Most orchardists in the Pacific Northwest's arid fruit growing areas have not had to worry much about apple scab, a common disease in the more humid East and Midwest.
But overhead sprinklers can create the conditions in which the disease thrives, Dr. Mike Willett, Washington State University Cooperative Extension horticulturist, told growers at the Columbia Basin Hort Day at Ephrata, Washington, in January.
The fungus Venturia inaequalis overwinters on infected leaves in duff on the orchard floor. In the spring, during wet periods, the fungus's fruiting bodies begin to explode, and spores shoot out and infect leaves. These are primary infections. Spores produced as a result of primary infections on the leaves are responsible for secondary infections.
The Mills table was developed at Cornell University, New York, to help orchardists predict infections and determine the need for controls. It relates hours of wetness and temperature to the chances of infection.
"In most orchards in the Pacific Northwest, we have not had to worry about light infection periods," Willett said. "This would not be the case in an orchard with overheads and a history of apple scab. That's just like farming back in Michigan."
Last season was a particularly bad year for scab in Washington. In Wapato, near Yakima, eight moderate to heavy infection periods were recorded. That compares with only four periods in 1984, the last bad scab year.
Willett said it is important to control scab early in the season. The fruiting bodies that develop on leaves on the orchard floor release spores when they are wet. Spore release peaks at about bloom. Leaves are most susceptible to infection two days after they are fully expanded.
After a time, the fruiting bodies will have discharged all their spores. If the orchard has not yet been infected when the fruiting bodies are empty, there is no longer any need to worry about infections later in the season.
"If you're clean at the end of June, you're probably okay, and subsequent infection periods are not going to be a great risk," Willett said.
When using the Mills table, orchardists may have some difficulty calculating wetness periods if the rain is intermittent, Willett said. Generally, if there are two wet periods separated by a dry period of less than eight hours, the two wet periods are added together, and the temperature of the two is averaged. If the dry period between them is more than eight hours, they are treated as two separate wet periods.
Temperature and wetness data is available from the PAWS (Public Agricultural Weather System), which updates reports every 15 minutes. Information from stations around the state is compiled at the Irrigated Agriculture Research and Extension Center (IAREC) at Prosser and made available on WSU's electronic bulletin board in Wenatchee. As an alternative, Willett said, orchardists can buy weather stations with built-in scab prediction software for their orchards.
There have been questions about the validity of the Mills table, as few spores are released at night. Willett said the small percentage that is released at night could still be sufficient to cause an infection. "We're not ready to recommend not counting a wet period at night," he said.
Infections later in the season can spread to fruit. The black lesions may be seen at harvest, or may not show up until after the fruit comes out of storage. Willett said scab does not spread to uninfected fruit in storage. Fruit that appeared to be clean when it went into storage had probably been infected before harvest.
Applying postharvest fungicides to fruit that is already infected will not have any effect, Willett warned, and certain export markets, such as Taiwan, do not accept apples treated with Ziram.
Willett said in the coming season he would recommend that orchards with overhead sprinklers and a history of apple scab receive some kind of protection in the spring. Eradicant fungicides, such as Rally (mycobutanil), Rubigan (fenarimol), or Procure (triflumizole) should be applied within four days after the Mills table indicates an infection period has occurred. If the orchardist is not able to cover the orchard in time, a preventive program, using protectant fungicides such as Captan, Ziram, or Dithane (mancozeb) might work, Willett said. These provide protection for about seven days.
Willett said it is important to control the disease early because control becomes more difficult as the season progresses. At higher summer temperatures, a shorter wetness period will trigger an infection, and irrigation may be all the wetness it takes.
"You don't want to control scab in the summer when you have overheads," he said. "It causes undue grief."
Copyright © 1996, Good Fruit Grower
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