A survey to determine the occurrence, diversity and pathogenicity of Pratylenchus species in South African vineyards
PROJECT TITLE: A survey to determine the occurrence, diversity and pathogencity of Pratylenchus species in South African vineyards
Project leader: Dr Rinus Knoetze (ARC)
Duration: 1 April 2022 – 31 March 2025
Project Summary:
Root-lesion nematodes (Pratylenchus spp.) are migratory endoparasites, causing severe damage by feeding and migrating through the root cortical tissue. They are among the most important nematode pests on grapevine along with root-knot, dagger, and ring nematodes. Since relatively large populations of lesion nematodes are recorded in some vineyards and since such nematodes are widespread across all soil types, they should be considered as being among the more important nematode pests of vines. Lesion nematodes are migratory endoparasites, causing severe damage by feeding and migrating through the cortical tissue. Vines, especially younger vines, infected with lesion nematodes have poor growth and yield declines gradually.
The scope of the problem in South Africa is currently unknown, as an in-depth survey of all the grape producing areas has not been done. Four Pratylenchus species (P. penetrans, P. vulnus, P. minyus and P. crenatus) were previously recorded in South African vineyards, but an updated survey will be needed to determine which species of Pratylenchus are present and which are the most prevalent and most pathogenic.
This project will include a survey of the main grapevine producing regions in South Africa and the identification of Pratylenchus spp. detected by morphological and molecular means will give an indication of the scope and severity of the lesion nematode problem in South African orchards. Also, the pathogenicity of the most prevalent species will be determined by evaluating the nematode reproduction and growth response of selected rootstocks to infestation by these lesion nematode species. Furthermore, the host status of the lesion nematodes species present will be determined on selected cover crop species to determine if they have the ability to increase the specific lesion nematode populations in SA vineyards.
The Western Cape is already seeing some effects of climate change, with increasing pressure on water resources, which means a reduction in the amount of water available for agriculture. To protect our water sources, agriculture needs crops that can be cultivated sustainably and profitably. Vines are hosts to various plant-parasitic nematodes that cause reduced vigour and yield. Nematode control in established vineyards is costly and untreated infestations can reduce the productive lifespan of a vineyard. Worldwide, the prevalent genera identified in local vineyards are Meloidogyne, Pratylenchus and Xiphinema. Endoparasitic nematodes, of which root-knot nematodes (Meloidogyne spp. and lesion nematodes (Pratylenchus spp.) are the most important, can be transmitted by infested rooted plant material. In a study by Smith (1977) lesion nematodes occurred in 86% of soil samples collected in vineyards of the Western Cape. The large number of Pratylenchus species as well as the vast number of hosts makes this genus very important from an economic perspective. Several Pratylenchus spp. have been reported from grapevine worldwide.
The species diversity of lesion nematodes is easy to over-estimate when considering morphological features because of the cryptic nature of these species. Regulatory and advisory diagnostic samples regularly contain very few specimens of lesion nematodes, which renders morphological identification inaccurate. Several Pratylenchus spp. have been recorded in SA grapevines, but an updated survey will be needed to determine which species of lesion nematodes are currently present and which are the most prevalent and most pathogenic. A similar survey led to the characterisation of lesion nematode species in apple orchards and improved knowledge of the scope of the problem. In this survey, a new species of lesion nematode, P. hippeastri, was identified, which occurs in 85% of apple orchards and was previously unknown to be a pest of apples (Knoetze et al, 2019). Recently P. hippeastri has been found to infest grapevine in California (Handoo et al, 2020) and we believe that the occurrence of this species on grapevine is a high probability in South Africa.
Most of the effort in incorporating host resistance to root-lesion nematodes in grapevine has been against P. vulnus (Ferris, 2012), but screening of grapevine rootstocks used in SA against the specific populations of lesion nematodes present is still lacking. Currently the most preferred method of orchard management is to plant a cover crop in the work row to improve fertility and soil structure, and reduce erosion. However, in choosing a cover crop it is important to ensure that it is not a good host for lesion nematodes.
The application of standard survey procedures with molecular identification techniques in this project will have an impact on our understanding of the diversity of lesion nematodes associated with grapevines in South Africa and our estimation of the damages caused by them. The optimisation of culturing techniques for lesion nematodes by the Nematology laboratory of ARC Infruitec/Nietvoorbij now enables us to mass produce nematode inoculum to use in screening test for the susceptibility of crops for different lesion nematode species.
Knowledge gained through this research will contribute knowledge and tools for the efficient management of lesion nematodes on grapevines.