The interaction between root knot nematodes (Meloidogyne spp.) and soft rot Enterobacteriaceae (Pectobacterium spp.) and their host Solanum tuberosum

Abstract

Meloiodgyne incognita, one of the most aggressive plant parasitic nematodes species on potato in South Africa, belongs to a group of plant parasitic nematodes commonly known as root knot nematodes (RKN). This group of nematodes is widely distributed throughout the world. Meloidogyne spp. cause major economic losses to important crops such as potato and therefore decrease their market value in many countries across the world. The second stage juveniles are the only mobile and infective phase of the root knot nematode. As they infect host roots, they create wounds that can be used by other plant pathogens to penetrate the host in large numbers. The most effective management strategy for root knot nematodes is the use of nematicides such as Temik and Methyl bromide. However, these have been banned due to adverse on the environment. Therefore, Meloidogyne spp. will inevitably become a big problem in the potato industry of many countries due to the lack of effective alternatives to banned chemicals. Pectobacterium carotovorum subsp. brasiliensis (Pcb) is one of the most important soft rot-causing agents in South Africa. This pathogen belongs to a group of pathogens commonly known as soft rot Enterobacteriaceae (SRE). Bacteria belonging to this group of pathogens are known to cause soft rot and blackleg diseases on potato and other crops. Pcb is known as an opportunistic pathogen that can only penetrate host root tissue through natural openings or wounds that result from a variety of agents. Post penetration, the bacteria will increase in number and cause soft rot and blackleg. As rotting plant tissue disintegrates the bacteria escapes into the soil where it serves as inoculum and can infect healthy hosts. Many interactions have been documented between Meloidogyne spp. and other plant pathogens but to our knowledge there are no interactions that have been reported between Meloidogyne spp. and Pectobacterium spp. Considering the life cycles of RKN and SRE, we hypothesised that there could be an interaction between the two pathogen groups. Since both RKN and SRE are potato pathogens, they share the same space in the rhizosphere. This likely can lead to synergies and complex formation between the two pathogens. Likely, the wounds created by RKN J2s as they penetrate plant tissue can potentially be used by opportunistic Pcb to infect various hosts. It is from these identified overlaps that the first part of this study focused on investigating the potential interaction between M. incognita and Pcb. The first objective was to determine whether Pcb can attach onto M. incognita J2s and, if this was the case, to determine whether the J2s can disseminate the bacteria as they move around in the environment. The second objective was to determine whether there is a synergistic interaction between RKN and SRE and the combined effect of the two pathogens on their host Solanum tuberosum cv Mondial. The results obtained in the first part of the study strongly suggested that Pcb can attach onto M. incognita J2s and can be disseminated as the J2s move around in the environment. Thus, this indicated that there is a synergistic interaction between M. incognita and Pcb as there was increased disease severity and incidence in plants inoculated with both pathogens compared to those inoculated with individual pathogens. Significantly higher Pcb concentrations were found in plants inoculated with both pathogens. There was no breakage of tolerance to Pcb-caused blackleg on an otherwise resistant cultivar, BP1. The second aim of this study was to determine whether the induction of natural resistance using environmentally friendly resistance inducing chemicals can potentially be used as an alternative to chemical control. To this end, the effect of three inducers at different concentrations, amongst DL-β-aminobutyric acid, Acibenzolar-s-methyl and Messenger on potato plants infected with RKN was compared. The most effective resistance inducer amongst the three was 20mM BABA as it was able to reduce the number of J2s that penetrated host tissue, the number of females in the roots and the rate of egg production. Furthermore, the galling index observed in potato roots was significantly lower when plants were treated with 20mM BABA. Additionally, the reduced rate of RKN infection in plants primed with 20mM led to a decrease in the rate of Pcb infection.