Effect of selected chemical applications (SevinXLR Plus, Accell) on the quality and yield of McIntosh apples

Abstract
The application of the mixture Accell + SevinXLR Plus increased the degree of thinning to a level comparable to that of the hand-thinned treatment. This was reflected in the total yield per tree, as trees treated with AS resulted in a yield comparable to the one of hand-thinned trees. Fruit weight increased significantly when the mixture of Accell and SevinXLR was applied as a thinning agent. The largest fruits were obtained after the application of the mixture and the weight of treated apples was even higher than the weight of hand-thinned ones. The fruit weight of apples on control trees was significantly lower than the weight of all other treatments. Accell + SevinXLR increased apple flesh firmness at harvest time, but it was not significant in 1998. Scanning electron microscopy might explain some of the increase in firmness. The micrographs (Figure 1) show two distinct cell layers for all apples : an outer more compact epidermal layer and an inner, parenchymatous layer. Generally hand thinned apples had a thicker outer layer followed by Accell+Sevin and Accell treated apples. The thinnest layer epidermal layer was observed in control apples. It seems that the use of Accell, SevinXLR or combination increase the fruit weight and quality and reduce the total yield, however the effect seems to be environmental dependent.

Introduction
Many varieties of apple trees can produce a heavy set of fruit. This results in overcropping with an increase in the number of under-sized fruits that are commercially undesirable. Overcropping may also affect the development of fruit buds for the following year and start a cycle of irregular cropping (Hugard, 1987). It is a common practice to remove a proportion of the immature fruits at an early stage of development by the application of a chemical thinning agent (Looney, 1986). Early season fruit thinning reduces the total yield but increases the number of higher-grade marketable fruits. Chemical thinning using Sevin (carbaryl), has proven useful for certain varieties of apples (Jonkers, 1979; Warnier, 1995). Sevin has the advantage, depending on the variety, of being reasonably consistent in its thinning effect and allowing some margin in time and rate of application (Looney, 1986). Although it is capable of thinning for a period of about 4 weeks from petal fall, it is normally used two to three weeks after the petals have fallen to permit an assessment of initial set and the need to thin (Warnier, 1995). Its main effect is in reducing the amount of small fruits with an increase in the yield of larger apples. There is a tendency for trees thinned with Sevin to bear a higher proportion of king fruits. Other thinning agents like Accell (6-benzyladenine (BA), Verdan, 6-bezylamino purine, gibberellins (GA ₄₊₇)) have been used commercially and have a similar effect to that of Sevin. Accell contains the active ingredient benzyladenine (BA) which is an effective thinner on diverse cultivars (Greene and Miller, 1984; McLaughlin and Greene., 1984), including McIntosh (Greene and Autio, 1989). Some chemicals have also been tested in combination in an attempt to improve spray thinning. Combination sprays of NAA and carbaryl (Looney and McKellar, 1984), NAA and NAD (Ferré, 1979), and NAD and carbaryl (Wertheim, 1985) were reported to result in greater fruit removal.

The objectives of this study were to compare the effects of SevinXLR Plus and Accell, alone and in combination, on the yield, harvest and storage quality as well as size of apples produced by 10 year-old McIntosh apple trees grown on M7 or M26 rootstocks.

 

Results and Discussions
The results of thinning were not very clear after the 1^st application year (Table 1, 1997). They were reflected in the number of apples produced as well as yield, but fruit weight was not affected. When applied alone, Accell and SevinXLR Plus did not significantly reduce the number of fruits as compared to the unthinned control (Table 1, 1997). However, the application of the mixture Accell + SevinXLR Plus (AS) increased the degree of thinning to a level comparable to that of the hand-thinned treatment (Table 1, 1997). This was reflected in the total yield per tree, as trees treated with AS resulted in a yield comparable to the one of hand-thinned trees. The latter were significantly lower than any other treatment, indicating that thinning was successfully carried when Accell and SevinXLR Plus were applied as a mixture. Although the average fruit weight increased when AS was applied, the difference between this treatment and the others was not statistically significant. In a previous report, we had recommended the necessity of gathering data from following years (after repeated applications of the chemicals) in order to be able to evaluate the effects of Sevin and Accell on apples.

In 1998, the second consecutive year, yields of apple trees were not significantly different whether treated with Accell, Sevin, or their combination, hand thinned or not thinned at all (Table 1). Although the number of apples per tree varied between 131 (trees treated with Sevin) to 166 (unthinned control), differences between treatments were not significant. However, fruit weight increased significantly when the mixture of Accell and Sevin was applied as a thinning agent (Table 1) : the largest fruits were obtained after the application of the mixture and the weight of treated apples was even higher than the weight of hand-thinned ones. However, the weight of hand-thinned apples was not significantly different from the weight of apples treated with either Accell or Sevin (Table 1). When Accell was applied alone, it resulted in the production of a high number of small apples (low weight/fruit). Although Sevin seems to perform comparatively to the mixture, the application of Sevin and Accell together did result in larger number of fruits per tree (139 vs 131), as well as bigger fruits (152g per fruit vs 150g). The fruit weight of control apples was significantly lower than the weight of all others.

Accell + Sevin increased apple flesh firmness at harvest time, although the increase was not significant in 1998 ((Tables 2 & 6), and the mixture kept its effect on apple stored for 152 days at 4°C (Table 4), or for 229 days under CAC (Tables 4 & 5). The difference in firmness was pronounced when apples were tested right after harvest in 1997 or after a 152-day storage at 4°C ((Tables 2 & 3). It was not significant when apples were stored under CAC (Tables 4 & 5). Scanning electron microscopy might explain some of the increase in firmness. The micrographs (Figure 1) show two distinct cell layers for all apples : an outer more compact epidermal layer and an inner, parenchymatous layer. Generally hand thinned apples (H) had a thicker outer layer followed by Accell+Sevin (AS) and Accell treated apples. The thinnest layer epidermal layer was observed in control apples (C).

There was no significant difference in the soluble solids content of apples after the second experimental year. In 1997, however, the soluble content of apples was highest when trees were hand-thinned, followed by those treated with AS (Table 2). Although apples treated with AS showed a significant lower soluble solid content than the hand-thinned ones when tested right after harvest, the difference was not significant after storage. Those apples showed a significant increase of SS (close to 1%) after storage (Tables 3 & 4). Whether tested right after harvest or after storage, apples thinned with either Accell or SevinXLR Plus alone showed a soluble solid content comparable to apples harvested from the unthinned control. The unthinned control showed the lowest increase in percent soluble solids, as did apples treated with Accell and stored at 4°C (Tables 2 & 4). These findings are in accordance with the literature, since Fisher and Kitson (1991) reported that under controlled environmental conditions, where relative humidity is kept at a high enough level to prevent dessication, there should be a gradual decrease in the levels of total sugars, resulting in a decrease in percent soluble solids, due to respiration. Since soluble solids content is associated with sweetness, apples treated with the mixture of Accell and SevinXLR Plus may appeal to consumers more than the others. Acidity, on the other hand, is associated with perceived sourness. According to Fisher and Kitson (1991), 0.3-0.4% acidity (medium range) in apples appear to be most accepted by consumers. McIntosh, with an average of 0.54% and an acidity range of 0.33 to 0.74%, is a high acid apple (Fisher and Kitson, 1991). In our case, all apples, whatever the treatment or lack of the latter, have a high percent of acidity (Tables 2-8). This might be due to the characteristics (environmental and physical factors) of the region where these apple trees are grown. The highest percent of acid content was found in apples from hand-thinned trees or trees thinned with AS. These were the apples that also showed the highest soluble solids content, meaning that sourness resulting from the high acid content may somewhat be masked by the sweetness of the soluble sugars.

When analyzed after the 1998 CAC storage as well as after CAC storage followed by 7 days storage at room temperature, the pressure, soluble contents, and percent acidity of treated apples were not significantly different from those of hand-thinned or control apples (Tables 7, 8). Only percent acidity after CAC storage showed some variability (Table 7). Apples treated with Sevin alone seem to be more acid than the ones treated with Accell alone. However, apples treated with the mixture of Accell and Sevin had an acidity comparable to those hand-thinned or untreated.

Some apples showed physiological disorders after storage under controlled environmental conditions, as well as after the 7-day storage at room temperature after that. It is difficult to pin point the cause of the vascular breakdown or the light senescence breakdown that we observed. The hand-thinned treatment seem to have resulted in the highest number of apples with vascular breakdown (Table 9). However, the unthinned control also resulted in apples with physiological disorders. These might be the result of storage conditions or handling.

Juice characteristics. No significant differences in total pectin and total sugar content, or pH, were found between the different treatments (Table 10). Only titratable acidity showed some variability, with juice from hand-thinned apples being the most acid, and the one from apples treated with Accell the least acid (Table 10). This correlates with the percent acidity obtained immediately after the 1997 harvest (Table 2), where apples treated with Accell or Sevin showed the least degres of acidity and hand-thinned apples the most.

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