Grapevines infected with leafroll virus are increasing. As the virus may have a negative effect on grape quality and the productive lifespan of the vines, it is important to research methods by which the spread of the virus can be stopped and its occurrence effectively reduced. The danger of the virus being spread from infected vines to adjacent healthy vines by inter alia grapevine mealy bug [Planococcus ficus (Signoret)] necessitates fast and effective removal of these vines as a source of virus contamination. Unfortunately the mechanical removal of the grapevines leaves live roots in the soil on which mealy bug may survive to contaminate the newly planted grapevines replacing the affected vines. The aim of this study was to identify a herbicide or herbicides, combined with the correct application technique, that will kill leafroll infected full bearing grapevines that have shown leafroll symptoms for four years or more.
Materials and methods
Phase 1 of the study was executed in a 14-year-old Chardonnay/99 Richter vineyard which showed symptoms of being infected with leafroll virus for at least the past four seasons. The vineyard was established on a medium textured soil at Nietvoorbij Experiment farm in Stellenbosch. Fourteen treatments were applied post-harvest on 26 April 2006 and repeated 26 March 2007 (Table 1). Another 14 treatments were applied on 6 December 2006 and repeated on 12 December 2007 during grapevine berry set, as described in Table 1. The treatments were replicated three times in a randomised block design. Each replication plot consisted of six infected grapevines and the two adjacent working rows. A vine row functioned as a buffer zone between treatments situated in different working rows and two vines buffered treatment plots situated in the same vine row. The leafroll virus and mealy bug status of the grapevines were determined visually before the treatments were applied. The trial was repeated over two seasons as explained above, to accommodate the effect that seasonal climatic conditions could have on herbicide efficacy.
To determine the grapevine control efficacy, the vines were monitored visually for re-growth. The roots of one grapevine per treatment plot in which no re-growth occurred, were inspected visually to determine if they are dead or alive. This was done by digging a profile pit 1.5 m long x 1.5 m wide x 1 m deep parallel to the vine row and by inspecting the four profile walls visually for any live roots. These measurements were done mid-October and mid-March, the two periods during which grapevine root growth is most active.
The effect of the different treatments on the occurrence of grapevine mealy bug on the grapevines, as well as the surrounding weeds and cover crops, was monitored visually once a month from August to May.
Results and discussion
Efficacy of herbicides applied post-harvest
Only the foliar application (FA) of Brush Off showed no above-ground re-growth up to one year after application (March 2007) (Table 2). However, active root growth was detected deeper than 10 cm in this treatment at this time. The live roots in the deeper soil layers resulted in re-growth occurring 24 months after application (March 2008). The application date for the 2007/08 season was moved forward to 26 March (directly after harvest) to try and improve herbicide efficacy, as the average temperature in April is higher than that of May (Table 3).
Brush Off (FA), Confront 360 SL (FA) and Plenum 160 ME (FA) resulted in no re-growth for up to one year after application (March 2008) (Table 2). The efficacy of these treatments improved when applied directly after harvest, with the exception of Brush Off. Visual evaluation of the roots in March 2008 confirmed the results of the 2006/07 season, with die-back of the roots in these treatments being limited to the 0 – 10 cm soil layer (Table 2).
Efficacy of herbicides applied during berry set
The above-ground growth of the grapevines was controlled effectively by Garlon 480 EC (FA), Timbrel 360 SL applied to the cut stump of the grapevines (CS), Plenum 160 ME (CS) and Brush Off (CS) up to the end of March 2007 (twelve months after application) (Table 4). Root studies during March 2007 revealed that active root growth still occurred throughout the soil profile (data not shown). Although re-growth occurred after four months (March 2007) where Confront 360 SL was applied (both CS and FA), the above-ground growth died 11 months after application (October 2007). However, active above-ground growth was observed in Brush Off (CS) from 11 months after application (October 2007) onwards, while the same trend was observed in all the treatments 16 months after application (March 2008).
In the case of Garlon 480 EC (FA), Timbrel 360 SL (CS), Plenum 160 ME (CS) and Brush Off (CS), the observations made four months after application (March 2007) was confirmed in March 2008 (Table 4). In contrast to the previous season, no re-growth was observed in the Confront 360 SL (CS) and the basal stem treatment of Garlon 480 EC. Similar to the previous season, active root growth still occurred in all the treatments after four months (data not shown).
Occurrence of grapevine mealy bug
In May 2006, 70% of the grapevines were infested with grapevine mealy bug, whereafter the above-ground level of infestation dropped to less than 1% where it stayed for the remainder of the study. No grapevine mealy bug was found in the soil, although the grapevine roots were still alive. This could be attributed to the fact that the farm practices to control mealy bug improved and that the natural enemies kept the infestation at acceptable levels.
The mechanical removal of the grapevines leaves live roots in the soil on which grapevine mealy bug may survive to contaminate newly planted grapevines. The aim of this study was to identify a herbicide or herbicides, combined with the correct application technique, that will kill leafroll infected full bearing grapevines that have shown leafroll symptoms for four years or more. None of the herbicides or application techniques gave total control of both the scion and rootstock of the grapevines which showed leafroll symptoms for four years or more. Grapevines that are heavily infected or have been infected with leafroll virus for four seasons or more can, therefore, not be controlled by the herbicides, herbicide techniques and dosages tested in the trial.
Some of the herbicide/application technique combinations showed the potential to control both the scion and rootstock of the 15-year-old Chardonnay/99 Richter grapevines heavily infected with leafroll virus. In a follow-up trial (Phase 2), selected treatments were applied in full bearing grapevines which showed leafroll symptoms for two seasons or less. These results will be discussed in another article (Phase 2).
The authors thank the ARC and Winetech for financial support. The following chemical companies are acknowledged 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; Villa Crop Protection for supplying MCPA 400 SL; and Monsanto for supplying Roundup.
– For more information, contact Johan Fourie at firstname.lastname@example.org.