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Three well-defined types of host-pathogen interactions occur for the P.
infestans in tomato: highly compatible, partially compatible, and incompatible
interactions (Gallegly and Marvel 1955). It was further observed that the moderately
resistant commercial hybrids ‘Ikram’, ‘Heinz H7155’ and Alambra F 1 ’ exhibited
susceptibility and the disease progress rate was higher comparing with the other
genotypes. In contrast some of these varieties; ‘Ikram’ and Heinz H7155 were
recorded to be moderately resistant in another study (Fiorini, 2008). This
discrepancy in the type of reaction may be due to racial differences of the pathogen
and structural change in the virulence genes of the pathogen or environmental
factors interactions.
3.1.1The analysis of variance for Y 50 , Y max and AUDPC
The analysis of variance of the three disease variables Y 50 , Y max and AUDPC,
showed that the great part of variation was attributed to genotypes that had highly
significant differences for these traits (Table 4) whereas there were no significant
differences for replicates (blocks) or between/within the plots. Analysis of variance
showed highly significant differences among the genotypes indicating the
occurrence of broad range of variability in the expression of resistant.
It can be observed from the analysis of variance that the coefficient of
variance in the case of severity at halfway epidemic (Y 50 ) had the highest value
(33.16) than the other traits; severity at the end of the epidemic (Y max ) and area
under disease progress curve (AUDPC). This was expected during the period of
evaluation, due to the difficulty of having homogeneity of the scores over all the
plants of the same treatment and at the same time, and may be attributed to the
disease progress rate, that was not equal over the replicates.
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