HORTICULTURE HORTICULTUREMark Lonstroth
Southwest Michigan District Extension Horticultural & Marketing Agent
Calcium nutrition has become more important in Michigan
orchards with the increasing demand for large fruit. These large fruit are increasingly at
risk for Bitter pit. Bitter pit is a physiological disorder of apple fruit. I am sure that
all apple growers have seen the dark sunken pits and brown corky tissue that are the
symptoms of the disorder. Research indicates that Calcium (Ca) metabolism is the key to
Bitter pit. 
It is generally believed that Bitter pit is a result of a calcium deficiency in the fruit. This is not always true since the tissue affected by Bitter Pit is often higher in calcium than the surrounding tissues. Unfortunately there is no quick fix for the problem. Calcium sprays are helpful in reducing the amount of Bitter pit, but other cultural practices have proved more effective in controlling the disorder. It helps to understand how calcium acts in the plant and its role in plant metabolism. Calcium is not very mobile in the plant and moves mainly in the woody xylem. It is carried in the transpiration stream from the roots to the leaves. The driving force for water movement through the tree is evaporation from the leaves and the movement of water to the leaves carries calcium with it. Calcium is also not very mobile in the phloem, which carries sugars from the leaves to the developing fruit. Therefore, Bitter pit can be viewed as a competition between the shoots and the fruit for calcium. Measures which shift the balance to the fruit and away from the shoots will reduce Bitter Pit.
The key to low Bitter Pit levels in your harvested fruit is to set large crops of moderately large fruit (2 3/4 to 3"). This requires efforts to reduce alternate bearing by fruit bud stimulation treatments, increasing the potential for cross pollination, having and maintaining moderate vigor, annual light pruning, frost protection to ensure annual crops and early thinning of excessive crops. Given the small size of last years apple crop it is likely that many growers will be faced with heavy crops. Bitter Pit will probably not be a problem this coming year, but we could set ourselves up for another small crop with Bitter pit if we do not manage our crops correctly this year. Bitter Pit is a complex problem that requires an overall management scheme to reduce the problem to acceptable levels.
Several factors can affect fruit uptake of calcium in the spring (10 to 45 days after petal fall). High levels of nitrogen, potassium, and magnesium all reduce the fruit uptake of calcium. This is because nitrogen favors shoot growth over fruit growth. Potassium and magnesium compete with calcium because they are all positively charged ions. High levels of available water will increase fruit calcium if shoot growth is not excessive. In general, lessening shoot growth increases calcium uptake by the fruit. Increasing root growth results in increased uptake of calcium. Finally, maintaining a high photosynthetic rate will increase calcium in the fruit as long as shoot growth is not excessive. Many of the practices which we employ as growers to maintain vigor in the tree canopy can come back to haunt us if we do not also maintain a large crop on the tree to manage vegetative vigor. I would like to discuss the practices we can use to control Bitter Pit.
Nitrogen:
Nitrogen levels need to be low enough to keep leaf nitrogen levels at about 2% in the
early spring. Fertilization of processed varieties like Northern Spy should be light in
the spring and light in late August or early September (10-30# of actual nitrogen for each
application). Fall applications may be needed for fruit set and early photosynthesis next
year. Fall applications of nitrogen to Red Delicious may cause ripening and coloring
problems. Spring applications should be ammonium fertilizers e.g. NH4SO4,
to lessen the stimulation of early growth and improve fruit bud set. In cold wet springs,
urea sprays at 10# to the acre may be useful at petal fall and at first cover.
Potassium:
Potash should be applied only when soil or tissue tests indicate a need for potassium.
Potash should only be applied in the fall and only at low rates, 60# Acre of actual
potassium is the maximum I recommend on Bitter Pit susceptible varieties. Annual
applications of 20 to 30 pounds would be better. Trees growing on sandy soils will be
especially sensitive to potash applications since potassium will reduce the availability
of calcium.
Magnesium:
Because magnesium and calcium are very similar in chemical activity, magnesium
competes with calcium for uptake into the tree. In fact, Doug Burmeister in Dr. Dave
Dilley's Lab at Michigan State University was able to induce a Bitter Pit like condition
by infusing magnesium into ripe apple fruit. Therefore Magnesium should only be applied as
dolomitic lime. If a tissue test indicates that magnesium is deficient then use low rates
of K-Mag or Sul-Po-Mag.
Calcium:
Growers who have consistent problems with Bitter Pit in their orchard blocks should
determine their soil pH with soil tests. The sandy soils of many of our orchards cannot
hold many nutrients and eventually following years of fertilization soil pH will fall and
calcium will become less and less available. Soil applications of lime are a cheap source
of calcium and increase the availability of most nutrients to optimum levels. Many growers
use calcium sprays which I will discuss later.
Boron:
Boron sprays have been inconsistent in controlling bitter pit. This is probably
because boron is necessary in only small amounts and we seldom see deficiency symptoms
here in Michigan.
Irrigation:
Few growers in Michigan routinely irrigate their orchards, but maintaining adequate
watering the soil will increase root growth and calcium absorption. Calcium is most easily
taken up near young shoot tips before they fully develop barriers to water loss. These
barriers are also effective in reducing calcium uptake. Woody roots do not take up much
calcium. Warm springs with little rainfall will reduce calcium uptake and also favor shoot
growth which promotes Bitter pit. Irrigation from pink through third cover at about 75 %
of the evaporative demand should help calcium uptake by the fruit under any weather
conditions. Calcium uptake depends on a healthy root system. Any activity that promotes
photosynthesis (energy production) and energy storage in the fall, before or after harvest
will promote root growth in the spring. Root growth occurs in 2 cycles in apples, in the
spring and again in the fall. Root deterioration begins in the spring and continues
through the summer. This means that calcium uptake from the soil during the summer is
severely limited.
Calcium Sprays:
The restriction of calcium from the roots and calcium's lack of mobility in the plant
are probably why calcium sprays have been so effective in reducing the symptoms of Bitter
Pit. Calcium sprays are necessary to increase fruit calcium in years when fruits are large
and the crop is light. No matter how much calcium is available in the soil, calcium
related disorders will occur in large fruit. Spray calcium as late in the season as
practical, but start early in the first or second cover. The addition of
small amounts of calcium to many sprays seems to be more effective than several large
dosage sprays. Growers should apply calcium 6 to 8 times and the total amount of calcium
should not exceed 20 pounds to the acre. Raising the calcium content of the fruit is not
easy. Essentially only the calcium which is absorbed through the waxy cuticle is used by
the fruit. Other benefits from calcium are increase fruit firmness in storage and
increased storage life. There may also be some benefit in disease resistance from
increasing cell wall strength and cell membrane integrity.
Control fruiting:
Large annual crops of moderately sized fruit are necessary to control shoot vigor in
Bitter pit susceptible varieties. This means carrying a large crop to harvest, but also
good thinning early so that as many spurs as possible set fruit buds for next year.
Setting fruit buds not only provides for next year's crop but also stops the growing point
controlling shoot vigor and the number of active growing points. Sprays of NAA and
Ethephon may be useful for fruit bud development and thinning. Use NAA at 1/2 of the
normal rate plus 200 ppm Ethephon, use 3 sprays at weekly intervals staring at petal fall
if a heavy set is anticipated. But growers should not use NAA and Accel on the same trees
as the two thinners seem to be antagonistic and do not work well when used in combination
or when different sprays of each material are applied. The NAA and Ethephon may help
calcium uptake in the young fruit by suppressing shoot growth for short periods. As far as
I know, no one has worked with the combination of Accel and Ethephon to determine the
effect on the trees. If growers do not have a heavy bloom this year then the aggressive 3
Ethephon spray program is probably not necessary. Many growers will be using Sevin
aggressively this year because it is compatible with Accel.
Pruning:
Annual pruning will result in good light penetration into the tree canopy and increase
the production of fruit buds. Drastic pruning every second or third year is counter
productive, since the heavy flush of shoot growth which follows in the spring will result
in Bitter Pit. Heavy pruning in alternate years also reduces the yield since much of the
wood removed is 2-year-old wood would have borne fruit. Trees that were heavily pruned
this last winter will certainly benefit most from calcium sprays.
Summary:
Bitter Pit can be managed by maintaining moderate vigor in your trees. Avoid heavy
applications of nitrogen or heavy pruning. Manage your crop by thinning the fruit
aggressively so that you have an moderate sized crop which will result in large numbers of
good sized fruit and many flowers next year. Avoid large applications of magnesium or
potassium which may reduce calcium availability to the fruit. Calcium sprays will help to
control Bitter Pit in light crop situations and reduce losses in varieties which are prone
to bitter Pit. But Calcium sprays will not correct mistakes made in orchard management.
Reduction and control of Bitter Pit requires that the orchard be managed to increase
calcium to the fruit.
Last modified February 8, 1997.
This is a revised version of an article which first appeared in The Great Lakes Fruit Growers News, Vol. 33, No 5, May 1994, as: Bitter pit management in apples.
Last modified: March 3, 1997
