The Secret To Less Bitter Pit in ‘Honeycrisp’ Apples
March 9, 2021 
‘Honeycrisp’ growers intent on preventing bitter pit might want to consider Bud-9 and G969 rootstocks because of their low rates of vigor.
Photo courtesy of Washington State University
A trial involving ‘Honeycrisp’ apple trees that were fruited for the first time in 2020 supports the premise that rootstock choice strongly affects bitter pit incidence in the popular but disorder-sensitive cultivar.
Lee Kalcsits, Associate Professor of Tree Fruit Physiology, Washington State University, concluded as much from a 2017 planting of ‘Royal Red Honeycrisp’ trees that were grafted onto one of 10 different rootstocks. The most successful of those rootstocks last summer were Bud-9 and G.969, both of which boast low rates of shoot growth, i.e., vigor.
“Rootstock heavily influences bitter pit incidence in young ‘Honeycrisp’ trees and is related to potassium-to-calcium ratio and rootstock vigor,” Kalcsits said in December at the annual meeting of the Washington State Tree Fruit Association. “Bud-9 and G969 are candidates for lower bitter pit incidence in the early production years, particularly where there are no limitations on tree vigor.”
The trial — supported by the Washington Tree Fruit Research Commission (WTFRC) and funded by USDA’s AppleRoot2Fruit project — involved trees that had been defruited in 2018 and 2019 before being fruited last year. They were thinned in June to a set commercial crop load target to ensure good fruit growth that season and good return bloom in 2021.
At harvest last year Kalcsits and his team picked two trees from each of three replicates and, after two months of storage, measured bitter pit incidence. The results spoke well for some rootstocks, not so well for others.
Bud-9 had the lowest incidence of bitter pit, followed by M9, G.969, and CG.4292. On the other hand, G.87, G.210, and G.814 rootstocks all had extremely high disease incidence.
“This is something that would be expected for a first-year crop,” Kalcsits said.
SHOOT LENGTH MATTERS
That pattern would then be reflected by measurements of shoot length. Bud-9, which had the lowest bitter pit incidence, had the lowest shoot length. Meanwhile, rootstocks such as G.814, G.87, and G.210, which had more vigor, presented the highest bitter pit incidence.
“That got us curious about whether crop load played a factor in that,” Kalcsits said. “So, what we did is express shoot length relative to crop load as a ratio and then plugged that in against bitter pit incidence. What we saw was this really clear non-linear relationship develop, where ratios that are lower have lower bitter pit incidence.”
For example, a shoot length of 10 inches and a crop load of five fruit per centimeter squared trunk cross sectional area (TCSA) would have approximately 25% bitter pit; whereas if one doubled the shoot length to 20 inches and had the exact same crop load, he or she would expect about 65% bitter pit.
“We see this relationship where, even though the crop load is similar, just by changing the vigor (it) changes the bitter pit incidence,” Kalcsits said. “And we see that rootstock confers differences in vigor — we know that — and different rootstocks with more vigor have more bitter
pit incidence.”
NUTRIENT COMPOSITION
The correlations do not stop there. Bitter pit incidence and potassium-to-calcium ratio of the fruit are also connected. This had previously been documented by data from two WTFRC trials of commercial ‘Honeycrisp’ orchards that involved 11 commercially available rootstocks. Bud-9, M9-T337, G.969, and G.11 had the lowest such ratios and lowest bitter pit incidence; G.30, G.41, G.210, and G.890 had the highest potassium-to-calcium ratios and highest bitter pit incidence.
“These were also the rootstocks with the highest amount of vigor, so we see this relationship between fruit and the nutrient composition and vigor and bitter pit incidence,” Kalcsits said.
In weaker growing sites, such as re-plant areas and those with lighter soils, more vigorous rootstocks might be necessary, Kalcsits said. “You need to adjust accordingly.”
FURTHER ADVICE
Also with regard to controlling vigor, bienniality, and bitter pit, proper crop load is “really critical,” Kalcsits added. Where it’s too light, a grower could have higher vigor and larger fruit with more bitter pit. And where it’s too heavy, a grower could get low return bloom and poor color development.
“That’s one of the most important things you can do outside of rootstock choice to control bitter pit in ‘Honeycrisp’,” Kalcsits said.
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