Good Fruit Grower / April-1st 2012 / Check at harvest for ­little cherry disease

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Check at harvest for ­little cherry disease

Small, bad-tasting cherries are a reliable symptom of the viral disease.

There is more than one reason why cherries in an orchard might be small, but if the cherries also taste bad, then it’s a sure bet that the limb or tree is infected by little cherry disease.    

Symptoms tend to be more noticeable in growing seasons when the spring was cool, Tim Smith, Washington State University extension educator explained during the recent North Central Washington Stone Fruit Day.

The spring of 2011 was extremely cool, with below-average temperatures on many days, providing ideal conditions for seeing little cherry disease in orchards. Trees don’t typically look diseased at all, so the only way to ­diagnose the problem is by tasting the fruit.

The disease, as its name implies, results in small, sometimes odd-shaped fruit on certain limbs or on whole trees. Fruit development is delayed and the fruit tastes nasty, with an almost bitter flavor, Smith said.

“No one would want to eat a second cherry if the first one tasted like that. If cherries are small for some other reason, they’re usually edible.”

Harvest

Because the symptoms are most obvious in the fruit, the best time to identify infected trees is around harvest. After warm springs, fruit size and color might be closer to normal, but the cherries still won’t taste good enough
to sell.

Smith said if there are limbs or trees in the orchard that still have pink fruit when the rest are turning red or mahogany, the grower should taste the fruit, and if it has an unpleasant flavor, the trees should be flagged for removal later. There is no treatment or cure for the ­disease.

“If they have this, they’re going to have it until you cut them down,” Smith warned. “Hope springs eternal, but they will not recover. The only way to stop it is to remove infected trees. It’s critical to your orchard, your neighbor’s orchard, and the industry.”

The disease typically starts in one limb and then spreads to the rest of the tree. Eventually, it will spread down the row and across the row to nearby trees, and it spreads more rapidly than any other serious cherry disease, Smith said. The longer infected trees are left in the ground, the greater the number of trees that will have to be removed.

The disease was first found in British Columbia in 1933, when 60,000 cherry trees were lost and the industry was almost wiped out. It is now recognized worldwide. The disease is caused by one of two viruses—Little Cherry Virus-1 or Little Cherry Virus-2—which can be transmitted on noncertified budwood or rootstocks. In the orchard, it is spread by mealybugs or through fused roots of neighboring trees. It cannot be spread by pruning, Smith said.

Over the past couple of years, Smith and WSU plant virologist Dr. Ken Eastwell have been sampling commercial orchards in the state that had fruit symptoms and testing fruit. Many of the orchards proved to have Little Cherry Virus-2, and growers have been encouraged to remove trees to prevent its spread.

When infected trees are removed, the freshly cut stumps should be treated with Roundup (glyphosate), which will soak into the root system and spread to any trees that are root grafted to the original tree. This will enable other infected trees to be quickly identified before they would normally show symptoms.

“Until you get ahead of this thing, it will daisy chain down your orchard,” Smith warned. “Glyphosate symptoms will show up in neighboring trees the next spring.”

Smith said some growers have been reluctant to pull infected orchards. “I can understand not wanting to pull a 20-year-old tree, but you have to be brutal with this ­disease, or it will continue to spread in your orchard and your neighbor’s.”

For more information, check Eastwell’s Web site at http://healthyplants.wsu.edu/ research/little-cherry/.