Agriculture Reference
In-Depth Information
and severity of these diseases. Below 90 percent relative humidity, the patho-
gens do not grow.
Management.
The cultural practices we have discussed to maintain open
canopies, good airflow, and low humidity are critically important in man-
aging these diseases. Diligently thin apples and pears to reduce humidity
around the fruit clusters and improve spray coverage. Do not use overhead
irrigation. In humid regions, keeping alley and in-row cover cops mowed
short can help reduce humidity in the canopy.
Fungicides are an important step in managing these diseases in humid
regions. As humidity and precipitation levels increase, so do the number of
sprays needed to control sooty blotch and flyspeck.
Black Rot, Frogeye Leaf Spot, and White Rot
Black rot (on fruit) and frogeye leaf spot (on leaves) are common and some-
times serious diseases on apples. Both are caused by the same pathogen,
Botryosphaeria obtusa
. Girdling of the twigs and branches causes stunting,
defoliation, and dieback. Infected fruits are small, may drop early, and can rot
on the trees or in storage. Yields are poor. Unfortunately, no apple varieties
appear to be resistant to the disease. 'Jonathan' and 'Winesap' are especially
susceptible. White rot (
Botryosphaeria dothidea
) is a closely related disease
and produces similar symptoms.
Symptoms.
The first symptoms on the fruit usually appear on the blossom
ends. The pathogens enter through natural openings or through wounds
caused by pests, hail, or mechanical damage. One light brown spot usually
develops on each infected fruit. These spots gradually darken and expand
to form firm, leathery, bull's-eye-shaped rotted areas that can cover the en-
tire fruit. Infected fruits shrivel into black mummies that may remain on
the trees for more than a year. Fruits infected with white rot can take on
a bleached appearance, especially in red-fruited varieties. The rotted areas
