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May 5, 1999 Volume 8 No. 7

Diseases
Fire Blight
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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

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

Fire Blight

The first open flowers on Bartlett pears at the Hudson Valley Lab were noted on Tuesday, April 27, and most pear varieties were in full bloom by May 3. The epiphytic infection potential for fire blight (as determined from degree hours >65°F) increased rapidly over the weekend and is expected to reach the minimum required for infection by Tuesday afternoon or Wednesday. Thereafter, a wetting event on any day with an AVERAGE temperature >60°F (max plus min divided by 2) will be sufficient to trigger a fire blight infection period.

The first apple blossoms appeared in the lower Hudson Valley only a day or two after the first pear blossoms, so apples will also be at risk for fire blight infection later this week. Highly susceptible apple cultivars such as Ginger Gold, Gala, Fuji, Mutsu, Idared, Jonathan, Monroe, Paulared, and Greening should be protected with streptomycin if there is any history of fire blight in the vicinity of the orchard.

Streptomycin should be applied as a protectant just ahead of predicted wetting periods that might trigger infections. Streptomycin only protects open flowers. If streptomycin is applied too far in advance of infection periods, additional flowers will open after the streptomycin application and will be unprotected at when infections occur.

If susceptible orchards are not protected prior to an infection period, a post-infection spray of streptomycin can reduce potential losses even though post-infection sprays are generally less effective than protectant sprays. When necessary, a post-infection spray of streptomycin should be applied as soon as possible after the infection event. Effectiveness of post-infection sprays varies depending on the severity of the infection event, temperatures during the period immediately following infection, and the duration of the post-infection delay. A post-infection spray applied within 24 hours after the infection period may provide reasonable control of fire blight under moderate conditions, and a spray within 48 hours after infection is probably still better than nothing.

Remember that if warm wet weather persists, follow-up sprays of streptomycin may be needed at 3-5 day intervals to protect blossoms that open after the first application.

Drought Stress and Phytophthora Crown Rot

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In eastern NY, April of 1999 was one of the driest Aprils on record. In the Albany region, this was the driest April of this century and the second driest since record keeping was initiated around 1825. Our weather this year is eerily similar to that of 1982 when apple trees were stressed by dry conditions during bloom. The drought in 1982 was broken by more than 5 inches of rain during the first week of June. Hudson Valley apple growers experienced an unusually severe epidemic of crown rot during August and September of that same year.

Three factors may have contributed to the crown rot epidemic of 1982. First, drought stress during bloom added to the natural "stresses" associated with flowering and fruit set. Stressed trees presumably are less able to resist infection by Phytophthora. Second, the drought conditions were alleviated by a deluge that provided ideal conditions for infection by Phytophthora. And finally, there were large numbers of young orchards where Phytophthora infections resulted in rapid girdling of the trees. On larger-diameter trees, an infection on one side of the tree may be arrested before the tree can be girdled by Phytophthora, whereas small trees can be girdled fairly quickly following an infection.

It is impossible to predict whether the spring drought of 1999 will contribute to an epidemic of crown rot. However, if the current drought is followed by excessive rainfall, young trees should be considered at risk and treatment may be warranted to prevent crown rot. Ridomil Gold is labeled for controlling crown rot, but its label restricts application to early spring and after harvest. Therefore, Ridomil Gold is useless for situations where a midseason application is needed. That leaves a foliar spray of Aliette as the only viable option for controlling crown rot when unusual conditions develop during the growing season.

Effectiveness of Aliette for preventing crown rot on apples has not been very well researched, and sprays of Aliette are rather expensive. However, tree losses to Phytophthora can also prove expensive. Given what we know, it would seem prudent to apply Aliette to orchards considered at risk for crown rot if and only if the current drought is followed by rainfall that results in flooded soil conditions. Aliette applied after the rain event should help trees wall off infections before they can cause extensive damage. Follow instructions on the label for applying Aliette. If copper was applied to trees at Green Tip for fire blight, be aware that Aliette can solubilize copper deposits and cause phytotoxicity to fruit. (See the warning on the Aliette label.) Trees sprayed with copper at Green Tip should be exposed to at least 3 inches of rain before they are sprayed with Aliette.

In determining which orchards might be at risk, both tree age and rootstock should be considered. As noted earlier, small trees can be girdled quickly, whereas older trees with a larger trunk diameter can sustain some damage from Phytophthora and still recover. The M.26 rootstock seems to be the most sensitive rootstock in commercial orchards today. MM.106 rootstock is technically more sensitive to crown rot than M.26, but most trees on MM.106 trees either died from crown rot long ago or they are now mature and therefore will be less susceptible to girdling. M.9 and MM.111 rootstocks are relatively resistant to crown rot. M.7 rootstock is more susceptible than M.9 and MM.111, but less susceptible than M.26. Although M.9 and MM.111 rootstocks are relatively resistant, they are not immune to infection. During the epidemic of 1982, young orchards on both of these stocks suffered occasional tree losses to crown rot.

Not all "natural" disasters can be ascribed to "acts of God". In young orchards with trickle irrigation, the events that contributed to the 1982 epidemic of crown rot can be re-created by delaying the start of trickle irrigation until trees are well stressed and then compensating for that error by overwatering. Several years ago, I was called to an orchard where a number of 3-yr-old trickle-irrigated trees were lost to crown rot. The trickle irrigation had been delayed because of equipment problems, and the trickle line delivered water directly to the tree crowns. Possible solutions in trickle irrigated blocks are to begin irrigation before trees are stressed, avoid overwatering of stressed trees, keep trickle lines at least several inches away from tree crowns, or treat trees with Aliette to prevent invasion by Phytophthora.

Past Disease columns: 4/5 | 4/12 | 4/26

5/3 Insects