Submitted to: European Journal of Plant Pathology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 4/6/2011
Publication Date: 8/22/2011
Citation: Spies, C., Mazzola, M., Mcleod, A. 2011. Characterisation and detection of Pythium and Phytophthora species associated with grapevines in South Africa. European Journal of Plant Pathology. 131:103-119. Interpretive Summary: The fungal like oomycetes belonging to the genus Pythium and Phytophthora form diverse associations with plants. Results from this study have improved our knowledge concerning the role of these oomycetes in grapevine health in South Africa and improved our ability to detect these organisms in plant tissues. Among those found present in vineyards the most common species detected and known to be pathogenic to grape included Pythium vexans, Pythium ultimum, Pythium irregulare, Phytophthora cinnamomi and Phytophthora niederhauserii. These species were shown to function in the grapevine decline syndrome individually, or when associated with other pathogens. These oomycetes could aggravate and induce symptom expression in existing infections that had been initiated by other trunk disease pathogens such as Phaeomoniella chlamydospora and Phaeoacremonium spp., which are known as severe pathogens on stressed plants. The possible involvement of non-symptomatic nursery material in the distribution of pathogenic Pythium and Phytophthora species in South Africa is important, and is likely due to vines only being examined visually. Therefore, our development of a sensitive assay using a DNA-based quantitative polymerase chain reaction (Q-PCR) method will be valuable for the screening of nursery material as a means to limit the dispersal of these pathogens. The Q-PCR assays can also be invaluable in research applications such as the evaluation of management strategies aimed at minimising the contribution of Pythium and Phytophthora to grapevine decline. Certain pathogenic Pythium species were only detected at low levels in grapevine roots. Therefore, future studies should assess the utility of rhizosphere soil as the sample unit to determine if a better correlation exists between pathogen DNA concentrations in this substrate and vine growth stunting. Sometimes these associations result in damage to plants resulting from infection of the host root system. Pythium spp. have been implicated as pathogens of numerous perennial crops including grape vines and apple, and isolates identified as Pythium vexans are commonly isolated from these crops in South Africa. However, isolates grouped in this species are often diverse and may represent multiple species. Studies were conducted using molecular DNA-based methods (for example internal transcribed spacer [ITS] sequence data) and morphological characteristics to examine the diversity of “P. vexans” recovered from grape vine and apple in South Africa and to determine whether these isolates comprised multiple species. Using these methods three groups (A-C) were identified among the P. vexans isolates studied. These groups are currently not considered as distinct species based upon the data obtained in these studies including the fact that the known ITS DNA sequence diversity in P. vexans is larger than that observed for groups A-C identified here. An important conclusion from results obtained in this study is that two of the three officially described species in the P. vexans ITS groupe, i.e. P. indigoferae and P. cucurbitacearum, should be considered illegitimate until suitable neotype strains for these species can be found.
Technical Abstract: Replant and decline diseases of grapevines not only cause quantitative and qualitative yield losses, but also results in extra costs when vineyards have to be replanted. This study investigated the role of Pythium and Phytophthora in the decline syndrome in South Africa by determining the (i) species associated with nursery and established vines, and (ii) pathogenicity of Ph. sp. niederhauserii and P. vexans relative to known grapevine pathogens. Quantitative real-time PCR (qPCR) assays were also developed for detection of the most prevalent oomycete groups. In total, 26 Pythium and three Phytophthora species were identified from grapevine nurseries and established vineyards. The most common infections in nurseries were caused by P. vexans (16.7%), followed by P. ultimum var. ultimum (15.0%) and P. irregulare (11.7%). In established vineyards, P. irregulare (18.0%) and P. vexans (6.2%) were also among the three most prevalent species, along with P. heterothallicum (7.3%). Three Phytophthora species were also identified from vineyards, of which Ph. cinnamomi (5.1%) was predominant, followed by Ph. sp. niederhauserii (1.1%). In established vineyards a higher incidence and more diverse species composition was observed in spring and winter, than in summer. Pathogenicity studies showed that some Ph. sp. niederhauserii and P. vexans isolates had comparable virulence to the known grapevine pathogens Ph. cinnamomi and P. irregulare. Sensitive qPCR assays were developed for the detection of P. ultimum var. ultimum, P. irregulare, P. vexans and the genus Phytophthora. These assays will be invaluable in limiting pathogen dispersal through screening of nursery material. This is especially important since pathogenic species were also isolated from healthy looking vines in nurseries.