Issue link: https://read.dmtmag.com/i/514197
36 JUNE 2015 GOOD FRUIT GROWER www.goodfruit.com A mong the many attributes of Honeycrisp that make it grower unfriendly: the cultivar is excep- tionally susceptible to fruit rot organisms that may require control measures that go beyond regular fungicide applications. Honeycrisp is not only more susceptible to summer rots that occur in the orchard, but is very susceptible to postharvest decays from blue mold and gray mold. That was the message from Dr. David Rosenberger, plant pathologist and professor emeritus at Cornell University's Hudson Valley Laboratory. He spoke to growers during a Honeycrisp intensive workshop put on by the International Fruit Tree Association in Halifax, Nova Scotia, in February. The message was especially cogent for Nova Scotia growers, whose orchards are recovering this year from a severe outbreak of fire blight caused by Hurricane Arthur last July 5. Fruit rot pathogens can colonize dead and dying wood, Rosenberger said, and then sporulate profusely. "They may be especially prevalent following fire blight outbreaks because shoots killed by fire blight are often rapidly invaded by one or more of these pathogens," he said. "They also colonize prunings left beneath trees throughout the summer." Three rots The three most common summer fruit rots are black rot (Botryosphaeria obtusa), white rot (B. dothidea) and bitter rot (Colletotrichum species). The Botryosphaeria pathogens also colonize fruitlets that are dead but persist on trees after poor pollination or thinning sprays. "The first infected fruit during summer often are located beneath a dead fruitlet that provided the inoculum for the fruit infection," Rosenberger said. Most mature orchards have an abundance of inoc- ulum for all three of these pathogens and fruit must be protected with fungicides throughout summer, he said. In orchards where bitter rot has caused severe losses, rotted fruit should be removed from beneath trees after harvest to reduce the level of overwintering inoculum. "Fungicides alone are sometimes unable to control bitter rot if inoculum levels are high and hot summer weather favors fruit infections," he said. White rot and black rot are fairly easy to control with commonly used fungicides if coverage is main- tained throughout summer, he said. Late-season rains can deplete fungicide residues, leaving fruit vulnera- ble. Growers should continue sprays even as harvest approaches because that gap in coverage can lead to decay in storage during the cool-down period. "Postharvest fungicide treatments usually will not arrest infections that were initiated in the field," he said. Fruit in the center of the stack in large storages is most vulnerable because they are slowest to cool down. Bitter rot Honeycrisp are very susceptible to bitter rot, Rosenberger said, and for reasons unknown, fungicide treatments sometimes do not control it very well. Rosenberger said he suspects that bitter rot outbreaks sometimes occur when sun-exposed fruit sustains heat injury without developing typical sunburn. Initial decay on injured fruit may generate an abundance of inocu- lum that then initiates additional infections. In most Northeastern and Great Lakes regions, sun injury is not a big problem, and bitter rot used to be considered "a southern disease," he said. But it has reached Honeycrisp in the Hudson Valley and even further north. Heat injury in the Northeast and Great Lakes areas has not been well-researched, he said. It may be that heat stress inactivates host defenses. Trees under water stress New Varieties Among the cultivar's many problem features is exceptional susceptibility to fruit rots. by Richard Lehnert Those rotten HONEYCRISP Call us FIRST for the largest selection of trees and rootstocks available Future contracts for cherries, pears, & apples; ALL ROOTSTOCKS. NEW APPLE rootstock! 1-800-421-4001 Phone: 503-538-2131 Fax: 503-538-7616 E-mail: info@treeconnect.com Web: www.treeconnect.com INC. Representing Over 30 Leading Nurseries in the U.S. and Europe From the breeders of Bud 9: • Vigor between M-9 T337 and M-9 Pajam®2 • Yield efficiency similar to M-9 T337 • Dwarfing • Cold hardy • Disease resistant • Fireblight tolerant B10 ® cv. Mich 96 USPP 21,223 Services are FREE TO GROWERS!