Behaviour of tomato cultivars grown in soil infected by Pyrenochaeta lycopersici (Snyder & Gerlach) and pathogenesis mechanisms

Tomato cultivars for fresh market were grown in greenhouse on soil naturally infected by Pyrenochaeta lycopersici (Snyder et Gerlach). The behaviour of the cultivars toward the Coelomycetes fungus causing corky root was studied. The compared cultivars were F-1 hybrids ''Cencara'', ''Fortara'', ''Italdor'', ''Max'', ''Minizano'', ''Monika'' and ''Nikita''. The resistance was observed detecting the symptom severity and the disease diffusion; the first was rated according to the following evaluation scale: 0 = healty roots; 1 = 1-9% of brown and sloughing root surface; 2 = 10-24%; 3 = 25-60%; 4 = over 60%; the second was noticed by using the McKinney index that consideres the weighted average evaluation scale values and the number of infected plants. For each cultivar, the root observations were carried out every month from 22nd March to 1st August 1995. Laboratory pathogenesis tests were performed by using plantlets and a toxin obtained by culturing the fungus on liquid media. Yield, earliness and number of harvests was registered. For each cultivar, the correlation and regression between plant age and infection degree and between last one and yields were calculated. Cultivars showed significative different behaviour towards corky root with effects on some productive parameters. Period in which the disease caused yield decrease was detected. The hybrids that showed the disease symptoms too early (up to 121 days after transplanting); such as such as ''Minizano'', ''Nikita'' and ''Fortara'', gave lowest yields; ''Cencara'', ''Italdor'' and ''Max'', root lesions later than other hibrids (about 152 days after transplantation), showed the highest yields; intermediate values were registered for ''Monika''. Referring to laboratory trials, the hybrids gave high susceptibility to P. lycopersici culture filtrate: the infection index of plantlets, grown three days on absolute culture filtrate, ranged from 2.4 to 3.8. At high dilution of toxin, the damage degree was differentiated and the laboratory essay became accurate predictors of the cultivar behaviour in greenhouse towards fungus metabolites in the soil. For each hybrid, the linear relationship between plant age (days from transplantation) and McKinney index showed high positive correlation (the coefficients ranged from 0.885 to 0.947) while regression coefficients ranged from 0.291 to 0.913. During stay of the plant roots in the soil, the damage and the epidemiology varied differently for the compared hybrids. The study of the correlation between the infection degree and the yields proved that if the symptoms on the roots appeare late (about 150 days after transplantation) corky root will not induce appreciable yield decreases.