In view of the fact that Aster yellows infects wine, table and raisin grapes, the disease poses a serious threat to the entire viticultural industry and especially the availability of clean plant material.
| Aster yellows is a phytoplasma disease which was first identified in the Vredendal area in 2006. Meanwhile the disease has also been recorded in the Waboomsrivier, Robertson and Montagu areas and classified as a quarantine disease. Serious crop losses and dieback of grapevines occur in all these areas and within two to four years infected blocks may deteriorate to such an extent that they have to be replaced in totality. No direct control measures for this disease exist anywhere in the world.
Various symptoms are associated with Aster yellows disease. Early in the season leaves have a wrinkled appearance and shoots have short nodes and may display delayed growth. Later in the season the leaves of white cultivars become yellow and those of red cultivars show a reddish discolouration. Affected leaves are thicker than normal, brittle and curl downwards. Shoots lignify only partially or not at all, with dieback of shoots from the growth tip and partial dieback of bunches. Infected grapevines deteriorate and eventually die. Overseas Aster yellows is transmitted by leafhoppers and this plays a big role in the spread of the disease within blocks and to other blocks in the immediate vicinity. Only one leafhopper species, Acia lineatifrons, is a known pest in South African vineyards. It causes hopper burn, a condition where leaf edges dry out and the leaves fall prematurely. It is a sporadic pest which only attains pest status in certain years. Surveys were conducted in vineyards infected with Aster yellows in Vredendal, Waboomsrivier and Robertson to determine the following:
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Aster yellows symptoms on white cultivar. Yellow leaves have a wrinkled appearance, leaves are thicker than usual, and the growth tip dies back. |
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To determine whether leafhoppers occur in the vineyards infected with Aster yellows, a motorised backpack-type vacuum sampler was used. The leafhoppers were identified under a stereo microscope and the numbers of each genus or species recorded.
At least five sticky, yellow traps were hung in various locations in each block and after two to three weeks leafhoppers were identified and counted under the microscope. Identifications were confirmed by M. Stiller, an expert in the field of leafhopper taxonomy at the ARC Biosystematics Division in Pretoria.
Altogether 26 leafhopper species in 24 genera and five families were found in the three Aster yellows infected areas. Apart from Acia lineatifrons which is a well-known grapevine pest, only a few of these species were previously known to occur in South African vineyards. Although all genera occurred on grapevines and on weeds in the vineyard during the growing season, the numbers of Acia lineatifrons and Mgenia fuscovaria are high on grapevines and low on weeds.
Molecular methods were used to determine whether Aster yellows phytoplasma occurs in the digestive tract of the various leafhopper species. The University of Pretoria conducted transmission studies on the leafhopper species in which the phytoplasma was found. These studies proved that Mgenia fuscovaria can act as a vector of Aster yellows phytoplasma in grapevines.
Field trials in the three viticultural areas where high infestations occurred, showed that the contact insecticides Steward (active ingredient: indoxacarb) and Dursban (active ingredient: chlorpyrifos) gave excellent results with regard to control of leafhoppers. Products should be applied when leafhoppers become active in the vines and, if necessary, repeated after 14 days. When applying these products, their withholding periods should be borne in mind. Dursban should not be applied within four weeks after bud burst, because it is phytotoxic to young vine leaves.
Commercial applications with the systemic insecticide Kohinor (active ingredient: imidacloprid) also offered very good control up to 17 months following application. This product has a particularly long withholding period (112 days) which has to be managed very carefully. Steward, Dursban and Kohinor are currently registered for the control of leafhoppers on wine grapes.
Although no product is known to kill the phytoplasma in grapevines with Aster yellows, the grapevines may be managed to curb losses. Shoots with yellow leaves and the infected cordons must be removed regularly throughout the season. Post-harvest removal of the entire vine with its roots will considerably reduce transmission of the disease.
Summary
Altogether 26 leafhopper species were identified during a survey in vineyards infected with Aster yellows. Further studies showed that the indigenous leafhopper Mgenia fuscovaria is able to act as a vector of Aster yellows phytoplasma in grapevines.
The results of field trials led to the registration of Steward, Dursban and Kohinor for the control of leafhoppers on wine grapes. To curb losses in infected vineyards, shoots with yellow leaves and the corresponding cordons should be removed regularly during the course of the season. Post-harvest removal of the entire infected grapevine, with roots, helps to limit the spread of the disease.
– For more information, contact André de Klerk at annette8@telkomsa.net or Roleen Carstens at CarstensR@arc.agric.za.
