The mechanical removal of grapevines leaves live roots in the soil on which grapevine mealy bug Planococcus ficus (Signoret) may survive to contaminate the newly planted grapevines replacing the affected vines. The aim of this study was, therefore, to identify a combination of herbicide and application technique that will kill full bearing grapevines that have shown leafroll symptoms for two years or less.Based on the results of Phase 1, and in liaison with the agrochemical companies which supplied the herbicides, the dosage of the most successful herbicides were increased in Phase 2 to determine whether full bearing grapevines that showed leafroll symptoms for two years or less can be controlled chemically. Care was taken not to exceed the dosages currently used to control exotic trees growing in river beds, agricultural land or forests.
Materials and methods
The study was executed in a six-year-old Chardonnay/Ramsey vineyard which showed symptoms of being infected with the leafroll virus for two years or less. The vineyard was established on a medium textured soil at Nietvoorbij Experiment farm in Stellenbosch. The treatments applied during the 2009/10 and 2010/11 seasons are described in Table 1. The treatments were replicated three times in a randomised block design. Each replication plot consisted of three infected grapevines that were buffered from the other treatments as described in the article reporting on Phase 1.
The grapevines that were controlled effectively above-ground and partially below-ground were removed during early June 2010 and early July 2011, and replaced by Cabernet/Richter 99 vines planted on 3 August 2010 and 30 August 2011, respectively. A red scion cultivar was used, as leafroll symptoms are much easier to identify on red grape cultivars.
The grapevines were monitored visually for re-growth. The roots of one grapevine per replication plot of a treatment in which no re-growth occurred, were inspected as described in the article reporting on Phase 1.
Occurrence of leafroll virus in newly planted grapevines
The new grapevines replacing the old ones were monitored visually for one (planted 30 August 2011) and two (planted 3 August 2010) growing seasons. Leaves were sampled from grapevines showing no visual symptoms during early April 2012. Indirect ELISA for specific or simultaneous detection of GLRaV-1, GLRaV-2 and GLRaV-3 in grapevines as developed by Dr. Goszczynski was done on these vines to verify that they were not contaminated with leafroll viruses.
The newly planted vines were monitored visually for the first two seasons after planting for any symptoms that may indicate herbicide uptake from the soil. Soil samples were taken from the 0 – 30 cm soil layer of each replication near the trunk of the grapevines planted end of August 2011. A bio-assay was done on these samples using tomato seedlings to determine whether herbicide residues were present in the soil.
Results and discussion
All the post-harvest applications made during 2009 and 2010 gave 100% control of the above-ground growth of the recently infected six-year-old Chardonnay/Ramsey grapevines during October 2009 and November 2010 (Table 2 and 3). However, only the grapevine roots of the foliar application (FA) of the 0.75% solutions of Garlon 480 EC and Plenum 180 ME caused the grapevine roots to start disintegrating during October 2009 and November 2010. These were the only treatments in which no above-ground growth occurred during February 2010 and February 2011 and in which the roots were also controlled partially.
The basal stem (BS) treatment with a 2% solution of Garlon 480 EC was the only berry set treatment that controlled the above-ground growth and killed the grapevine roots to a depth of 20 cm (Table 2 and 3).
Occurrence of leafroll virus in newly planted grapevines
No visual symptoms indicating leafroll virus infection or herbicide phytotoxicity were detected on the grapevines planted 3 August 2010 and 30 August 2011. One year after these vines were established (vines planted 30 August 2011), one vine in Plenum 160 ME (FA) tested positive for leafroll. Two years after these vines were established (vines planted 3 August 2010), one vine in 0.75% Garlon 480 EC (FA) and 2% Garlon 480 EC (BS) tested positive. In all three cases, the Chardonnay/Ramsey grapevines adjacent to the affected Cabernet/Richter 99 vines did not show leafroll symptoms when the new vines were planted, but showed visual symptoms during the April 2012 visual evaluation. It is possible that the full bearing Chardonnay/Ramsey vines could have facilitated the contamination of the newly planted grapevines.
The mechanical removal of grapevines leaves live roots in the soil on which mealy bug may survive to contaminate the newly replanted grapevines. The aim of this study was, therefore, to identify a combination of herbicide and application techniques that will kill full bearing grapevines that have shown leafroll symptoms for two years or less.
The 2% Garlon 480 EC in diesel applied post-harvest as a basal stem treatment took a full season (from March to March) to kill all roots to a depth of 20 cm or 30 cm. Depending on the season, most roots were also controlled in the 30 – 90 cm soil layer. A post-harvest foliar application of Plenum 160 ME may also be considered as, within the above-mentioned time frame, the grapevine roots were controlled to a depth of 20 cm or 30 cm with some roots being controlled as deep as 50 cm, depending on the season.
Research that is currently being executed, will determine to which depth the mealy bug can survive on grapevine roots in the soil in the absence of above-ground growth.
The current results were submitted to the agrochemical company that holds the patent to the above-mentioned two herbicides to help facilitate registration of these chemicals for the control of leafroll infected grapevines.
The authors thank the ARC and Winetech for financial support. The following chemical companies for supplying the herbicides and technical advice: Dow AgroSciences for supplying Confront 360 SL, Garlon 480 EC, Plenum 160 ME, Timbrel 360 SL; E I Du Pont de Nemours & Co for supplying Brush Off.