Postharvest rots often affect the calyx end of the fruit. The skin turns brown and the flesh mushy. Symptoms of speck rot and sphaeopsis rot look similar, and the fungus has to be isolated to be identified. (Geraldine Warner/Good Fruit Grower)
Plant pathologist Dr. Parama Sikdar has joined the faculty at the Tree Fruit Research and Extension Center in Wenatchee, Washington, where she is working on research designed to help Washington apple growers maintain access to the China market.
Washington apple growers were shut out of the Chinese market for two seasons because of the Chinese government’s concerns about importing fruit infected with bull’s-eye rot (Neofabraea perennans), speck rot (Phacidiopycnis washingtonensis) and sphaeropsis rot (Sphaeropsis pyriputrescens).
The market reopened last fall under a new, more stringent protocol that requires Washington growers to manage those fungal diseases in the orchard and certify they have done so.
China also asked that research be done to verify the effectiveness of the systems approach, which involves pruning out cankers and diseased twigs from crab apple trees and applying postharvest fungicides. Fruit must also be inspected for decay at the packing house.
Bull’s-eye rot is well known to the Washington tree fruit industry, but it’s only within the last decade that P. washingtonensis and S. pyriputrescens were first recognized as causes of postharvest decay in Washington apples.
Dr. Chang-Lin Xiao, who was a plant pathologist with Washington State University in Wenatchee before moving to California three years ago, discovered that Manchurian crab apple, a common pollenizer in Washington orchards, was a host of speck rot and sphaeropsis rot and was a source of inoculum that spread the diseases to neighboring commercial fruit trees.
The cold-loving fungi overwinter in cankers that form on dead or weak wood. Spores disperse in the spring but can remain dormant on trees for a long time until an opportunity for infection arises.
Xiao recommended removing dead limbs, overwintering cankers, and diseased fruit (mummies) from the crab apple trees to reduce the risk of infection spreading in the orchard. Many growers had stopped pruning the crab apple trees because sharp thorns on the prunings can puncture tractor tires and also because of the additional expense. (Watch a video on crab apple pruning)
Sikdar worked with Xiao on this research while a graduate student at WSU from 2009 to 2013. When Xiao left for California, Dr. Mark Mazzola, plant pathologist with the U.S. Department of Agriculture in Wenatchee, took over as advisor for Xiao’s graduate students, including Sikdar and current graduate student Christian Aguilar, who is working on bull’s-eye rot.
In early 2014, after earning her doctorate, Sikdar took a postdoctoral research associate position at Tennessee State University, where she began studying biological control of powdery mildew in flowering dogwood.
TASC grant
Last year, the Washington apple industry received a $1.9 million grant from the U.S. Department of Agriculture’s Technical Assistance for Specialty Crops (TASC) program for crab apple research.
Part of the funds will be used to verify that the systems approach works and to develop a long-term strategy for mitigating the postharvest decays. The grant will also fund separate research into alternative pollenizers that could be used in place of Manchurian crab.
Since Xiao left, WSU has had no tree fruit plant pathologist and so Mazzola helped to initiate the systems-approach part of the research. Sikdar was recruited to head the project because of her experience with both speck rot and sphaeropsis rot.
Trials
Dr. Parama Sikda
Sikdar will continue field trials that began in 2014 in four locations—Malott, Yakima, Quincy, and Othello—to demonstrate the efficacy of pruning and postharvest fungicide applications in controlling the diseases.
Apples harvested last fall from the four locations have been held in storage, and Sikdar, who began work in December, is comparing the incidence of fruit rot on treated and untreated trees. Preliminary results show no rot where the pollenizers were pruned and sprayed.
The project has an extension component, and results of the first year of the trial will be shared at meetings and demonstrations this spring.
Growers will learn how to identify and manage orchards at a high risk of infection from speck rot and sphaeropsis rot. Sikdar plans to repeat the experiments for two more years, but do the pruning and the fungicide application in separate treatments so she can assess the effectiveness of each.
Alternative pollenizers
It is estimated that Manchurian crab apple trees are planted in 60 to 75 percent of the state’s orchards at a rate of 5 to 10 percent of the total trees.
WSU horticulturist Dr. Stefano Musacchi and extension specialist Karen Lewis are heading the other aspect of the TASC project, which is to identify alternatives to Manchurian crab.
Sikdar will screen new pollenizer material for susceptibility to speck rot and sphaeropsis rot, as well as other common diseases, such as scab and fire blight.•
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