PHENOTYPIC-EXPRESSION OF QUANTITATIVE AND QUALITATIVE COMPONENTS OF PARTIAL RESISTANCE TO POWDERY MILDEW (SPHAEROTHECA-FULIGINEA RACE-1) INMELON (CUCUMIS-MELO) GERMPLASM

A study was conducted to investigate the expression of four components of partial resistance to Sphaerotheca fuliginea race 1 in selected me lon (Cucumis melo L.) lines viz. infection frequency, latent period, s pore production, and disease-severity score. Those components were eva luated at two developmental stages of the host: the cotyledon stage an d the stage of the first two true leaves. Detached plant parts (disks of cotyledons and true leaves) were inoculated using a vacuum-operated settling tower. All four components showed significant variation amon g genotypes, and correlations between components at both developmental stages were large and significant. The line 'CNPH 83-095' (without an y major resistance gene to powdery mildew) presented the highest level of partial resistance in both vegetative stages for almost all compon ents evaluated. The lines 'W-6' (Pm1Pm1, Pm2Pm2),'Cinco' (Pm1Pm1, Pm2P m2), and CNPH '84-147' (Pm1Pm1), even though carrying the major gene P m1 for complete resistance to race 1 of the fungus, showed slight but significant differences for quantitative components of partial resista nce at the cotyledonal stage. Different levels of partial resistance m ay be expressed, even in lines with a major race-specific resistance g ene to powdery mildew, in specific developmental stages of the melon p lants.