FLUORESCENT PSEUDOMONADS ASSOCIATED WITH DISEASES OF WHEAT IN SOUTH-AFRICA

A survey of fluorescent pseudomonads associated with diseased wheat was conducted in South Africa during 1987 and 1988. Phenotypic features of 87 local strains were compared with those of 10 reference strains. Five groups were distinguished. Group 1 (nine reference and 16 local strains) was classified as Pseudomonas syringae pv. syringae. Group 2 (four local strains) was similar to group 1 but did not produce levan on nutrient sucrose agar. Group 3 (one reference and 33 local strains) also resembled P. s. pv. syringae, but did not elicit a hypersensitive reaction on tobacco. Group 4 (20 local strains) was mostly isolated from plants with atypical symptoms (total melanism) found in a single geographical region (Villiers) within South Africa. These strains had uniform characteristics, but failed to induce melanism on inoculated test plants. Group 5 (14 local strains) was not uniform. Twenty-eight representative local strains, selected from each of the five groups, and the 10 reference strains were used in pathogenicity and virulence tests. The four most virulent local strains were used to screen 14 wheat cultivars grown commercially in South Africa. Five of the cultivars were susceptible to these strains. Symptoms on leaves of naturally-infected plants corresponded with those already described, but the typical ear symptom (basal glume rot) was absent.