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 White Rot. White rot is caused by the fungal plant
pathogen Botryosphaeria obtusa. It survives from season
to season as mycelium and fruiting structures in cankers,
dead bark, and mummified fruit. Current season fireblight
strikes are often colonized by the fungus and are also an
important source of inoculum.
Effective white rot control requires a combination of
good sanitation and preventative sprays. All dead limbs,
cankers, and mummies should be removed from the trees.
Also, prunings should not be left in the orchard as they
can become colonized and serve as an inoculum source
during the same growing season. Use a 14-day schedule of
1 pound of captan plus 0.5 pound of Benlate starting at
first cover and continuing until harvest. The Rome
Beauty, Jerseyred, and Julyred varieties are most
susceptible to this disease.
The incidence of these disorders may be reduced by
the nutrient sprays described in the section on "Orchard
Nutrition." A description of these problems is presented
below:
Cork. Fruit spots vary in size from 1/4 to 1/2 inch
in diameter, with the larger size more common. Spots
become visible between the time the fruit is half-grown
and harvested. The spots are greenish at first; later they
turn blackish-brown. The affected skin tissue is firmer
than adjacent healthy tissue at harvest.
The spongy, brown tissue that underlies the spots on
the skin extends deeply into the flesh. Individual fruits
rarely contain more than three to five cork spots. The
spots are not associated with the fruit lenticels.
Bitter Pit. Fruit spots vary in size from 1/8 to 1/4
inch in diameter and normally develop after the fruit is
harvested. They never approach the size of large cork
spots. The spots are initially dark brown and quickly
become blackish brown. The affected skin tissue is less
firm than adjacent healthy tissue.
The affected tissue beneath the spot on the skin is
shallow and spongy in texture. Individual fruits frequently
contain 30 or more spots, with most of the spots located
around the calyx end of the fruit. The spots are not
associated with the fruit lenticels.
Jonathan Spot. This is often a serious problem in
the Jonathan variety, and it more closely resembles bitter
pit than cork spot. The fruit spots are almost always
associated with the lenticels. The flesh adjacent to the
lenticels is soft and dark in color, but the fruit spots do
not extend deeply into the flesh. Affected tissue frequently
becomes infected with one or more fungi, which cause an
increase in size of the spots during storage. Several other
varieties show similar spots that are generally classed as
Jonathan spot. A few include Rome Beauty, Wealthy,
Grimes Golden, and Twenty Ounce.
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Many factors determine the successful control of
postharvest diseases and disorders. Some of the factors are
a good spray program during the growing season; proper
maturity at harvest; proper handling of fruit; and proper
storage, sanitation, and chemical treatment.
Scald. Either Stop Scald or DPA will provide
control of scald if properly used. Each can be used as a dip
or spray treatment. The rates that provide adequate
control, except on the Rome Beauty variety, are:
Stop Scald
(ethoxyquin) 3 pt/100 gal
of water,2,700 ppm OR;
DPA (liquid or dry)
2,000 ppm (2 qt of
31%)
With the Rome Beauty variety, adequate control is
obtained with the following rates:
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Neither of these materials is as effective on cold fruit
as when used on warmer fruit. Of the two materials, DPA
is more apt to injure fruit. Injury is minimized if the
treated apples are dried before storage.
DPA is the preferred choice on Red Delicious,
Stayman, Winesap, and Cortland. The risk of injury with
DPA is greater if used on Rome Beauty, Baldwin, or
Golden Delicious. Stop Scald is preferred on these three
varieties, and it can be used successfully on Red Delicious
and Stayman.
Additional factors to consider:
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Liquid formulations are preferred. Keep solution well
agitated if DPA is used.
The 100-gallon Stop Scald mixture or the 120-gallon
DPA mixture should be sufficient to dip or roller-
spray 1,000 bushels. If the solution gets dirty, change
it. Replace solution at least every other day.
If dipping, submerge for no more than 30 seconds,
remove, and tilt box to drain solution that has
collected in the stem or blossom end of the fruit.
These materials are most effective if applied within 1
day after harvest. Apples stored 1 to 2 weeks can still
be treated, but fruit temperature must rise to 60F or
above to obtain proper coverage and con-trol. This is
especially true for Red Delicious, which is difficult to
cover at lower temperatures.
Solution temperatures must be 50o to 80F to obtain
the most effective results.
To prevent possible reduced control and fruit injury,
do not allow fruit to set in the sun aftertreatment.
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