Bacterial wilt of potato (Ralstonia solanacearum race 3, biovar 2): disease management, pathogen survival and possible eradication

Potato brown rot, caused by Ralstonia solanacearum race 3 biovar 2 (Phylovar II, sequevar 1), is a serious endemic disease in the Nile Delta of Egypt. It is a quarantine disease in the EU, and export of potatoes fromEgyptis restricted to pest-free areas in the desert. This thesis is directed at survival and eradication of the pathogen in areas that may become contaminated and at brown rot control in infested areas. R. solanacearum survival and disease suppression were studied for soils differing in origin (Dutch versus Egyptian soils), type (sand versus clay), and management (organic versus conventional). Effects of amendment of conventional soils with NPK and organic soils with compost or cow manure were compared with non-amended controls. The pathogen survived longer in Dutch than in Egyptiansoils,and in clay than insandy soils from both countries. Survival was never longer than 180 days and in many cases much shorter, especially in Egyptiansandy soil. Organic management reduced pathogen survival in Egyptian soils, especially in thesandy soil, but not in Dutch soils. Pathogen survival was positively correlated with dissolved organic carbon (DOC) in soil, negatively with organic matter (OM) content at similar DOC content, and negatively with bacterial diversity estimated from denaturing gradient gel electrophoresis (DGGE) of eubacterial 16S rDNA extracted from soil. NPK fertilization and cow manure amendment reduced pathogen survival in most soils while compost was not effective. There was a clear shift in microbial community after manure amendment, a weak shift by compost amendment, and no shift after NPK fertilization.Disease development was highest in Dutchsandy soils and similar in all other soil types. It was positively correlated with DOC (substrate availability), and negatively with K and Ca contents in soil, which may increase plant resistance. The disease was slightly suppressed in organic compared to conventionalsandy soils fromEgyptwhile organic management increased disease incidence in all Dutch soils. This was related to a high DOC content in organic Dutch soils. There were no differences in bacterial diversity and composition in different rhizosphere soils, and no correlations between disease development and bacterial diversity.Manure fertilization suppressed brown rot in most soils with a clear shift in rhizospere bacterial community. Stenotrophomonas maltophilia , isolated from the rhizosphere of eggplant in the Egyptian Delta, was antagonistic to R. solanacearum in vitro ; its antagonistic activity was not Fe-siderophore dependent. A selected S. maltophilia strain survived longer and reduced R. solanacearum survival more in Egyptian than in Dutch clay soils, and suppressed potato brown rot in Egyptian soil (by at least 36%) but not in Dutch soil. Survival and suppressive activity of S. maltophilia were positively correlated withOM, which was highest in Egyptian organic clay soil.The effect of biological soil disinfestation (BSD) was tested in glass vessels, microplots, and in an accidentally infested commercial field. BSD is based on production of toxic organic acids through anaerobic digestion of fresh organic matter. BSD was accomplished by incorporating grass or potato haulms in soil and covering the soil with airtight plastic.Survival of R. solanacearum in soil and potato tubers was significantly reduced in the BSD treatment (>93%), and not in separate grass amendment -or plastic cover treatments. This thesis may contribute to regulatory decisions in the EU concerning the time required for keeping accidentally infested fields out of potato production and to recommendations for management of potato brown rot in areas where the disease is endemic.