Dessert Quality and Storability of Jonagold Apples
Jonagold, a controlled cross of Jonathan by Golden Delicious, has aroused a great deal of interest in British Columbia (B.C.) during the past 2 or 3 years. There were approximately 5,000 Jonagold trees planted in 1984 and 10,000 trees planted in 1985. There is a projection that 15,000 Jonagold trees will be planted in 1986 and 25,000 trees in 1987. The prospect of growing and marketing Jonagold apples has been discussed at the British Columbia Horticultural Forum (Proceedings of the 16th Annual British Columbia Fruit Growers Association (BCFGA) Horticultural Forum, 1984). However, experience in growing, harvesting, and storing this cultivar in air and CA is limited. Below is a preliminary report on current research aimed at evaluating the dessert quality, harvest indices, and storability of B.C.-grown Jonagold apples. This information may help the B.C. industry decide whether Jonagold is a pro mising cultivar for commercial production in this area.
Dessert QualityJonagold has a juicy and fine texture. Its acids and sugars are higher than those in Golden Delicious, Spartan, and Red Delicious (Table 1). Even after storage, until May, it still retained acid levels (Table 2) higher than those of Red Delicious, or comparable to those of Golden Delicious and Spartan at the time of harvest (Table 1). It generally receives a higher taste rating than other B.C. apple cultivars.
Like most triploid (3N, that is, 3 sets of chromosomes) varieties, Jonagold trees are relatively vigorous and fruit size is generally large. Jonagold apples harvested on October 10, 1985 from 10 to 11 year old trees (M111 rootstock) at the Summerland Research Station, averaged 8.8 to 10.6 ounces (250 to 300 grams) (box size = 72 to 62, XL to XXL) with 2.5% of the fruit afflicted with bitter pit. Fruit picked from the same trees, which did not receive any fruit thinning during the 1985 growing season, were still fairly large, averaging 7.5 ounces (213 grams) in weight (box size = 88) on October 10, with no bitter pit. It will be important to find out if these unthinned trees bear a full crop next year.
Jonagold fruit are striped with yellow ground color. Most of the red color appears to develop at Summerland between October 1 and 10 (Table 3). Like any apple cultivar, color development in Jonagold varies from year to year. On October 10, for example, well exposed fruit had only about 50% red color in 1984, compared with as much as 80% in 1985 (Table 3).
Later picking improves red color in the fruit, but this may also lead to a higher incidence of Watercore at harvest, resulting in flesh browning and storage breakdown. It appears that a proper level of nitrogen fertilizer and selective picking rather than once?over harvesting (most exposed fruit first and then least exposed fruit a week later) would improve grower returns, and minimize water core, flesh browning, and storage breakdown. Summer pruning might also help to improve red color. In a preliminary study in cooperation with Mike Sanders of the British Columbia Ministry of Agriculture and Food (BCMAF), Alar applied in late-spring and mid-summer of 1985 did not appear to significantly improve red skin color in Jonagold (data not shown).
Physiological and Storage Disorders
Bitter pit--Because of its large fruit size (typical of a triploid) which generally results in low fruit calcium, bitter pit can be a problem in some years (Table 2).
Watercore--Jonagold developed Watercore in the first part of October with as much as 12%,53%, and 68% of the fruit afflicted with Watercore on October 10,21, and 28, respectively. Although Jonagold is less susceptible to Watercore than is Red Delicious the presence of Watercore will predispose the fruit to flesh browning and storage breakdown.
Flesh browning and breakdown--Jonagold is quite susceptible to such storage disorders as flesh browning and flesh breakdown, particularly in late-picked, Watercored, and oversized (low in fruit calcium) fruits.
Fruit Maturation and Ripening (1985)
Changes during the maturation and ripening of Jonagold were followed between September 19 and October 21 of 1985, in an attempt to identify some useful maturity indices for harvesting this cultivar for maximum dessert quality and storage potential; and to evaluate the performance and potential problems of this cultivar in air and CA storage, in relation to cultural practices and harvest maturity.
The patterns of maturation and ripening of Jonagold between September 19 and October 21, 1985 (Table 3) can be summarized as follows:
As Jonagold matures and ripens, there is an increase in fruit weight (0.25 ounce [7 grams] per week), yellow ground color, red skin color, soluble solids (0.37% per week), and the amounts of ethylene and watercore (large increase after October 10). There is a concomitant decrease in green ground color, starch (higher starch index), flesh firmness (0.35 pound per week), and titratable acidity (0.1% per week).
Starch hydrolysis (as indicated by the higher starch index) occurred during late-September, but the internal ethylene concentration did not exceed 0.5 ppm until mid-October, indicating that when used in conjunction with other harvest parameters, starch index is a good index for predicting harvest date for this cultivar.
Preliminary studies (1984 and 1985) revealed that Jonagold is ready to be picked for CA storage around October 10, when the fruit has attained a reasonable size and color (50 to 80% red color), firmness (16.5 to 17.0 pounds), soluble solids (13.5 to 14.5%), starch (starch index of 6 to 7 on the Ottawa/BC scale of 9), and acids (0.7 to 0.8%). The fruit should not be allowed to develop Watercore (<15%) or evolve ethylene (<1.5 ppm internal ethylene or <30% of the fruit with internal ethylene >1 ppm) which may drastically reduce the length of Jonagold storage.
Storage Performance (1984)
A limited storage study conducted in 1984 revealed that Jonagold apples lost firmness and titratable acidity very rapidly in 32°F (0°C) air storage (Table 2). At harvest the firmness and titratable acid levels were 16.1 pounds and 0.78%, respectively. They were reduced to 13.6 pounds and 0.49% on February 25, and to 12.6 and 0.34%, May 31, respectively. This resulted in fruit that is quite soft and bland in taste. Flesh breakdown was found in 8% (range: 0 to 26% breakdown) of the air-stored fruit in February, and in as much as 23% (range: 0 to 38%) in June.
Comparable lots of fruit stored in 32°F (0°C) CA (1.5% oxygen + 1.5% carbon dioxide) were, on the average, 2.1 pounds firmer and 0.17% higher in acidity than those stored in air. CA stored Jonagolds did not develop breakdown as late as June (Table 2). CA stored fruit received high firmness, taste, and flavor ratings from the taste panelists in the late spring.
Jonagold does not keep well in regular air storage. However, CA storage technology exists which can keep Jonagold apples in good condition until late spring or early summer.
The potential problems of Jonagold apples and their corrective measures are outlined in Table 4. They should be considered carefully before Jonagold is grown in B.C. in commercial quantities.
I extend my appreciation to Agriculture Canada Research Station, Summerland for providing the fruit samples and to R. Yastremski, M. Bacon; R. Potter, and A. Yastremski for their valuable assistance. Special thanks are also due to Dr. D. Lane for critically reading this article.
Dr. O. L. (Sam) Lau, Postharvest Physiologist
Industry Research Program, Okanogan Federated Shippers Association, Canada
Post Harvest Pomology Newsletter, Vol 4, No. 3