Bitter Pit in the 2005 Apple Crop

With reports of very heavy crops from some growers and on the other hand rather light crops in some areas, it’s easy to reconcile that the probability of bitter pit showing up this year was pretty good on light cropping trees. Historically, this disorder is far more predictable in years that are warmer/hotter than usual and on trees that don’t have as much crop as they could conceivably carry. The trees with a high leaf to fruit ratio seem to get it the worst. A vigorous tree canopy can easily out-compete a crop of fruit for available calcium if that crop is considered to be light.

We know from experience that some cultivars are inherently more susceptible to pitting than others and also that in some situations we can expect to see some bitter pit. This is very evident on cultivars like ‘Honeycrisp’ where a planting is just getting established. We know that a ‘Honeycrisp’ planting that is more mature and settled as far as cropping history has a diminished chance of showing this disorder.

With young ‘Honeycrisp’ trees, this is understandable since we have a very weak scion and a root system on the tree that could be considered as immature. I have heard of rootstock/scion combinations that are almost guaranteed to produce bitter pit regardless of the type of growing season. Something like a Spy on an M27 for example will almost certainly show a textbook case of this disorder and is a mismatch of root system and cultivar.

It was not uncommon to see some of the first crops of ‘Honeycrisp’ grown in Ontario from young immature trees looking pretty good and relatively free of bitter pit at harvest, only to show it 2-3 weeks later in a storage environment.

Calcium sprays have been shown to reduce the severity of this disorder. However, calcium sprays alone may not be the solution to long term planning for bitter pit control. Dr. Randy Beaudry of MSU in Michigan has shown that bitter pit tissues are often higher in calcium than surrounding tissues. Calcium is involved but the problem, however, can be more complex.

We usually consider that the soils we grow on in Ontario have far more exchangeable calcium in them than we could ever use in a lifetime of orcharding. The problem arises in the extraction of the calcium from the soil and the delivery to the crop. I understand that a single ion of calcium can take as long as 2 years to get to the crop from being picked up in the soil.

The ability of a tree’s root system to pick up calcium is directly related to the proliferation of new root initials, which in turn is intimately related to available soil moisture. This in turn is related to the spring and fall root growth cycles when trees put out new feeder roots. These roots will only explore soil that is sufficiently moist.

Apple tree roots do not actively seek moisture as some believe. They will only grow into soil that has sufficient moisture and this is one of the keys to long-term success with the extraction of calcium from the soil environment.

Wild fluctuations of available soil moisture in the orchard can inhibit the ability of tree roots to do their thing by absorbing and transporting exchangeable calcium. The problem could get even worse if the use of magnesium and potassium is excessive and the soil pH is too acidic. A mineral soil with a pH of 7.5 could easily have several times the amount of available calcium when compared to an acidic soil.

Soils with low pH problems can benefit from the use of lime and in fact lime is probably the most economical source of added calcium to the orchard environment. Addition of calcium-containing materials, including gypsum, to the soil may not necessarily add more calcium to the fruit. In some cases these soil treatments will only add more calcium to the leaves and wood of the tree. Even if the calcium manages to get into the fruit it can accumulate in the stem end and be deficient in the calyx end of the fruit where bitter pit usually shows up.

With the demand for larger apples on the market the problem is even more complex. Excessive thinning and too much nitrogen getting to the tree at the wrong time could make potential problems with this disorder even worse.

Don’t forget about boron (B), a micronutrient that can be hard for trees to pick up, especially in drier soil environments. If boron is low in an apple orchard, it may enhance the effects of calcium related disorders. For a more complete story see the OMAFRA factsheet on Bitter Pit Control in Apples.

Figure 1 – Bitter Pit Symptoms at Harvest

Related Links

• Bitter Pit Control in Apples, Order No. 00-009