Sustainable control of pea bacterial blight : approaches for durable genetic resistance and biocontrol by endophytic bacteria

Key-words: bacterial blight, biological control, biodiversity, endophytic bacteria, L-form, pea, PDRl retrotransposon, Pisum sativum, Pisum abyssinicum, Pseudomonas syringae pv. pisi, race specific resistance, race non-specific resistance, Spanish landraces.Pea bacterial blight (Pseudomonas syringae pv. pisi) occurs worldwide and can cause severe damage under cool and wet conditions particularly at the seedling stage in wintersown crops. Seven Ps. syr. pv. pisi races are currently recognized. There are no resistant cultivars to race 6, which is becoming increasingly important. Current disease control measures include disease avoidance through seed testing and the deployntent of resistant cultivars with race specific resistance gene(s). In the present study two novel control measures were investigated with the potential for integration to give a durable and sustainable disease control. The first was breeding for resistance based on race non-specific resistance present in Pisum abyssinicum, which confers resistance to all races, including race 6. Its mode of inheritance was investigated through a crossing programme with Pisum sativum cultivars. Resistance was controlled by a major recessive gene and a number of modifiers. Progenies of crosses between resistant F5 populations and commercial cultivars are now available. Molecular markers for race non-specific resistance based on a pea retrotransposon marker system were developed. It is suggested that the combination of race specific and race non-specific resistance provides the optimal genetic background for the maximum expression of resistance to all races of the pathogen in all plant parts and under field conditions. The second measure was biological control by endophytic bacteria. Studies on the ecology of endophytic bacteria in pea and identification of efficient indigenous colonizers for potential application in biocontrol have been made. Endophytic population levels were in the range 10 3 -10 6 CFU/g fresh tissue in roots and stems. There was a predominance of Gram-negative bacteria, particularly Pseudomonas sp. and Pantoea agglomerans. Arthrobacter sp. and Curtobacterium sp. were the main Gram-positive bacteria. Factors such as soil type, plant genotype and crop growth stage may significantly influence the diversity and population levels of endophytic bacteria. Future research should focus on the combination and testing of elite breeding lines with selections of disease suppressive endophytic isolates under a variety of field conditions in order to obtain an efficient and durable performance in commercial agriculture.