Franz Niederholzer, UCCE Farm Advisor, Colusa, Sutter and Yuba Counties
Fruit skin cracking results in off-grade fruit that can easily be infected with brown rot. There are two general kinds of fruit cracking: side-cracking and end-cracking. Each is caused by different combinations of similar fruit growth and environmental conditions and requires different practices to manage.
Prune fruit growth drives fruit cracking. Prune fresh fruit goes through a rapid, final expansion from late June through early July, on average. Once that volume growth is finished, roughly by mid-July, the fruit only grows in dry weight as sugar increases; there is no more fresh fruit size growth. Rapid fresh fruit growth lasts about three weeks and can be tracked by comparing fruit diameter from cheek-to-cheek vs suture-side.
End-cracking: When water stressed orchards are irrigated in June through mid-July, the rapid fruit rehydration increases internal pressure at the tip end (away from the stem end) of the fruit faster than the fruit skin can stretch and splitting occurs. Maintaining adequate orchard moisture through the spring and early summer (May and June) is the key to reducing end-cracking. After mid-July, re-irrigating dry orchards does not increase end-cracking.
Side-cracking: Sun-exposed prune fruit skin is less elastic than shaded skin. During the roughly 3 weeks of rapid fresh fruit growth in late June/early July, skin side-cracks form on the sun exposed side of fruit during cool nights with high humidity. The fruit skin absorbs moisture from the humid air and cracking occurs, driven by the internal pressure of the expanding fruit. Once the rapid fruit growth period has passed, so has the risk of side-cracking. Lighter crops, with a tendency towards larger fruit size, commonly see more side-cracking than heavier crops with smaller fruit sizes.
To reduce side-cracking as much as possible, there are several factors to consider and attempt to manage. A key monitoring tool is fruit diameter, both cheek and suture. When fruit cheek diameter exceeds suture diameter, rapid fruit growth and side-cracking risk is a week away. Good vegetative growth from adequate irrigation and nutrition management provides more shading for fruit that under poor vegetative growth conditions. Mild ochard water stress as rapid fruit growth timing begins reduces side-cracking. Reducing or delaying irrigation, especially flood or wide-coverage sprinkler irrigation, as rapid fruit growth begins can reduce orchard humidity and fruit end-cracking. However, sudden weather shifts from hot to cooler can’t be managed and will promote higher humidity at night and a greater risk of end cracking. Attention to all these factors will help reduce losses to fruit side-cracking.
In a difficult marketing year, delivering a good quality crop is essential to maximizing grower income. Careful attention to irrigation amounts and timings as well as fruit growth patterns through June and into July can help reduce fruit cracking and grower income losses.