Genetic Origin Affects the Reduction of Fruit Load in Young Grapevines Sprayed with an Organosilicone Surfactant and Ethephon

1. Introduction

Young fruit trees in the nursery and in the first few years after transfer from the nursery to the field often produce minor non-commercial yields that can reduce the development of the full scaffold necessary for maximal commercial production (1). Growers often manually remove fruitlets or mature fruit from young trees and vines to allow further development of the fruit scaffold for increased commercial yields (2,3,4). Surfactants have been used to thin fruit crops in mature trees (5-10) , but their use in young (less than three years old; Leviticus 19:23) trees for total removal of the crop has not been thoroughly examined. Dilwet (Adama Inc., Ashdod, Israel) and other organosilicone surfactants have been applied at concentrations of 0.5-1.0 percent to open flowers of grapes (11), apples (12), almonds (Klein and Cohen, unpublished), loquats (13), plums (14,15) and peaches and nectarines (16), and can reduce this undesirable yield significantly. Spraying Dilwet during full bloom, with or without the addition of ethephon (11), significantly reduced the number of Cabernet Sauvignon and Merlot grape clusters, but it has not been tested on other red grapes and not at all on white wine grape varieties.

In 2012, two experimental vineyards were planted in the Negev desert at Ramat HaNegev (elevation 300 m) and at the outskirts of Mitzpe Ramon (Kerem Ramon, elevation 900 m), in order to investigate the effect of hot dry climates on grape production and wine quality (Eran Harcabi, personal communication). Each vineyard included 19 red wine grape varieties and 10 white wine grape varieties. Most originated from the Mediterranean region, with the exception of Riesling and Gewurtztraminer, which originate in Germany (17). We examined the effectiveness of Dilwet spraying, with and without the addition of 0.5 and 1 percent ethephon, as a means of reducing fruit yield of young vines in a wide range of wine grape varieties.

2. Materials and Methods

In preliminary experiments (11), spraying combinations of Dilwet, an organosilicone surfactant also known as Silwet 408, and ethephon, reduced both number and size of Merlot and Cabernet Sauvignon grape clusters. The two cultivars responded differently to combinations and concentrations of the two compounds. Although effects of the treatment on Cabernet Sauvignon were noticed after 3 weeks, effects on Merlot were noted only much later (11).

In order to examine the effects of Dilwet with ethephon on reducing number and size (weight) of grape clusters in a wider range of wine grapes, we tested combinations of Dilwet and ethephon on 19 red wine cultivars and 10 white wine cultivars in two separate vineyards. The cultivars originated in Germany, Italy, France, Portugal, Egypt and Israel. The grapevines were grafted on Ruggeri rootstock and planted in four replications for each variety, with 8 (Ramat Hanegev) or 9 (Mitzpe Ramon) vines per replication. The central two (Ramat Hanegev) or three (Mitzpe Ramon) vines of each of three replications were treated. In late April (Ramat HaNegev) and early May (Mitzpe Ramon) 2014, we sprayed 0.5 and 1.0 percent Dilwet with or without 0.04 percent ethephon during the peak bloom of each variety at each site. Since the growers in both vineyards were eager to remove the grapes to allow development of the young vines, we counted and weighed clusters 5-6 weeks after spraying. We also examined the effect of ethephon alone, as well as an ‘overdose’ of 2 percent Dilwet with 0.08 percent ethephon on five white varieties and four red varieties in the Mitzpe Ramon vineyard.

All sprays were applied using a mechanical Solo backpack sprayer (15 liters). Based on preliminary experiments, we lightly pruned foliage in the flowering area of the vines before treatment. This allowed optimal penetration of the spray. We applied 0.5 liters of solution per vine, which is equivalent to 1000 liters of spray per hectare. Six to eight weeks after the treatments, we counted all the clusters on the middle vine in each triplet of vines/repetition and weighed five of them. At the same time, we rated leaf damage on a scale of 1 to 4, where 1= no damage,2= minor, 3= moderate and 4 = severe damage.

3. Results

Despite the differences in elevation, there were no significant differences between the Ramat HaNegev (300 m.a.s) and the Mitzpe Ramon (900 m.a.s.) vineyards in yield or in response to spray treatments. Data from the two vineyards were combined.

Number of clusters

The number of clusters per vine ranged from 46 (Muscat Alexandria) to 86 (Dolcetto) and averaged 66 (Table 1). Geographic origin of the grapes did not usually affect cluster number, although in the case of white grapes the two Muscat cultivars from Egypt and the Riesling from Germany all had relatively few clusters. Application of 0.5% Dilwet was mostly ineffective in reducing cluster number, except in French-origin white grapes and in 2 of 3 Spanish red cultivars. Doubling the Dilwet concentration to 1% further reduced the overall number of clusters by more than 20% only in French-origin red grapes and in Muscat cultivars. The addition of 0.04% ethephon to 0.5% Dilwet reduced cluster number to an average of 12 and 18% of control in red and white varieties. Adding 0.04% ethephon to 1.0% Dilwet further improved cluster removal in both red and white grapes of French origin, as well as in Temperanillo (ES) and Argaman, which is a cross derived from the Spanish cultivar Carignan (18), but did not substantially improve cluster removal in the other cultivars. However, the number of clusters over all cultivars was reduced to 6% of controls as a result of spraying 1% Dilwet with 0.04% ethephon (Figure 1). The effect was more pronounced in red wine grape varieties than in white wine grape varieties.

Cluster Weight

Grape cluster weight ranged from 68 grams (Cabernet Sauvignon) to 133 grams (Tempranillo) and averaged 88 grams (Table 2). On average, Dilwet alone at either 0.5 or 1.0% reduced cluster weight by less than 20% compared to control. Adding 0.04% ethephon to 0.5% Dilwet reduced cluster weight by more than 60%, while adding it to 1.0% Dilwet further reduced cluster weight by another 10-20% (Table 2 and Figure 2 and Figure 3).

Leaf and Fruit Damage

Dilwet application, with or without ethephon, did not cause significant leaf damage compared to the control. Leaf damage ratings averaged less than 2 (minor) in all cases. The overdose spray of 2% Dilwet and 0.08% ethephon resulted in severe (average rating 3.5) damage to leaves of white grapes, but did not cause significant damage to leaves of red grapes (Table 3). We could not let clusters remain on the vine until ripe. However, damage to unripe fruit (scarring, browning) was non-existent to minimal.

Spraying grapes with 0.04% ethrel alone had no effect on grapevine flowers, fruit clusters, or leaves (data not shown).

3. Discussion

Dilwet and other organosilicone surfactants are effective alone in reducing the number of fruits set in a number of tree and vine crops (11-16). We have now shown that the effect can be enhanced by adding 0.04% ethrel to 0.5% Dilwet, and that results obtained are similar to those found with 1.0% Dilwet alone (a considerable savings). The combination of 1% Dilwet with 0.04% ethephon reduced fruit load the most, but 0.5% Dilwet with ethephon was also quite effective.

It is important to note that spraying did not significantly affect fruit weight as much as it affected fruit formation. This suggests that the significant reduction in cluster number did not have as substantial an impact on individual fruit size in remaining clusters, at least not early in the season when we removed the clusters for analysis, despite the spray’s effectiveness as a selective blossom thinner that reduces the formation and development of fruits (11-16). Unless sprayed at more than twice the recommended concentration, Dilwet did not cause significant leaf damage, as has been noted with apples (12), peaches (16) and almonds (unpublished data).

The many articles about differential effects of wine grape thinning on yield (19-21) and on biochemical (19-21) and organoleptic (24-27) quality indices of grapes and of resulting wine ascribe the contradictory results to varietal differences without further investigation. Recently the contrasting and sometimes contradictory results of variety, yield, and quality indices resulting from wine grape thinning have been ascribed to the interaction of fruit and vine characteristics with varying amounts and timing of irrigation (28). Results presented here were derived from nearly 30 grape cultivars that were grown under identical soil, water, and horticultural conditions, Despite this uniformity of cultivation, we also found that the effectiveness of Dilwet varied among different grape varieties, and that the degree of responsiveness to the treatment could be ascribed at least partially to currently unexamined factors connected to the origin and berry color of the cultivars. French-origin cultivars seemed more resistant to the chemical effect of Dilwet than were Portugese or Spanish-origin varieties (Table 1 and Table 2). White grape cultivars were more responsive to the spray treatment than were red cultivars (Table 1 and Table 2). This was further highlighted in the much greater sensitivity of leaves of white grape cultivars than those of red cultivars to a Dilwet+ethephon overdose (Table 3). It should be noted that cultivar differences in response to thinning compounds have also been noted for apples (3), peaches (7) and pears (10). These results emphasize the need to test a range of cultivars and genetic origins of a given crop when developing and testing new agrichemicals and combinations.

5. Conclusions

Dilwet applications can reduce the number of clusters in a wide range of wine grape varieties, thus permitting young vines to develop more fully without being burdened in their initial growing seasons. The combination of 1% Dilwet with 0.04% ethephon was most effective in fruit thinning (Table 1). However, in many cases 0.5% Dilwet with ethephon performed similarly to the higher concentration, at a savings in cost and in potential environmental contamination. Dilwet treatment did not reduce remaining cluster weight to the same extent that it reduced cluster number (Table 2) nor did it cause damage to leaves or to the fruit scaffold.

These findings provide valuable information for grape growers and vineyard managers seeking to control fruit yield at an early stage of vine development in a range of wine grape varieties. Further studies are recommended to investigate the long-term effects of Dilwet spray application on return yield, and to evaluate its effectiveness in other grape varieties, particularly those originating in other geographic regions or those that are the result of inter-regional crosses.