Materials and methods
A five year study was conducted in a seven year old Shiraz/101-14 drip irrigated (from December to March) vineyard, established on a sandy to sandy clay loam soil near Stellenbosch. The rainfall during the study is shown in Table 1. The fourteen treatments were replicated five times and consisted of two soil management practices, namely CC and MC (Table 2), applied to canola, Caliente 199, Nemat, white mustard, Pallinup oats (cover crop frequently used in vineyards), a treatment in which no cover crop was sown (Weeds) and a weeds treatment in which a nematicide was applied in the vine row during grapevine bud break (Weedsnem) (Figure 1). The nematode status of the soil was determined just before the management practices were applied (day 0), 15 days and 60 days thereafter, as well as at the beginning of April (before the cover crops were established).
Results and discussion
The ring nematode populations in the work row were in all cases very low (significantly lower than those in the vine row), which did not allow for specific conclusions or trends regarding the impact of the cover crops on these nematodes to be made (data not shown). The ring nematode populations in the vine row are shown in Figure 1.
Three periods were considered, namely:
- April to day 0 in a specific year, which indicates the impact that the actively growing cover crops and winter weeds had on the ring nematode numbers;
- Day 0 to day 60 for a specific year, indicating the impact of the cover crops after the management practices were applied; and
- Day 60 to April the following year, indicating the ability of the ring nematodes to recover during the ‘inter-treatment period’.
April to day 0
During 2010, the ring nematode population declined in canola (CC) and Caliente 199 (MC) (Figure 1a & b). This trend was observed for all treatments during 2011 and 2012, with the exception of Nemat (CC), white mustard (MC) and Weedsnem (CC) during 2011 (Figure 1a – g). During the relatively wet winter of 2013 (Table 1) this trend was observed in the two Nemat treatments, white mustard (CC), Pallinup oats (CC), Weedsnem (MC) and Weeds (CC) (Figure1c – g). The ring nematode population increases during the growing season of the cover crops and were on average 61% for canola, 35% for white mustard, 26% for the weeds, 20% for Pallinup oats, 17% for Caliente 199 and 7% for Nemat.
Day 0 to day 60
A continuous decline in ring nematode numbers from day 0 to day 60 was detected throughout the study for canola (MC) (Figure 1a). This was also observed for white mustard (MC) and Weedsnem (CC), with the exception of 2013 and 2009, respectively (Figure 1d & f). Pallinup oats (CC) resulted in the ring nematode numbers declining continuously from day 0 to day 60 during 2009, 2010 and 2011 (Figure 1e). As far as the other treatments are concerned, the ring nematode numbers were suppressed irregularly.
Day 60 to April
With the exception of canola (CC) in 2009/10, as well as the two white mustard treatments, Nemat (CC) and Weedsnem (CC) in 2011/12, the ring nematode populations repeatedly increased from day 60 to April the following year (Figure 1a – g). The rainfall during this period varied between 152 mm and 228 mm, which seemed sufficient to allow the ring nematode population to recover at least partially. The average percentage increase during this period was much higher than during the winter (April to day 0), namely 247%. This is an indication that a follow-up treatment with a nematicide is necessary in an attempt to maintain the downward trend achieved during the sixty day period after grapevine bud break. Biofumigation should, therefore, not be applied in isolation, but as part of an integrated nematode control program.
Day 0 2009 to day 60 2013
A significant decline in the ring nematode population was observed from day 0 in 2009 to day 60 in 2013, irrespective of the treatments applied (Figure 1a – g), with the exception of white mustard (MC), although the rainfall was higher during the 2012/13 and 2013/14 seasons than during the 2010/11 and 2011/12 seasons. It is possible that the roots of the permanent sward, which was maintained in the grapevine inter row before the treatments were applied by regular slashing, supplied an additional food source to the ring nematodes throughout the year. In contrast, the soil management practices applied during the trial removed this food source during spring and early summer (also in the Weeds treatment in which no biofumigation crop or nematicide were applied), which reduced the availability of roots during the grapevine growing season.
During winter, Nemat, Caliente 199 and Pallinup oats did not facilitate a ring nematode population increase to the same extent as the weeds growing in this region. In some years, the ring nematode population even showed a decline during winter, whether a cover crop was established or not. During the early grapevine growing season, the most effective ring nematode suppression was achieved when canola and white mustard were mechanically incorporated into the soil, as well as where a nematicide was applied or oats was sown and combined with full surface chemical control. However, the drastic increase in the ring nematode population during summer and early autumn indicates that a follow-up treatment with a nematicide is necessary in an attempt to maintain the downward trend achieved during the sixty day period after grapevine bud break. Biofumigation should, therefore, not be applied in isolation, but as part of an integrated nematode control program. Controlling the weeds/cover crops during the grapevine growing season may help reduce the ring nematode population over the medium term.
The authors thank the ARC, Winetech and Dried Fruit Technical Services for financial support, the staff of Soil and Water Science at ARC Infruitec-Nietvoorbij for technical support and Blaauwklippen Wine Estate for supplying the trial site and farm support.
– For more information, contact Johan Fourie at email@example.com.