Scaffolds 00 index

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

A severe hail storm moved through the lower Hudson Valley on the evening of May 18, leaving varying levels of damage in its wake. In orchards at the center of the storm path, hail damage to tree fruits was so severe that some growers will abandon the crop. Orchards on the edges of the storm path sustained varying degrees of damage. In general, late blooming apple varieties such as Delicious, Golden Delicious, and Rome sustained less damage than earlier varieties such as Mac and Empire. Late blooming cultivars apparently escaped some damage because they had not yet sized enough to provide a substantial target. The longer fruit stems on the later cultivars may also have allowed the fruitlets to deflect the hail impacts more than cultivars such as Empire that often have short stiff stems.

Pear blocks and high-risk apple blocks that were hit with hail should have been sprayed with streptomycin as soon as possible after the hail storm. Because of continuing rain and wind, most streptomycin applications were made 36 to 48 hours after the hail storm rather than within 24 hours as is usually recommended. Most growers with apple orchards that were free of fire blight for at least two years opted not apply streptomycin because they assumed that these orchards would be free of blight inoculum (a decision with which I concurred). Whether or not this was the correct decision should become evident within several weeks.

The risks of hail-induced trauma blight are presumably higher this year than they would be in most years because bloom-time weather was very favorable for fire blight. According to the MaryBlyt model, fire blight infections in the lower Hudson Valley could have occurred on May 6, 7, and 10, with "high risk" indicated for May 8, 9, 11, 14. (There was only a bit of rat-tail bloom left by the 14th). The model indicated that the first symptoms of blossom blight and canker margin symptoms should have been present about two days before the hail storm. No one has reported visible fire blight symptoms at this time. However, the prediction that early symptoms may have been present at the time of the hail storm means that secondary inoculum for blight could have been present in orchards where blossom infections occurred. Growers should monitor orchards carefully for blight symptoms during the next two weeks.

In hailed blocks where this year's crop will be abandoned, no more fungicide sprays should be necessary to protect foliage. The supply of apple scab ascospores has been depleted (1229 DD base 32°F.), and risks of further spread of apple scab are relatively small in orchards where trees were fully protected from primary scab up to this point. Where the crop will be harvested, continued fungicide protection is warranted to ensure protection against flyspeck, black rot, and the risk (albeit a small risk) of secondary apple scab infections.

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

Fabraea leaf spot is a perennial threat to quince and Bosc pears. Other pear varieties can also get Fabraea, but Bosc is the most susceptible of the commonly grown varieties in New York. Epidemics of Fabraea leaf spot are usually initiated between petal fall and July first. This disease is one of the most "explosive" diseases of tree fruits. It often seems to appear almost overnight during late June or early July, but epidemics are actually initiated much earlier than that.

Epidemics usually occur as a result of primary infections that become established during the three to four weeks after petal fall. These primary infections appear as nondescript, round leaf spots that usually escape notice. If fungicide protection is inadequate during June or early July, a few primary infections will provide the inoculum for a rapidly developing epidemic. Foliar symptoms can appear almost simultaneously on many leaves throughout much of the tree canopy during late June or early July.

Fabraea leaf spot lesions on immature quince fruit.

 

Fabraea can build up more quickly than diseases like apple scab because scab is able to infect only newly formed leaves on growing terminals, whereas older leaves and fruit never become resistant to infection by Fabraea. Leaves and fruit on quince and pear trees remain susceptible to Fabraea leaf spot right up until harvest. Thus, when Fabraea leaf spot epidemics develop in early summer, all of the existing leaves can become infected in a short time if inoculum is present and trees are left unprotected.

To avoid Fabraea epidemics, quince and pear trees should be protected with fungicide from petal fall through July 4. These sprays will prevent the primary infections that subsequently produce the abundant conidia that cause the epidemics. If trees are protected with fungicides applied on a 14—21-day interval through July 4, then the chances for late-season development of Fabraea are minimized.

Fabraea leaf spot lesions on Bosc pear fruit at harvest.

The mancozeb fungicides have been considered the most effective for controlling Fabraea, but their use is restricted by their 77-day preharvest interval. Until this year, ziram was the most commonly used fungicide for protecting pear orchards during summer. Now both Sovran and Flint are registered for use on all pome fruits, including pears and quince. Neither Sovran nor Flint have been evaluated for efficacy against Fabraea leaf spot and Fabraea therefore is not included on either label. However, both Sovran and Flint should control Fabraea if they are used during summer for other diseases for which they are labeled.

Pears and quince should be protected from Fabraea during June, even in orchards where the crop has been lost to hail. Fabraea can cause premature defoliation, and trees that defoliate in early summer will fail to set fruit buds for next year. Furthermore, orchards that develop Fabraea this year will pose control challenges for next year because of high inoculum carry-over.

> 5.22 Insects