Issue link: https://read.dmtmag.com/i/350705
www.goodfruit.com Good Fruit Grower AUGUST 2014 15 4.0 mg per 100 g might want to wait to pack that fruit until symptoms of bitter pit appear so it can be sorted out before marketing (see "Recommended calcium in Jonagold"). The most telling indicator of all-round fruit quality is the ratio of calcium to nitrogen, Wolk said. In Hon- eycrisp, neither nitrogen nor calcium alone correlates well with fruit quality, but there's a strong relationship between the nitrogen-calcium ratio and the percentage of bitter pit, for example. Potassium Potassium has a positive effect on fruit color, which is why hundreds of millions of dollars of potassium fertilizer have been sold to growers over the years, Wolk said. He has seen this positive effect on color regularly in Spartan and Jonagold, but not so much in other varieties. Phosphorus It is often held that you cannot have too much phosphorus in fruit, Wolk said. While it is desirable in fruit because it can help enhance firmness, it can also increase bitter pit in sensitive varieties if the level is high, particularly if it is out of balance with the calcium level. Boron Because British Columbia soils are low in boron, fruit levels were expected to be low. But most fruitlet samples had high levels, indicating that growers were applying too much boron. High boron levels can lead to early maturity (including stem bowl splits), fruit drop, and poor storability. "Growers are unaware of how easy it is to get boron into the tree and also unaware how sensitive the fruit can be to high boron," Wolk said. Nonmineral factors The density of the fruitlets, which is easy to measure, is related to firmness, bitter pit, stem-bowl splitting, and visual appearance. "We haven't used the density a lot," Wolk said. "But I'll tell you something: high-density fruit is really good fruit." Small fruit tends to be more dense, he said. "But if you have two size 88 apples and one is more dense than the other—I'm assuming it has smaller cells and more cells— that fruit is just going to be all around higher quality than the lower density fruit." Fruit weight (size) is related to firmness, internal breakdown, and bitter pit. Big fruit usually has lower cal- cium. A powerful predictor of fruit quality is the weight of the fruit divided by the milligrams of calcium per 100 grams of fruit weight. As well as helping individual growers, fruitlet analysis can give the industry a heads-up about the overall stor- age potential of the crop and indicate if it's going to be a high-calcium year or a low-calcium year. In addition, it can can red-flag grower lots that have specific problems, Wolk said. • <4.0 4.0-4.9 5.0-5.9 6.0-6.9 >6.9 Calcium level in mg per 100g fruit weight 16 14 12 10 8 6 4 2 0 % bitter pit in fruit after 60 days of cold storage and 7 days at room temperature Recommended calcium in Jonagold Research in British Columbia has shown that when the fruit calcium level six weeks before harvest is below 4.9 milligrams per 100 grams of fruit weight, the likelihood of bitter pit developing after harvest greatly increases. To allow a margin of safety, B.C. growers are advised to aim for a calcium level of 6.0 mg. SOURCE Dr. Bill Wolk, B.C. Tree Fruits Cooperative