Horticulturist Todd Einhorn says leaving cherries on the tree longer can result in higher sugar content, but this must be balanced against adverse effects
An Oregon State University horticulturist is doing research to help cherry growers optimize harvest timing and is looking at the feasibility of using skin color to predict other quality attributes of cherries.
Dr. Todd Einhorn, at OSU’s Mid-Columbia Agricultural Research and Extension Center in Hood River, is conducting a two-year project, in collaboration with Dr. Matthew Whiting and Lynn Long, looking at the changes in quality attributes of ten cherry varieties during the final fruit growth and ripening periods.
Tests were conducted in 2009 and 2010 at two orchards, one in Oregon and one in Washington. Each variety was harvested three times: at normal commercial harvest timing, a few days before, and a few days after. On each date, one tree was strip-picked and fruit quality analyzed. To classify the color of the fruit, Einhorn used the Ctifl scale that ranges from 1 (light pink) to 7 (black).
He found that, in general, as harvest is delayed, skin and flesh color, fruit size, and sugar content all increase, while stem retention force and fruit firmness decrease. Changes in all quality attributes over the harvest period were related to the progression in skin color. However, a fairly wide range in individual quality attributes was observed within a given color class.
Further, the values of those attributes change significantly from year to year, depending largely on the crop load and climate. Although skin color darkened with later harvest dates, the rate of darkening depended on the cultivar.
There was an amazing amount of variability of skin color on the tree at any one date, Einhorn reported to the Washington Tree Fruit Research Commission, which is partially funding the research. For example, at the midharvest timing for Skeena, the average color score was 3.9, but a third of the fruit fell into each of the 3, 4, and 5-6 color classes, and the sugar content increased by 10 percent with each increase in color class, while firmness was the same.
Fruit increased in size with subsequent harvest dates, up to a point. Not all varieties darken to a mahogany color. He found that as Bing and Regina cherries attained the darker color (7), they did not gain additional size. Although potential fruit size is determined by the cultivar, other factors, such as crop load, light, age of the wood, climate, and bloom timing, also affect fruit size. Flowers that opened earlier in the bloom period tended to produce larger fruit. This can probably be attributed to the longer time they are on the tree, but could be due to their advanced development.
Good horticultural practices (such as pruning and training to enhance light penetration and distribution, renewal of fruiting wood, and application of plant growth regulators) and new tree architectures (such as the Upright Fruiting Offshoots) could help reduce some of this variability, Einhorn believes.
Einhorn also looked at fruit softening and said a model using temperature data for the final weeks leading up to harvest might help explain differences from year to year. For example, firmness values in his trials were 20 percent lower in the warm growing season of 2009 than in 2010, when temperatures were lower. A predictive model could help growers with harvest timing decisions in hot weather. However, crop load is also a factor in softening. Einhorn said that in studies of crop load on Lapins, Sweetheart, and Skeena, he has seen significantly softer fruit with heavier crop loads.
Although soluble solids increase in direct relation to the darkening of skin color, differences from year to year also can be attributed to light, temperature, and crop load. Leaving cherries on the tree longer can result in higher sugar content, but this must be balanced against adverse effects such as fruit softening and, in some cultivars, lower stem retention force.
It’s possible, Einhorn said, that degree-day models could be developed to more accurately predict harvest date for any given year, site, and cultivar. However, research would need to be done to determine what attributes best define quality. Would that be the best quality for storing, shipping, or consumer acceptance?
Einhorn did tests with Sweetheart, Bing, and Lapins cherries to study their susceptibility to pitting relative to crop load. He found no relationship between the amount of pitting and crop load. For Sweetheart, skin color at two different harvest timings was unrelated to the severity of pitting.
He is also studying the quality of cherries in storage and observed in both 2009 and 2010 that fruit firmness either remained the same or increased for up to 28 days of storage, though the reason is not clear. This means that any disparity between the firmness of fruit when it goes into storage and when it arrives at the retail market can likely be attributed to temperature fluctuations during transportation or at the retail market. Soluble solid content was not affected by storage time as long as the fruit did not become dehydrated. In some varieties, acids degraded in storage.