Etiology of soft rot and blackleg on potatoes in South Africa

Abstract

Pectobacterium carotovorum (Pbc), Dickeya spp., Pectobacterium atrosepticum (Pba), and a new, atypical strain, Pectobacterium carotovorum subsp. brasiliensis (Pbcb) can cause potato blackleg, soft rot and aerial stem rot. To determine the impact and extent of these pathogens on the South African potato industry, samples were collected from 72 potato fields in 10 potato production regions during the 2006/7 production seasons. During these seasons, blackleg outbreaks occurred in commercial production fields causing severe economic losses. Bacteria isolated from diseased material that were Gram-negative, facultative anaerobic with pectolytic ability were identified using a Multiplex PCR targeting the 16S-23S region. Isolates were subjected to partial sequencing of the 16S-23S rDNA and a subsequent PCR-RFLP of the 16S-23S rDNA region. Comparison of RFLP patterns of isolates to reference cultures confirmed the identity of the South African blackleg strains as Pbcb. This is the first report of Pbcb in South Africa. A total of 128 isolates were obtained from 10 production regions. Of these 77% were shown to be Pbcb, 17% Pbc and 6% unkown. Dickeya spp. and Pba were not detected in South Africa. From imported tubers 14 isolates were obtained, 13 of which were identified as Pbcb and one isolate as Pbc. Thus Pbcb was shown to be the most important causal agent of blackleg and soft rot in South Africa and poses a threat to the South African potato industry. Through a questionnaire survey the farmers` knowledge of soft rot / blackleg disease complex and the need for research on these potato diseases in South Africa was determined. A total of 41 questionnaires were collected from potato growers and analysed. Estimates of economic losses experienced by growers due to soft rot / blackleg disease complex, ranged from 1 to 70%. It appears that in South Africa disease symptoms are mainly prevalent at temperate (10 – 20°C) to warm climates with prolonged wet or humid conditions. To reduce disease incidence, low generation seed tubers need to be planted and good crop rotation systems need to be followed. Blackleg is mainly a seed-borne disease and is therefore difficult to control. This prompted the search for a possible management strategy to increase plant / tuber resistance to blackleg and soft rot and to minimise losses. The study also focussed on the effect of a calcium silicate slag soil amendment on phenolic formation in cell walls of potato peels, and subsequent tuber resistance to Pectobacterium carotovorum subsp. brasiliensis (Pbcb) . Pot trials were conducted with the following treatments: Control with and without the pathogen; slag (30% Si) with and without the pathogen; lime treatment (CaCO3) as pH control with and without the pathogen. Results from pot trials show that phenol production in potato stems and tubers increased in both lime and slag treatments, compared to the controls. This suggests that soil pH may play a role in phenol production. This could, however be due to increased silicon absorption by plants at higher pH values. Results also indicate that the best Si source to use is Calmasil, since Si and Ca combined have a synergistic effect in enhancing tuber resistance. This is, however, the first study on the effect of Si on defence responses of potatoes and further research is required to elucidate the modes-of-action of Si in the potato plant. This study has enhanced the understanding of the etiology of soft rot / blackleg diseases in South Africa and opened up new possibilities for the use of Si in commercial production to improve plant health status. However the need for future research on this disease complex has been highlighted in this thesis.