Department of Microbiology and Plant Pathology, University of Pretoria, Pretoria, 0002, South Africa

Background and objectives
The potato industry constitutes46% of the total fresh-produce market in South Africa. Potatoes are susceptible to various bacterial diseases due to their high starch and sugar content. One of the most common diseases is caused by pectolytic erwinias (Erwinia carotovora (E.c.) subsp. atroseptica, E.c. subsp. carotovora and Erwinia chrysanthemi). Direct and indirect crop losses due to these pathogens are considerable, especially in certain production areas. Survival of the pectolytic erwinias and environmental conditions affecting disease incidence are important aspects when considering control strategies. In areas where soft rot is particularly prevalent it is important to plant cultivars that show resistance to the disease. Eleven common potato cultivars grown in South Africa were therefore assessed for bacterial soft rot resistance by planting infected tubers, using a point titration assay [1] and a detached-petiole inoculation technique [2]. Due to a lack of available registered chemicals for Erwinia control, alternative options were also evaluated. One such alternative is disinfectants, commonly used in the food industry. The following disinfectants, quaternary ammonium compounds, chlorines, chlorine dioxide, glutaraldehydes and ethyl alcohol, were evaluated against E.c. subsp. atroseptica, E.c. subsp. carotovora and E. chrysanthemi.

Results and conclusions
Isolations made from symptomless tubers showed that only 18% were infected with Erwinia spp. Of the 11 potato cultivars tested only three showed blackleg symptoms after planting: Up-to-date, Buffelspoort and Mnandi. These results were confirmed by point titration and the detached-petiole inoculation technique, which indicated that the cultivars could be divided into two groups based on susceptibility. Buffelspoort, Mnandi, Agria, Mondial, Vanderplank and Astrid constituted the more susceptible group, whereas Up-to-date, Late Harvest, HoŽvelder, Hertha and BP1 were less susceptible. During a survey to determine spread and importance of Erwinia spp. in all the potato-producing areas of South Africa, growers reported that blackleg and soft rot occurred mostly on Buffelspoort and Up-to-date (unpublished data). The contrasting results found with Up-to-date can be due to environmental conditions that are more favourable for the pathogen. A study in Florida showed that the severity of bacterial soft rot appeared to be more influenced by rainfall or irrigation.

The use of disinfectants to control Erwinia showed promising results. Chlorine dioxides (Oxine) at a concentration of 0.1% and a contact time of 1 min proved highly effective in controlling Erwinia. Oxine is approved by the FDA and USDA for use on all food contact surfaces not requiring a water rinse. It is also approved for treatment of drinking water. The use of disinfectants during tuber washing is particularly important due to the fast spread of Erwinia spp. in wash water as well as during cutting of tubers. Other disinfectants controlling Erwinia spp. were glutaraldehyde (EcoSanitizer) and quaternary ammonium compounds (Sporekill(+)). Disinfectants tested in vivo were most effective when applied 24 h before the pathogen.

1. Lojkowska E, Kelman A, 1994. American Potato Journal 71, 99-113.
2. Bisht VS, Bains PS, Letal JR, 1993. American Potato Journal 70, 611-616.