Shedding light on core rots
Dry core rot develops in the orchard; wet core rot is a postharvest disease.
Moldy core and core rot can be serious problems in exported apples, because the symptoms usually can’t be seen from the outside when the fruit is shipped.
Dr. Maryna Serdani, plant pathologist at Oregon State University’s Mid-Columbia Agricultural Research and Extension Center in Hood River, said that in most apple growing regions around the world where Red Delicious and Fuji are produced, moldy core and core rot can affect between 5 and 8 percent of the fruit. When shipments are rejected in export markets, significant financial losses can result.
Moldy core (also known as dry core rot) begins to develop while the fruit is on the tree. It is caused by Alternaria and other species of fungi, which enter the fruit and grow in the seed cavity. Dry, spongy brown lesions extend from the core, but rotting symptoms do not develop. However, the fruit might drop from the tree prematurely.
Moldy core tends to develop when the fruit set is light and when dry weather in early summer is followed by heavy rains, Serdani said. The fungi can colonize the flower parts as soon as the blossoms open and can enter the fruit through an open calyx tube. The mite Tarsonemus confusus can carry the fungal spores into the fruit through the calyx. The spores germinate during rain.
Fuji has a large calyx opening, which makes it particularly susceptible to moldy core, Serdani said during a fruit quality meeting presented by Washington State University Extension this summer. Braeburn and Granny Smith have small calyx openings and are not very susceptible.
Serdani said the Alternaria fungi produce mycotoxins that are toxic to bacteria, animals, and humans when consumed in large amounts.
To control the disorder, she recommends removing leaf debris from the orchard floor. There is no effective fungicide because once the fungus is inside the core, it is protected. Good mite control might stop mites carrying the spores into the apple.
Opening the tree canopy to improve airflow might slow the development of the molds, she suggested.
In Tasmania, Australia, scientists have experimented with biological control, using an antagonistic fungus Gliocladium roseum, which was carried to the apple blossoms by bees. In Spain, the bacterium Pseudomonas syringae strain CPA-10 has been tested as a biocontrol.
“I think biological control is something we should look into in the future,” Serdani said.
Wet core rot
Wet core rot is exclusively a postharvest disease, Serdani said. It can be caused by a number of fungi, including Penicillium expansum, the fungus that causes blue mold, and Mucor piriformis, the fungus that causes mucor rot. It enters through the fruit’s open calyx when it is immersed in contaminated water during dumping and flotation. A light brown, wet rot develops around the core of the apple. The rot can produce mycotoxins. This is not a problem in fresh apples, but it can be in juice apples, as the toxins survive pasteurization, Serdani said.
“It’s important to control this disease.”
Wet core rot must be controlled in the packing house and can be accomplished through good sanitation, adding chlorine or SOPP (sodium o-phenylphenate) to the water, or avoiding immersing susceptible varieties in water. After fruit is drenched, the bin should be tipped to drain solution from the fruit cavities, Serdani said.