Effects of salinity on severity of infection by Phytophthora parasitica Dast., ion concentrations and growth of tomato, Lycopersicon esculentum Mill.

The response of tomato (Lycopersictm esculentum Mill., cv. UC82B) to salinity, alone and in combination with Phytophthora parasitica Dast. (a fungal pathogen causing root rot) was investigated in a field study. Three salinity regimes were established: 1) a low salinity control, 2 medium salinity, where 75 mm total salts (NaCl and CaCU in a 4:1 molar ratio of Na:Ca) were added to the irrigation water to give an electrical conductivity (EC) of approximately 8 dS m-1 , and 3) high salinity, where ISO mM total salts (4:1 molar ratio of Na: Ca) gave an EC of approximately 16 dS m-1 . Half of the plots were inoculated with P. parasitica. and the remainder were treated with a selective fungicide to inhibit the pathogen. Soil salinity markedly increased the incidence of Phytophthora root rot in both years of the study. The combination of salinity and enhanced disease severity led to significant reductions in fresh fruit yields, fruit size and, to a lesser extent, total above ground biomass. Fruit size was affected to the greatest extent and showed a strong interaction between the effects of disease and salinity, suggesting that the import of water by fruit was more sensitive than dry matter production to the combination of these stresses. Net root growth (0-50 cm depth) was greatly reduced (by 40-50%) in the presence of salinity, whereas P. parasitica had no discernable effect even when more than 50% of the root system showed severe root rot lesions. In spite of the reduced root system, leaf water potential was not affected by disease in the 1989 growing season. During the fruit-fill period in 1988, however, leaf water potential was more negative in inoculated plots. A marked degree of leaf ion homeostasis was maintained even under high salt and root rot stress. Excessive build up of Cl or Na concentrations in the leaves did not contribute substantially to the observed reductions in plant growth and yield. The results suggest that a reduced root growth rate or an enhanced root death rate may be at least partially responsible for the increased disease severity at high salinity.