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Blackleg in South African Potato production: pathogens and impact

Jacquie van der Waals: University of Pretoria

<div>The sustainable production of potatoes in South Africa is hampered by many biotic and abiotic constraints. Among the most important biotic factors is the disease complex caused by the Soft Rotting Pectobacteria (SRPs). The symptoms caused by these bacteria include poor emergence, blackleg, wilt, aerial stem rot and soft rot of tubers post-harvest. Pathogens causing this disease in South Africa include <em>Pectobacterium carotovorum </em>subsp. <em>brasiliense </em>(Pcb), <em>P. carotovorum </em>subsp. <em>carotovorum </em>(vPcc) and to a lesser extent <em>P. parmentieri </em>and <em>Dickeya dadantii</em> (Dd). The most important and widely spread causal agent of this disease is Pcb. This bacterium has resulted in significant losses in field, primarily due to pre-emergence soft rotting of seed, leading to poor stands. In the period between 2006 and 2010, Pcb made up 77% of the SRE population isolated from potatoes. This percentage decreased between 2011 and 2013, to about 40%. The percentage of virulent vPcc strains increased from about 10% of the SRP population between 2006 and 2010, to 35% in the period from 2011 - 2013. This population shift from dominance of primarily Pcb to both Pcb and vPcc coincided with a decrease in blackleg incidences in seed potato fields from the early 2000s to after 2010. Currently, Pcb is still the most important blackleg-causing pathogen in South Africa. Dd is the only reported member of this genus in South Africa and constitutes a small proportion of the blackleg-causing pathogens in the country. Despite the decrease in blackleg in seed plantings, the prevalence of the disease in ware potato plantings has continued to increase, resulting in financial losses to growers. Warm temperatures during the planting seasons, irrigation, countrywide transport of seed, contaminated irrigation water and planting of susceptible cultivars could have contributed to this steady increase in disease.</div>

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