Studies on the Resistance of Tomato Cultivars Against Pseudomonas syringae pv. tomato Races 0 and 1

Abstract

Screening the degree of resistance of tomato plants to bacterial speck by monitoring symptom development coufd be confirmed by determining bacterial multiplication in planta. The growth of P. tomato, race 0, was strongly inhibited in resistant tomato leaves, so that the finaf concentration was 1/100 of that in susceptible leaves. However, race 1 was only very weakly inhibited.

In order to study resistance mechanisms, susceptible and resistant tomato leaves were inoculated with P. syringae pv. tomato races 0 and 1. At different intervals -1, 3 and 5 days post inoculation — the intercellular washing fiuid (IWF) was obtained from these leaves and fiiter-sterilized. The presence of bacteriostatic or bactericidal components in the IWF was tested by inoculating the IWF with low concentrations of P. syringae pvs. tomato ana phaseoficoia. Constantly, P.s. pv. tomato races 0 and 1multiplied a little less in the IWF from resistant leaves compared to susceptible leaves, when the IWF was obtained 3 and 5 dpi, but not in IWF from 1 dpi. It was concluded from these results that the resistance mechanisms operating in tomato leaves against P.s. pv. tomato are only marginally due to bactericidal or bacteriostatic components, such as phytoalexins.

Unexpectedly, P. syringae pv. phaseolicola was strongly inhibited in IWF from susceptible tomato leaves which had been infected by P. syringae pv. tomato, but not in IWF from contrai leaves (water-treated). The underlying mechanism is unknown.

A comparison of quantitative and qualitative production of extracellular polysaccharides (EPS) by several strains of P. syringae pv. tomato revealed considerable variation in total amount (280 — 1370 mg/l), however, there was no correlation with virulence. The main component of the EPS was levan, and only a small portion of alginate was produced (in most cases less than 10%).