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Production Perils 2
(The Tart Cherry Industry - continued)
Diseases
The two most destructive diseases affecting tart cherries are leaf
spot and brown rot (Fogle, et al.). Both are fungal diseases. Leaf
spot primarily attacks the leaves, causing various degrees of
defoliation. Brown rot can infect the blossoms, leaves, shoots, and
fruit.
- Cherry Leaf Spot
Cherry leaf spot overwinters on fallen leaves. In the spring, leaf
spot spores are carried by the wind to new leaves, where they
germinate and promote new infections. These infections then produce
additional spores which spread the disease to adjacent leaves and
trees. The infected leaves eventually turn brown and drop from the
tree.
In mild cases, cherry leaf spot may affect only a small number of
leaves. During damp or rainy weather, however, the disease can
“explode” in the orchard, infecting nearly all the leaves and
defoliating the trees before the crop is harvested. Fruit on
defoliated trees fail to mature normally, and are light-colored and
low in soluble solids.
Rotary mowing the orchard after leaf drop in the fall helps to
control leaf spot by hastening the decay of fallen leaves and
reducing chances for the fungus to overwinter. The main control,
however, comes through the use of fungicides.
- Brown Rot
Brown rot is common to all stonefruit and frequently causes heavy
losses among cherries. This disease can destroy blossoms, fruit, and
stems. The most significant losses result from blossom blight and
destruction of the fruit. Fruit may be infected in the orchard, in
transit, or while being held for processing. Brown rot can develop
rapidly, and a few infected fruits can lead to the destruction of all
surrounding fruit.
Brown rot infection of blossoms appears as a sudden wilting and
browning of the flower parts. If conditions are moist, the dead
flowers are soon covered by grayish brown fungal spores that then
infect the healthy fruit. Infections on the fruit begin as small,
circular brown spots that rapidly increase in size and develop into a
soft rot.
Infected fruits that remain on the tree shrink and dry into firm
“mummies,” which become a source of infection in future seasons. The
brown rot fungus also infects twigs, causing oval-shaped cankers.
The fungus lives over the winter in infected twigs, in fruiting
spurs, and in mummified fruit on the tree or on the ground.
Three methods are used to control brown rot. Cultivating around the
trees to bury the infected mummies and pruning out infected twigs
during the spring and early summer helps to eliminate sources of new
infections. Providing open space around the trees to assure good air
circulation helps eliminate excessively moist conditions that favor
brown rot. As with leaf spot, however, the main control comes from
the use of fungicides.
- Other Fungal Diseases
Diseases including black knot, powdery mildew, leaf rust, scab, and
verticillium wilt occur to some extent on tart cherries. These
diseases are usually less serious than either leaf spot or brown rot.
Most of them are held in check by the fungicides applied to control
leaf spot and brown rot.
Viral Diseases
Several viral diseases affect tart cherries. The most widespread are
ringspot, sour cherry yellows, X-disease, and pink fruit. Viral
diseases tend to gradually reduce tree vigor and, consequently,
yields over a period of years.
Prevention is the best control. The most effective preventative
measures include the avoidance of potential sources of infection,
such as planting diseased stock or selecting sites near orchards with
infected trees. Promptly removing infected trees in a young orchard
also helps reduce the chances of spreading the infection to healthy
trees.
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