ScientiaHorticulturae, 35 (1988) 207-215 207 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
Effect of Gibberell in A4÷ 7 o n Stem- and Calyx-End Russet ing in 'Golden Delicious' Apples
J. STEENKAMP and INA WESTRAAD
Fruit and Fruit Technology Research Institute, Private Bag X5013, SteUenbosch, 7600 (Republic of South Africa)
(Accepted for publication 7 October 1987)
ABSTRACT
Steenkamp, J. and Westraad, I., 1988. Effect of gibberellin A4+7 on stem- and calyx-end russeting in 'Golden Delicious' apples. Scientia Hortic., 35: 207-215.
Successive gibberellic acid and promalin sprays resulted in a significant reduction of calyx-end russet during 1983/84 and 1984/85, respectively. Four sprays of 10 mg l - 1 GA4 +7 gave the greatest reduction in both calyx- and stem-end russet, and should therefore be considered as the best treatment. It was also found that larger apples were more susceptible to calyx-end russet, while stem-end russet was more evenly spread amongst the various size groups. Although these treat- ments increased calyx-end opening, no correlation with core rot (an internal disorder mostly caused by Alternaria spp. ) was found.
Russet has a detrimental effect on the appearance of 'Golden Delicious' ap- ples and is therefore considered a major problem of this cultivar. Russet symp- toms become apparent when the cuticle and epidermis are replaced by a protective periderm layer (Skene, 1982 ). The symptoms may be limited to the stem- or calyx-end of the fruit, or may cover the whole surface under severe russet conditions. The incidence of russet may fluctuate among years, or- chards, and even among individual fruit on the same tree. The sensitivity to, and intensity of, russeting depends indirectly on the chemical (Meyer, 1944) and/or physical (De Vries, 1968; Faust and Shear, 1972) composition of the wax layer, which influences the fruit's ability to form cracks.
Water, in the form of humidity and rain, is most important in the etiology of russet (Brown and Koch, 1962 ). Therefore orchards situated in dry regions are usually less subject to russet than those in humid and/or cool areas (Faust and Shear, 1972; Creasy, 1980). Russet is also more prevalent in humid micro-
climates such as along rivers (Eccher and Boffelli, 1981 ), dams or where morn- ing mists are frequent (Tukey, 1960).
The period from full bloom to approximately 6 weeks thereafter, when the fruit surface is under severe tension because of rapid growth, is apparently critical in the development of russeting (Simons, 1962, 1965; Wertheim, 1982 ). The interaction of factors involved in the mechanism of russet development is highly complex, and the intensity must therefore be seen as the sum total of a great number of climatological factors (temperature, sunshine, etc.), cultural practices (fertilizing and spraying materials), different clones and interac- tions among these (Tukey, 1960; Spruits and Peerbooms, 1983).
The best long-term solution would be the propagation of clones naturally resistant to russet. As it is practically impossible to control environmental or genetic factors over the short term, Eccher (1978) has suggested the growth regulator manipulation of fruit growth (at the time when the fruit are vulner- able ) to eliminate the formation of cracks. GA4+7 seems to be the most prom- ising of the wide range of chemicals evaluated over the past few years for the control of russet (Taylor, 1975, 1978; Katschner, 1978). GA4+7 applied ap- proximately 8 days after full bloom is known to stimulate a uniform build-up of the cuticle and promote cell elasticity, therefore rendering the developing fruit more resistant to surface tension and eventually decreasing the incidence of russet (Eccher and Boffelli, 1981 ).
In this study, preparations containing gibberellic acid were evaluated to de- ;ermine their effect on calyx- and stem-end russeting, fruit size and yield.
MATERIALS AND METHODS
The trials, following upon previous work (Steenkamp et al., 1984), were conducted for a further two seasons on 16-year-old 'Golden Delicious' apple trees on Merton 793 rootstocks, trained as a palmette. All trees received nor- mal cultivation practices and were chemically thinned with naphthylacetam- ide (0.70 g 1-1) 4 and 7 days after full bloom (31 October and 26 October) during 1983 and 1984, respectively.
The experiment was laid out in randomized blocks with 15 single-tree rep- lications per treatment. Ten days after full bloom, each tree was sprayed with 10 1 of solution with hand guns at weekly intervals (Table 1). During the 1983/84 season, 0.03% Agral-90 was used as a wetting agent, while it was omit- ted during the 1984/85 season. As this study was a continuation of previous work, the same trees received the same t reatment as before, to eliminate trans- mission effects or interactions which might have developed amongst the dif- ferent treatments. GA4+7 concentrate was supplied by Abbott Laboratories, North Chicago. The ratio of GA4 and GA7 was 1:1 throughout, except where specified. GA4+7 (3:1) was not available during the 1984/85 season. Promalin (6-benzyladenine and GA4+7, in an approximate 1:1 ratio) was eventually in-
TABLE 1
Time and number of treatments applied in 1983/84 (X) and in 1984/85 (Y)
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Treatment Date of treatment (weeks after full bloom)
0 1 2 3 4
Control (water) XY XY XY XY GA4+v (10 mg1-1) XY XY XY Y GA4+v (10 mg 1-1) X X X X GA4+v (10mg1-1) X X X X GA4+7 (5 mg 1-1) XY XY XY XY GA4+7* (10 mg 1-1) X X X X Promalin ( 1.25 ml 1-1 ) XY XY Promalin (1.25mll-1)/GA4+7 (10mg1-1) X(P) X(P) X(GA) X(GA)
X XY X
P = date on which promalin was applied GA = date on which GA4+7 was applied. *75% GA4+25% GAT.
c luded to serve as a source of GA4 + 7, a l though it was r epor t ed by Tay lo r (1975) t h a t benzy laden ine may increase the level of russet ing.
All f ru i t were p icked on the commerc ia l ha rves t ing date. T h e f rui t of each t ree were subsequen t ly graded in to d i f fe ren t commerc ia l size-groups, and the apples in each group were sor ted in to four calyx- (Fig. 1) and s t em-end (Fig. 2 ) russe t categories.
A russe t ing index (RI ) was ca lcu la ted by mul t ip ly ing the n u m b e r of f ru i t in Ca tegory 0 by 1, t h a t in Category i by 2, t h a t in Category 2 by 3, and t h a t in Ca tegory 3 by 4. T h e s e figures were added and t h e n divided by the to ta l n u m b e r
iii~i~iii~:~,~ ¸
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Fig. 2. Categories of s t e m - e n d russe t on 'Golden Delicious ' apples. 0 = complete ly smooth ; 1 = slightly russeted; 2 = mode ra t e ly russeted; 3 = severely russeted.
of fruit. The advantage of such an index value is that the counts in the separate grades are combined into a single figure and may therefore be regarded as an overall indication of the intensity of russeting.
The calyx-end opening and the percentage core rot were determined after the apples were cut longitudinally by measuring the opening just below the sepals and counting the number of apples affected by core rot.
To test all comparisons between pairs of t reatment means, Tukey's least significant differences (LSD) were calculated according to Snedecor and Cochran (1980).
R E S U L T S
All treatments, except four 10 mg l-1 GA4+7 sprays during 1983/84, resulted in a significant reduction of calyx-end russet, as compared with the control during 1983/84 and 1984/85 (Tables 2 and 3). During 1983/84 it appeared that the 5 mg 1-1 GA4+7 t reatment was as effective as any of the 10 mg 1-1 GA4 + 7 treatments. This tendency also occurred during the 1981/82 and 1982/83 seasons (Steenkamp et al., 1984). The higher index in the control during the 1984/85 season indicated that the intensity of russeting was much greater in comparison with the previous three seasons mentioned above. In the size-groups 83 to < 64 mm, the calyx-end russet index average decreased gradually from 1.39 to 1.14 and from 1.69 to 1.44, during 1983/84 and 1984/85, respectively (Table 3). However, while it was found that larger apples were more suscep- tible to calyx-end russet, stem-end russet was much more evenly spread be- tween the different size-groups (Table 4).
It was found that under South African conditions, four 10 mg 1-1 GA4+7
Effect of various spray treatments on stem-end russeting indexes (RI) 1 for different size-groups of 'Golden Delicious' apples during the 1984/85 season
1See Table 3. *Significantly different from the control at P = 0.05.
TABLE 5
Effect of various spray treatments on the percentage of 'Golden Delicious' apples in different stem- end russet categories and on the calyx-end opening during the 1984/85 season
*Significantly different from the control at P = 0.05.
sprays also decreased the occurrence of s t em-end russet significantly on 'Golden Delicious ' apples dur ing 1984/85 (Tables 4 and 5). A l though this reduc t ion was small, as differences only occurred in two of the size-groups, the to ta l rus- set index was s ignif icant ly different. Th i s f inding conf i rmed results ob ta ined by Van Rooi jen (1983), who found t h a t GA4+7 also decreased the n u m b e r of s t em-end cracks on 'Discovery ' and 'Sep te r ' apples.
A l though all t r e a t m e n t s increased the ca lyx-end opening (Table 5 ), no core rot was found.
DISCUSSION
The russet index of 1.72 dur ing 1984/85 was 17.8% higher t h a n t h a t of 1.46 of 1983/84. I t is ev ident t h a t dur ing years of high incidence of ca lyx-end russet,
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the t reatments were much more effective, confirming results reported by Scholtens and Bootsma (1981). During 1984/85 (a severe calyx-end russet year) the percentage improvement by these t reatments was 67.9% compared to the 18.9% of the previous season.
The variation in calyx-end russet control by the same t reatments in different seasons is difficult to explain under the present circumstances.
During these experiments, it has also become evident tha t under South Af- rican conditions, 'Golden Delicious' apples are much more susceptible to stem- end russet than to calyx-end russet. Russet indexes of 2.35 and 1.72 were ob- tained for stem-end russet and calyx-end russet, respectively, during the 1984/85 season.
The results indicate tha t four sprays of 10 mg 1-1 GA4+7 gave the greatest reduction in both calyx- and stem-end russet, and should therefore be consid- ered as the best t reatment.
At the time of writing, apples in calyx- and stem-end russet Categories 0 and 1 are acceptable for most overseas markets. In practical terms this means tha t 9.2 and 19.4% of the South African 'Golden Delicious' crop could not be ex- ported during the 1983/84 and 1984/85 seasons, respectively, due to calyx-end russeting. The corresponding figure for stem-end russet during 1984/85 was 47.3%. The economic implications of such a loss are considerable.
Although the gibberellic acid t rea tments did not control russeting com- pletely, these chemicals should prove a useful tool in apple production areas where russet is a problem.
ACKNOWLEDGEMENTS
We would like to thank J. Hadlow and all s taff members of the Elgin Exper- imental Farm for their assistance in this research.
REFERENCES
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