Biology of the aster yellows vector Mgenia fuscovaria
The leafhopper Mgenia fuscovaria is the only confirmed vector of aster yellows (AY) phytoplasma of grapevines in South Africa to date, but its biology is still largely unknown. The objectives of this research were to identify its indigenous host plants, study its seasonal abundance and life cycle on these plants and determine whether any of these plants can serve as a reservoir for aster yellows phytoplasma.
To identify native host plants, vegetation in and around vineyards where M. fuscovaria were previously found, were sampled for the presence of the leafhopper, using a vacuum sampler and yellow sticky traps. Seasonal abundance of Mgenia on indigenous host plants was determined by means of yellow sticky traps. Adult leafhoppers were collected in the field with sweep nets and confined on potted host plants in the insectary to study the life cycle and shoots of wild host plants were inspected during the season to find leafhopper eggs. Wild hosts near vineyards infected with aster yellows phytoplasma were sampled to determine if they harbour the phytoplasma. Two evergreen, indigenous rambling plants that grow near watercourses were identified as host plants for M. fuscovaria, namely wild bramble (Rubus sp.) and Cliffortia odorata (wilde wingerd).
• Seasonal occurrence on wild host plants: numbers peaked in spring (Sept/Oct) and again in May/June, but very few leafhoppers occurred in the adjacent vineyards. Numbers were much lower in 2016/17 than in 2015/16, probably reflecting the effect of the drought on host plant quality and leafhopper survival.
• Life cycle: adult leafhoppers survived on the wild bramble and Cliffortia odorata plants in the insectary, but did not to lay eggs, consequently the life cycle could not be studied. Eggs were found in shoots of wild hosts, but they turned out not to be eggs of M. fuscovaria.
• Rubus plants adjacent to the AY-infected vineyard tested negative for the phytoplasma, therefore AY transmission studies with M. fuscovaria and the indigenous host plants were not carried out.
Two evergreen, indigenous rambling plants were identified as indigenous host plants for M. fuscovaria, namely wild bramble (Rubus sp.) and Cliffortia odorata (wilde wingerd). These plants grow near watercourses and if present in an area, it is likely that M. fuscovaria, the vector of Aster Yellows phytoplasma, would be present as well. This means that great care should be taken not to bring plant material infested with AY into the area, as it could be transmitted by the vector. To date it does not seem likely that wild brambles are reservoirs for AY phytoplasma. Data on seasonal occurrence concurred with studies on grapevines by other researchers and showed that leafhopper numbers peaked during spring and again in autumn. As found in previous studies, M. fuscovaria does not appear to be an abundant leafhopper in healthy vineyards, but research has shown that it is attracted to grapevines infected by AY.
The finding that M. fuscovaria is not a very abundant leafhopper in healthy vineyards and the absence of aster yellows phytoplasma in its wild bramble host lead to the conclusion that AY can be successfully contained and eradicated if infected vines are destroyed, weeds are properly managed and only certified disease-free plant material is used.