Agriculture Reference
In-Depth Information
Many fruit varieties are resistant to powdery mildew, and planting resist-
ant varieties in areas where powdery mildew is a problem is a key to con-
varieties. Fungicides effectively control powdery mildew. Bicarbonate fun-
gicides can give some control of early infections, and highly refined horti-
cultural oil has proven surprisingly effective. Sulfur fungicides may also be
used, but not within 10 to 14 days of applying horticultural oil. Sulfur can
also russet sensitive fruits. Do not apply sulfur products to apricots.
Viruses and Phytoplasmas
Many kinds of viruses and phytoplasmas attack stone and pome fruits. A few
include apple mosaic, apple proliferation, flat apple disease, rubbery wood,
flat limb, cherry mottle leaf, cherry twisted leaf, leafroll, little cherry, and
Prunus necrotic ringspot. In some cases, the viruses are carried by aphids,
leafhoppers, mites, or nematodes. Most viruses can be transmitted during
grafting or budding if using infected rootstocks or scions.
Symptoms.
Signs of a problem include distorted limbs and fruits, leaf spots,
distorted leaves, and even decline and death of trees. In some cases, the vir-
uses kill the cambium at the graft union, causing the trees to die. Quite often,
trees will harbor latent viruses that produce no symptoms.
Management.
Start with virus-free rootstock and scion wood. Deal with
reputable nurseries that test their planting stock for viruses. Once a tree is
infected, it can never be cured. Where the virus is a threat to other trees in
the orchard, rogue out infected trees as soon as you find them. Grind the
stumps to speed decay of the roots and reduce the likelihood of the virus
spreading through root grafts to other trees. An exception might be X-dis-
ease in cherries grown on Mazzard rootstocks where many trees in an orch-
ard are infected. Infected cherries on this rootstock seldom die and can pro-
duce marketable crops for some years.
