Pathogenicity studies, biochemical characterisation and use of enzyme-linked immunosorbent assay (ELISA) for the detection of Pseudomonas spp.

Abstract

The pathogenicity, identification and serology of Pseudomonas syringae pv. tomato (Okabe 1933), Young et al. 1978, P. cichorii (Swingle 1925) Stapp 1928, and P. corrugata (Scarlett et al. 1978) were studied. Low humidities (50-60%) and high temperatures (30°-37°) restricted development of symptoms in tomato plants. Both P. sy. pv. tomato and P. cichorii behaved as compatible pathogens. The population increased logarithmically for two days, reached a stationary phase after five days and then declined. P. corrugata was tomato leaves, and had a lag phase unable to induce disease symptoms in of two days. In tomato stems, P. cichorii and P. corrugata moved upward and downward from the inoculation point. The bacterial population was highest at the inoculation point, and decreased with distance from that point. Bacteria could be recovered from symptomless stem sections. An infection population threshold for P. cichorii and P. corrugata at 1.4 x 10⁵ colony forming units (CFU)/g was required for symptoms to become visible.

The three pathogens could be differentiated biochemically from most other bacteria isolated from tomato plants. P. corrugata was identifiable by the green diffusible pigment when cultured on glucose-yeast- chalk agar, and nutrient agar plus 5% sucrose.

The double antibody sandwich form of enzyme-linked immunosorbent assay (ELISA) enabled sensitive detection of 10³ - 10⁴ CFU/ml, with immunogloblin G (IgG) and IgG-enzyme (alkaline phosphatase) conjugate used at 1.0 µg/ml and 0.5 µg/ml. The three pathogens could be distinguished from Pseudomonas spp. and species of Corynebacterium, Erwinia, and Xanthomonas bacteria isolated from tomato plants.

Numbers of bacteria in infected tomato tissues were quantified using ELISA. Extracts of healthy tomato leaf, stem, fruit, seed and flower reacted weakly; but root, soil and potting mix extracts reacted strongly. These non-specific reactions were decreased by dilution. Presence of plant and soil components decreased the reactivity of the three Pseudomonas spp. but this effect was minimised by dilution.