Effects of inoculum concentration, wetness duration and plant age on the development of tomato early blight were evaluated in relation to host susceptibility under controlled environmental conditions. The main effect of early blight was premature defoliation, which was linearly related to the percentage of leaf area showing symptoms. As ln(inoculum concentration, conidia mL 21 ) increased from 6´2 to 11´5, the percentages of leaf area affected and of defoliation increased linearly. Four h of leaf wetness after inoculation were sufficient to initiate the disease on plants of hybrid Skala RZ but not on those of cv. Rio Rojo, for which at least 6 h leaf wetness were needed. As wetness duration increased up to 24 h, there was an increase in the percentage leaf area showing symptoms and in the percentage of defoliation, but thereafter there was no significant increase in either parameter. Tomato plants were susceptible to Alternaria solani at all growth stages, but susceptibility increased as plants matured. There were no significant differences in susceptibility between tomato cultivars and hybrids.

Early blight of tomato (Lycopersicon esculentum) caused by Altemaria solani has the potential to become one of the most serious diseases throughout the tomato-producing regions of Greece.Controlled environment experiments were conducted to study the virulence of A. soluni isolates and the susceptibility of commercial tomato cultivars and hybrids to early blight. The isolates used, derived from naturally infected tomato plants during the period 1997/1998, differed significantly (P>O.O5) in the rate of mycelial growth as well as in their ability to sporulate in vifro. No correlation (R2 = 0.33) was found between mycelial growth and conidia production.Isolates of A. solani were virulent to young tomato plants (cv. Ace 55VF), although they differed significantly (P >0.05) in the intensity of symptoms produced on leaves, stems, petioles and flowers. Defoliation was linearly related (R2 = 0.87) to the percentage of leaf area with symptoms. Twenty-three tomato cvs. or F, hybrids were evaluated for their susceptibility to early blight. The cultivars or hybrids were arbitrarily categorized as immune, highly tolerant, tolerant, moderately tolerant, susceptible and highly susceptible based on a percent disease index range: 0%, 1-9%, 10-24%, 25-49%, 50-74% and 75% or more respectively. None of the cultivars or hybrids tested was immune or tolerant to A. soluni infection.