Survival of Ralstonia solanacearum race 3 biovar 2 (phylotype II sequevar 1) in Egyptian soils and compost was studied under laboratory and field conditions. Survival of the pathogen under laboratory conditions varied with temperature, water potential and soil type, with temperature being the major determinant of survival of the pathogen. The effects of temperature and moisture content were variable between different experiments, but survival was generally longer at 15°C than at 4, 28 and 35°C respectively. Survival was also longer when moisture levels were constant compared with varying moisture levels at all temperatures. In experiments to compare the effects of progressive drying in sandy and clay soils there was a difference in survival times between the two soil types. In sandy soils, the pathogen died out more rapidly when soil was allowed to dry out than in controls where the soil was kept at constant water potential. In clay soils there was little difference between the two treatments, possibly due to the formation of a hard impermeable outer layer during the drying process, which retarded water loss from within. Survival in mature composts at 15°C was of the same order of magnitude as in soils but shorter at 28°C, possibly owing to increased biological activity at this temperature, or a resumption of the composting process, with concomitant higher temperatures within the compost itself. The maximum survival time recorded over all soil types and conditions during in vitro studies was around 200 days. In field studies, the maximum survival time in both bare sand and clay was around 85 days at depths up to 50 cm. The survival time was reduced in field experiments carried out in summer to less than 40 days and in one study when the ground was flooded for rice cultivation, the bacterium could not be detected 14 days after flooding. The maximum survival time of R. solanacearum in infected plant material or in infested soil samples incorporated into compost heaps was less than 2 weeks. At the culmination of field soil and compost experiments, no infection was detected in tomato seedlings up to 10 weeks after transplanting into the same soils or composts under glasshouse conditions at a temperature of 25°C.

Surveys over three seasons of irrigation, drainage and artesian well water throughout the major potato-growing areas of Egypt indicated that Ralstonia solanacearum bv. 2 race 3 (phylotype II sequevar 1), cause of potato brown rot, was limited to the canals of the traditional potato-growing areas in the Nile Delta region, with positive findings more commonly associated with the network of smaller irrigation canals flowing through potato-growing areas. Pathogen populations in the canals of the Delta (~100–200 cfu l−1) were generally variable throughout the year with presence linked to potato cultivation in the immediate area. The pathogen was not detected in irrigation or drainage water associated with potato cultivation in the newly reclaimed desert areas (designated as Pest-Free Areas, PFAs) or in the main branches of the Nile upstream from these areas. In vitro studies showed that temperature and microbial activity were the main factors affecting survival of the pathogen in canal water. In experiments at temperatures of 4, 15, 28 and 35°C, survival was longest at 15°C and shortest at 35°C. Survival at 4 and 28°C tended to be intermediate between these extremes as was survival when the bacterium was grown at fluctuating temperatures. Aeration, solarisation and pH variation between 4 and 9 appeared to have little effect on survival. Survival in autoclaved or filter-sterilised canal water was longer than in untreated water irrespective of other factors with survival times exceeding 300 days at 15°C in some experiments. Evidence is presented indicating that survival in water-saturated sediment may be longer than in the overlying water suggesting that sediment may provide a protective niche for the pathogen in some circumstances. The maximum survival time in non-sterile Egyptian canal water at high inoculum pressure was estimated to be up to 300 days at optimum temperature for survival (15–30°C) suggesting the potential for long-distance spread in Egyptian surface waters from sources of contamination.