New research reveals that faster growth rates in Honeycrisp apples increase both the incidence and severity of the “bitter pit” disorder, which is responsible for large-scale fruit yield losses and avoidable economic impacts for apple producers and storers. This study also reveals that these faster growth rates are associated with disruptions to the transport of calcium through the xylem, which the study suggests being a major cause of bitter pit.
Bitter pit is a calcium-deficiency disorder observed in apples. Affected fruits have dark spots of dead tissue that can occur in the skin or the flesh, resulting in an unpleasant appearance and bitter taste that makes the fruit unmarketable. Post-harvest storage can also greatly increase the severity of bitter pit in apples.
“Given the scale of commercial production of ‘Honeycrisp’ in the US, and its relatively high susceptibility to bitter pit, a conservative estimate of annual revenue lost from non-marketable fruit is easily in the tens of millions of dollars,” says study author Dr Todd C. Einhorn from Michigan State University, USA.
Previous research has shown that larger apples are more susceptible to bitter pit, but the biological mechanisms driving this relationship were unknown until now.
“We wanted to challenge the dogma that mature fruit size was somehow ‘responsible’ for bitter pit,” says Dr Einhorn. “This perception did not provide insight or direction as to when or how growth affects bitter pit incidence.”
These findings, presented in the Journal of Experimental Botany , demonstrate how it is actually the growth rate of apples that drives bitter bit development, rather than size, with faster growth disrupting the transport of calcium through apple tissue towards the peel.
To investigate this relationship, the research team created three growth conditions for Honeycrisp apples by altering the number of apples growing on each tree and manipulating the relative resources available to each apple. This resulted in low, medium and high sink:source resource ratios, where low-ratio apples were able to grow faster and larger due to an abundance of resources, while high-ratio apples grew slower and smaller.
They found the highest incidence and severity of bitter pit amongst low-ratio apples that exhibited the fastest growth rates.
They also found that xylem transport in the low-ratio apples dropped by 50% almost two weeks earlier than the high-ratio apples, resulting in a similar decrease in peel calcium content and development of bitter pit.
In a second experiment, apples were artificially shaded to temporarily slow down their growth rate to test the hypothesis that growth rate was directly affecting transport through the xylem transport system. Once the shade was removed, the shaded apples then had rapid surges in fruit growth rate compared to non-shaded apples, which were associated with higher rates of xylem transport loss.
The severity of bitter pit development in storage was also found to be consistent with the incidence at harvest. “Our data imply that bitter pit is primarily established in the pre-harvest interval where potential mitigation strategies should be employed,” says Dr Einhorn. “The discovery of a growth rate function and characterization of a timeframe informs when horticultural and physiological strategies should be imposed.”
Based on these findings, the study authors recommend practical solutions that include the timely use of irrigation to slow down growth rates, root pruning and the use of photosynthesis inhibitors. “The economic potential associated with the development of practical management strategies to reduce fruit growth rate is high,” says Dr Einhorn. “So long as these strategies do not reduce fruit quality below the market/consumer expectations, profitability and apple supply will be improved.”
The Journal of Experimental Botany is a partially open access journal published on behalf of the Society for Experimental Biology by Oxford University Press. The aim of the Journal of Experimental Botany is to publish papers that advance our understanding of plant biology.
Journal of Experimental Botany
Experimental study
Not applicable
Fruit growth rate influences cortical xylem transport, calcium delivery, and bitter pit incidence of apple
23-Oct-2026