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June 1, 1999 Volume 8 No. 11

Diseases
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Scaffolds is published weekly from March to September by Cornell University -- NYS Agricultural Experiment Station (Geneva), and Ithaca -- with the assistance of Cornell Cooperative Extension.

New York field reports welcomed. Send submissions by 3 p.m. Monday to:

Scaffolds Fruit Journal

Editors: A. Agnello, D. Kain

Dept. of Entomology, NYSAES

Geneva, NY 14456-0462

Phone: 315-787-2341 FAX: 315-787-2326

Scaffolds 99 index

GENERAL MALAISE

(Dave Rosenberger dar22@cornell.edu, Plant Pathology, Highland)

Rust Diseases

Cedar apple rust lesions are now apparent on leaves of susceptible cultivars that were not protected with fungicides. Lesions appear as bright yellow or orange dots on the upper surface of the leaves. Infections this year are unusually severe because of the heavy inoculum load in cedar trees. The intermittent but extended wetting periods that we had between May 3 and May 9 provided ideal conditions for infection. Heavy rains tend to wash spores out of the air whereas the intermittent wetting optimizes the process of spore production, release, and dissemination.

Severe quince rust infection is also evident in our test plots at the Hudson Valley Lab. Quince rust causes deformed fruitlets, but does not cause any symptoms on leaves. Fruit are most susceptible to quince rust from Tight Cluster until the time blossoms open. On unsprayed Jerseymac trees, we noted that most of the fruitlets infected with quince rust are dropping from the trees as our thinning treatments take effect. The selective thinning of quince rust-infected fruit probably resulted from the fact that most infections occurred on side flowers rather than on king flowers. The critical infection period for quince rust was probably the split wetting period of May 3-4 when early-flowering varieties were at King Bloom. Because the king flowers were already open, the king fruit was less susceptible to infection during this critical infection period. The thinning treatment (carbaryl plus NAA at 5 ppm) effectively removed most of the side flowers and in the process eliminated much of the quince rust. Had the infection period occurred several days earlier, I suspect more king flowers would have been infected and the thinner would not have removed the quince rust.

In years with heavy cedar apple rust infection, rust-resistant cultivars often develop leaf spots that are associated with cedar apple rust infections. Rust-induced leaf spotting can occur on cultivars such as Empire, Cortland, and Liberty even though the trees of these cultivars will never develop typical cedar apple rust lesions. (These cultivars occasionally have normal lesions of hawthorn rust, but hawthorn rust is much less common than cedar apple rust.) If rust-resistant cultivars are sprayed with a fungicide that does not control rust (e.g., captan), then the rust spores landing on leaves will germinate and begin the infection process before the fungus dies. Infections may kill only a few cells, or they may appear macroscopically as a pin-point yellow or orange spot on the upper leaf surface.

See IPM site's information on cedar apple rust.

Rust-induced leaf spots develop after the rust fungus dies. The cells that were killed or damaged by the rust infections are then invaded by Botryosphaeria, Alternaria, or Phomopsis. These fungi use the dead or dying cells in the original rust lesion as a food-base that allows them to enlarge the original rust lesion. The resulting leaf spots are indistinguishable from frog-eye leaf spot except that rust-induced leaf spots are usually more uniformly distributed throughout the tree canopy. Sometimes the original orange-yellow rust lesion is visible in the center of the brown leaf spots initiated by rust infections.

As noted in the last issue of Scaffolds, rust diseases have no secondary cycle in apples trees. Therefore, there is no reason to adjust current-season spray programs if rust lesions or rust-induced leaf spotting appears in trees. The damage for this season has been done and will not get any worse.

Fire Blight

The MaryBlyt model predicted that blossom blight symptoms should appear in the Hudson Valley around May 23 if any infections occurred during the high risk periods for blight infection that were registered May 6 and 7. Pears were at greatest risk during that period. The hot weather during the past few days should cause terminal shoots to turn black and wilt if blossoms were infected in early May.

Nectria Twig Blight

This disease, which can easily be confused with fire blight, is just beginning to appear in Rome trees. Nectria causes terminal shoots to wilt and die, with shoot tips forming "shepherd's crooks" similar to those caused by fire blight. The disease occurs when the fungus Nectria cinnabarina invades pulled or broken stems after harvest and progresses into the twigs below the stem. After invading pulled or broken stems, the fungus continues to grow into the fruiting node and eventually girdles the twigs. Terminal shoots beyond the infection point wilt suddenly after the twig is girdled. During late June and July, bright orange fruiting structures 2-3 mm in diameter will erupt through the bark of the node just below the pulled stem. Nectria twig blight is most common on Romes and other cultivars that have enlarged nodes where flowers are produced.

Nectria twig blight can be differentiated from fire blight by the fact that the Nectria cankers are usually associated with pulled stems. Also, cankers caused by nectria twig blight rarely extend more than an inch or two back into the tree from the affected node, whereas fire blight cankers can "run" considerably further. When affected twigs are sliced open, there is usually a sharp transition between healthy wood and necrotic tissue in a Nectria twig blight canker, whereas fire blight cankers frequently have indistinct canker margins at this time of year. Later in the summer, the orange fruiting structures that are unique to Nectria provide a critical diagnostic sign. Diagnosis can be complicated when both diseases are present in the same orchard. The presence of Nectria twig blight makes scouting for fire blight very difficult.

Although the wilted and dying shoots can make a tree look sick, Nectria canker generally causes minimal damage to trees. No fungicides have proven effective for limiting spread of this disease. Nectria twig blight occurs sporadically, but infections are usually most common and abundant in low-lying blocks with poor air-drainage (i.e., blocks subject to cold damage.)

Past Disease columns: 4/5 | 4/12 | 4/26 | 5/3 | 5/10 | 5/17 | 5/24

Next in this issue: 6/1 Insects