured. Stem puncture inoculation of young seed lings is cost-effective in terms of time and space, and is therefore r ecommended for screening of cocoa for wilt resistance, especially in l arge-scale breeding programmes.