Thus far, the indigenous leafhopper Mgenia fuscovaria is the only confirmed vector of the phytoplasma that causes aster yellows in South Africa.
Aster yellows is a phytoplasma disease which causes serious yield loss. Severely infected grapevines may have to be replaced within two to four years, therefore aster yellows has been classified as a quarantine disease. Phytoplasmas can be transmitted by leafhoppers, psyllids and planthoppers.
What does the vector look like?
It is a fairly large, robust leafhopper (adults are 4 – 6 mm long). Adults can be recognised by the whitish spot in the centre where the forewings join, as well as by the black insides of the hind legs (Photo 1). The legs of live leafhoppers are pale green, but the colour fades to brown when dead (Photo 2). Immature leafhoppers or nymphs are pale to bright green and in older nymphs the wing buds and dark colouration on the insides of the hind legs are already visible (Photo 3).
PHOTO 1. Adult Mgenia fuscovaria with characteristic whitish spot (a) in the middle of the forewings and black insides of the hind legs (b).
PHOTO 2. Adult Mgenia fuscovaria: The green colour has faded to brown, but the whitish spot on the wings, black insides of the hind legs and stippling pattern on the fore wings are still visible.
PHOTO 3. Mgenia fuscovaria nymph with wings buds (v) clearly visible. The insides of the hind legs are also starting to appear black.
Researchers at ARC Infruitec-Nietvoorbij in the Western Cape (Wellington, Slanghoek and Wabooms River) found that the leafhopper occurs year round on two evergreen plants, namely wild vine or Cliffortia odorata and wild bramble or Rubus spp. which usually grows along watercourses. In the Olifants River Valley (Vredendal vicinity) researchers from the University of Pretoria found that this leafhopper occurs on various weeds in and around vineyards, and that they also overwinter on weeds, such as nettles (Urtica sp.), henbit dead-nettle (Lamium amplexicaule) and yellow sorrel (Oxalis pes-caprae).
Data from yellow sticky traps adjacent to wild host plants in the Wabooms River Valley and Wellington area showed that Mgenia numbers generally peaked in spring (September/October), with a subsequent peak in autumn or early winter (May/June). Very few to no Mgenia were found on traps in the adjacent vineyards, even when numbers peaked in spring. Furthermore, M. fuscovaria was not the most abundant leafhopper – other leafhopper species that do not transmit aster yellows were generally more numerous. The phytoplasma which causes aster yellows was not detected in wild vines or wild brambles adjacent to the vineyards in the Western Cape.
In the Olifants River Valley sticky trap catches revealed that Mgenia numbers peaked in September and February on grapevines and in June on weeds in vineyards. The numbers were generally considerably higher than in the Western Cape, particularly when the vineyard was heavily infected with aster yellows and M. fuscovaria was the most abundant species. Research at the University of Pretoria showed that M. fuscovaria preferred grapevines infected with aster yellows to healthy plants and that the leafhopper can also transmit the phytoplasma to wheat, maize and the cover crop triticale.
Leafhoppers should be monitored weekly throughout the year, using commercially available yellow sticky traps, for timing of control measures, such as weed management in winter or timing of foliar sprays during the growing season. In areas of high risk for aster yellows, at least two sticky traps should be used per vineyard block.
The numbers of M. fuscovaria in vineyards in the Western Cape were very low. Since the first aster yellows infection was reported in the Wabooms River Valley, no new infections have been reported.
In the Olifants River Valley, where new infections are still being reported, producers are advised to monitor vineyards for the presence of M. fuscovaria. If the leafhopper occurs, particularly where aster yellows infected vines are present in the same or adjacent vineyard, leafhoppers should be controlled with a registered chemical to prevent spreading of the disease. Weed control, especially during winter when leafhoppers overwinter on the weeds, is very important to reduce leafhopper numbers.
Only certified, disease-free planting material should be used when establishing new vineyards. In the Western Cape aster yellows can be successfully limited and eliminated by destroying infected vines and only using certified, disease-free planting material.
The indigenous leafhopper Mgenia fuscovaria is the only confirmed vector of the phytoplasma which causes aster yellows. New infections are still being reported in the Olifants River Valley. Regular monitoring of leafhoppers with yellow sticky traps, control of leafhoppers, weed control, as well as the use of certified, disease-free planting material for establishing new vineyards, are required to contain the disease in this region.
This research was funded by Winetech, the ARC and NRF – projects WW 05/23, UPKK01, UPKK02, UPKK03, THRIP Technology and Human Resources for Industry Programme (THRIP), as well as TROPICSAFE, the European Union’s Horizon 2020 research and innovation programme under grant agreement number 727459, project “Insect-borne prokaryote-associated diseases in tropical and subtropical perennial crops” TROPICSAFE.
– For more information, contact Elleunorah Allsopp at email@example.com.
Article written by: Elleunorah Allsopp & Kerstin Krüger