Root knot nematodes and soft rot Enterobacteriaceae , two emerging problems of potatoes.

Abstract

Abstract The soft rot Enterobacteriaceae (SRE) and root knot nematodes (RKN) are major pathogens of potatoes and other crops globally; one of the most threatening emerging species of SRE is the newly identified Dickeya solani biovar 3. Using advanced molecular-based technologies for identification and evaluation of genetic diversity of many pathogens, new and threatening species of RKN and SRE are constantly being identified. In our research, PCR-based diagnostic tools have been used to conduct surveys of the different SRE and RKN infecting potatoes in South African potato fields. Results from the survey indicate that there are other SRE which had previously not been reported and are emerging problems of potatoes globally. These include Pectobacterium wasabiae which was identified in some potato-producing farms in South Africa. A countrywide survey of RKN using PCR-based diagnostic assays identified the presence of the three main tropical species, namely Meloidogyne javanica (24% of the samples tested), Meloidogyne incognita (23%) and Meloidogyne arenaria (17%). The study further established the presence of more temperate species such as Meloidogyne chitwoodi (3%) and Meloidogyne hapla (1%) in the samples tested. Of significant interest was the identification of Meloidogyne enterolobii (constituting 13% of the tested samples) in some potato-producing regions. A phylogenetic tree was constructed using partial sequences of the mitochondrial DNA and intergenic spacer (IGS) region to determine and compare the genetic diversity of South African RKN to those isolated from other parts of the world. In the survey, RKN samples were also profiled according to cultivar from which they were isolated. The highest number of RKN were isolated from Mondial (61%) followed by Up to date (14%). RKN samples were isolated from other cultivars including Sifra, Buffelspoort, Argos, Fianna, Valor, BP1 and Van der Plant (1-5%). RKN are known to form synergies with other pathogens in the soil, leading to increased incidences of disease. To determine whether such synergy exists between RKN and SRE, a Pectobacterium strain tagged with reporter fluorescent mCherry protein was developed. With the aid of this tool, the presence of RKN in the soil was shown to facilitate entry of Pectobacterium spp. into potato tubers, leading to latent infection.