Handbook for Irrigation of Wine Grapes in South Africa

Figure 6.24 out the possibility that the high crop load could have increased ET, compared to grapevines bearing no grapes. In fact, micro-sprinkler irrigated Colombar grapevines bearing a full crop load of 39.4 t/ha extracted more soil water during a drying cycle, compared to ones where the crop load was reduced by 50% to 19.9 t/ha (Van Zyl, 1984). In this case, it was hypothesized that the higher crop load absorbed more soil water during the night, and that this water was lost via transpiration in the day. This suggests that the higher crop load could have enhanced transpiration losses dur ng th initial stages of the drying cycle in the sandy loam to sandy clay loam soils with a high level of PAW. explanation could be that the full crop load became a more competitive sink for water at the cost of transpiration, thereby reducing Ψ S . However, this does not rule

SWC (mm/60 cm)

103

32

-1 8 -1 6 -1 4 -1 2 -1 0 -0 8 -0 6 -0 4 -0 2 0 0

a a a

a

ab

Ȍ S (MPa)

b

No bunches 1 Bunch per shoot Full crop load

FIGURE 6.24. Effect of crop load and SWC on midday Ψ S during berry ripening in Mourvèdre in a sandy soil near Rawsonville (unpublished data). For each level of SWC, bars designated by the same letter do not differ significantly (p ≤ 0.05).

6.3.8 LEAF DAMAGE BY PESTS Erinose mites are a common pest which may cause extensive leaf damage in vineyards (Fig. 6.25). A pilot study was carried out by measuring midday Ψ L in 20 damaged, as well as undamaged Cabernet Sauvignon leaves from four different grapevines. The damaged leaf area was determined by measuring the mean diameter of 100 felt galls on the ad-axial leaf sides. Leaf damage caused by erinose mites does not seem to affect grapevine water status (Table 6.1). Although there was substantial variation in the number of blisters per leaf, there was also no relationship between Ψ L and the percentage leaf damage (data not shown).

166 CHAPTER 6 – GRAPEVINE WATER STATUS