A survey to determine the occurrence, diversity and pathogenicity of Pratylenchus species in South African stone fruit orchards

Root lesion nematodes (RLN) are among the most prevalent nematode pests on stone fruit, but the scope of the problem in South Africa is currently unknown, as an in-depth survey of all the stone fruit producing areas has not been done. This project included a survey of the main stone fruit producing regions in South Africa, followed by the characterisation of RLN populations by molecular means. Additionally, the screening of stone fruit rootstocks for resistance to local RLN populations has not been adequately addressed. Also, current orchard management practices, such as the use of cover crops, may inadvertently support nematode populations if those crops are good hosts. Thus, understanding the host status of commonly used cover crops is critical to effective nematode management.

Root lesion nematodes (RLN) are among the most prevalent nematode pests on stone fruit, but the scope of the problem in South Africa is currently unknown, as an in-depth survey of all the stone fruit producing areas has not been done. This project included a survey of the main stone fruit producing regions in South Africa, followed by the characterisation of RLN populations by molecular means. Additionally, the screening of stone fruit rootstocks for resistance to local RLN populations has not been adequately addressed. Also, current orchard management practices, such as the use of cover crops, may inadvertently support nematode populations if those crops are good hosts. Thus, understanding the host status of commonly used cover crops is critical to effective nematode management.

The majority of populations (90%) collected were identified as P. vulnus, but this study provides the first record of P. hippeastri parasitizing stone fruit in South Africa.

Variable and, at times, conflicting results were observed regarding the resistance of several rootstocks to root-lesion nematodes (RLN) encountered in this study.

The majority of cover crop cultivars evaluated in this and previous studies are poor hosts of RLN.

Lesion nematodes can be regarded as a prevalent plant-parasitic nematode in the stone fruit orchards sampled in this study, but their populations are generally below economically damaging thresholds. Their relative contribution to overall parasitism in stone fruit is limited when compared to ring nematodes. Variable and, at times, conflicting results were observed regarding the resistance of several rootstocks to root-lesion nematodes (RLN) encountered in this study. These results highlight the complexity of RLN-host interactions and emphasize the need for further multi-season, multi-site evaluations before definitive conclusions regarding resistance can be drawn. The majority of cover crop cultivars evaluated in this and previous studies are poor hosts of RLN. These cultivars do not support nematode reproduction and therefore have the potential to prevent the build-up of high, damaging RLN populations in orchard soils. A smaller percentage of cover crop cultivars screened were identified as good hosts, capable of supporting significant nematode reproduction. These cultivars may contribute to the establishment of harmful RLN populations and serve as primary sources of  infection.

Lesion nematodes are common in orchards but typically remain below damaging levels and play a far smaller role in parasitism than ring nematodes. Rootstock resistance to RLN is inconsistent and requires further multi-season testing. Most cover crops assessed are poor hosts and can help suppress RLN build-up, but a minority are good hosts that can drive harmful population increases and should therefore be avoided.