The Status of Resistance to Bacterial Diseases of Wheat

Abstract

Bacterial leaf streak and black chaff caused by Xanthomonas translucens pv. undulosa (XTU), basal glume rot caused by Pseudomonas syringae pv. atrofaciens (PSA) and bacterial leaf blight caused by P. syringae pv. syringae (PSS) are the most frequently reported bacterial diseases on wheat. Eight other bacteria have been described on wheat but have not shown severe outbreaks during the last years. Seed contamination by XTU, PSA or PSS has been demonstrated. It results from spike infection but in some cases also from contamination by bacteria released from diseased leaves of highly susceptible genotypes. Several pathovars are distinguished among X. translucens strains isolated from small grains on base of differences in host range. Transmission electron microscopy reveals in the compatible interaction, characterized by water-soaked lesions on the leaf, an abundant bacterial multiplication and the formation in intercellular spaces of a matrix with fibrillar material; while in incompatible reactions only limited bacterial multiplication and a restricted amount of matrix or fibrillar material is noticed. XTU designates strains pathogenic to several graminaceous hosts. XTU mutants affected in their capacity to infect some hosts can be obtained through transposon mutagenesis. DNA finger-printing of XTU isolates reveals various genotypes. Nevertheless, in pathogenicity tests only quantitative differences in virulence are noticed, except for a few South American isolates. Field screening under natural or artificial infection has been effective in identifying wheat genotypes with high level of partial resistance. Crosses have revealed the occurrence of several resistance genes as well as both additive and dominant genetic control for partial resistance. The ranking of the genotypes appears stable over time, although differences in inoculum level and environmental conditions induce variations in actual disease severity. Inoculation at the seedling stage under controlled conditions and rating of the water-soaking reaction is not a reliable means for screening wheat for XTU resistance. The discrimination of PSA and PSS is difficult because some PSS strains also show the capacity to induce basal glume rot. Both bacteria appear to have an asymptomatic epiphytic growth in the phyllosphere of wheat, other annual or perennial Gramineae and possibly unrelated hosts. Symptoms develop on susceptible cultivars after periods of very humid, cool weather and wind-driven rain. Quantitative difference in virulence among strains and occurrence of races are reported. PSA and PSS are producing syringomycine and other phytotoxic and surface tension active compounds. The differences in virulence require careful selection of strains for resistance screening of germplasm under controlled conditions. A range of inoculum concentrations has been used to identify genotypes with partial resistance among winter and spring wheat cultivars of various origins. Aegilops genotypes show a high level of resistance.