Washington researcher warns of little cherry disease devastation

February 06 , 2014

A leading Washington fruit tree specialist has warned that a cherry virus which stunts growth and causes bitter tasting fruit has potential to “take out the industry” and urged farmers to act now.Little cherry disease pic

In an interview with www.freshfruitportal.com, scientist Timothy Smith spoke about the devastating effect of the little cherry disease that has big consequences.

The Washington State University regional extension specialist, Smith, studies the little cherry disease that has been ravaging several cherry orchards across the Wenatchee region recently.

For undetermined reasons, the disease is making a comeback after previous appearances in the 1970s when it had a serious impact on the industry in British Columbia. This wasn’t the first time orchards were destroyed there – in 1933 almost all of BC’s 60,000 trees were wiped out because of the little cherry disease.

Over the last few years Smith has seen entire orchards wiped out because of the virus that has taken an upturn since 2010. The disease causes small tasteless cherries that are sub-standard and not good enough to market. Very few infected trees recover and the best remedy is to yank them out.

“The little cherry disease has caused serious problems in the past because it’s very difficult to detect visually and it spreads fairly rapidly across orchards. It’s been common in Europe and Northern America and is basically a threat anywhere with a cherry growing industry,” he said.

“It’s been an issue in the past in Washington where growers have lost out massively because the cherries are barely marketable. Now, we are trying to head off a worsening situation by emphasising this disease and having people find it in their orchards and then take the trees out before it spreads further.”

Although the disease does not affect the overall health of the tree, it does inflict extensive damage to the fruit.

“With the standards that we have in our industry, the fruit has to be at least medium to large and growers have to eliminate the small fruit. I don’t think much fruit from the little cherry virus actually gets into the market because they are eliminated in the sorting process,” he said.

One striking aspect of the little cherry disease, which is spread by at least two mealy bugs according to Smith, is that it’s enigmatic in nature and only becomes identifiable at harvest.

This can make his job of convincing a cherry grower to pull out trees suspected of infection very difficult because the trees themselves appear to be perfectly healthy.

“We’ve had some areas where it has sat there for a number of years, an unknown number of years, completely unidentified and it’s taken whole orchards under its influence.  In these situations, growers have had to pull out whole orchards rather than just a patch of trees, which is obviously devastating and very costly,” he said.

“If nothing is done with infected trees, things will not stay they way they are. They will get much, much worse.”

The lack of symptoms has complicated disease control and made it difficult to win over farmers’ confidence, Smith explained.

“Sometimes it can be difficult to convince a grower to take out a tree that looks very healthy. Cherry growers are accustomed to viruses and diseases which are common in cherry trees but they normally see symptoms for themselves, whether it be the way the tree is growing or the color of the leaves or other strange patterns and shapes, so they can usually tell when a tree has a virus and needs to come out,” the researcher said.

“However, this one is cryptic and the trees look just great and healthy as can be. There is only about a two-week period when unaffected trees will go ahead and ripen normally while the little cherry virus infected trees are flagging behind in the size of the fruit.”

Smith says the virus can be spotted and tracked at harvest time and although nothing can be done to salvage those unfit-for-sale cherries, action must be taken to prevent the spread of the disease and stop it from disrupting future harvests.

“You identify a tree that is infected by the small, poorly colored, tasteless, nasty fruit and compare it to the sweet delicious fruit that is next to it. The contrast is quite easy to pick out,” Smith said.

“Those cherries will often never ripen to a point that they are pleasant to eat but if growers don’t mark the trees during harvest and the fruit drops off the tree, naturally that tree will then be unidentifiable.

“It takes a couple of years for this disease to show up so growers who find it visible in their orchard must consider that it may be less visible in neighboring trees. When they cut the infected trees down it’s important to mark the neighboring trees and watch them for two to three years to see if they come down with it because the trees can graft roots together and the virus can pass through the roots.”

With no cure, Smith and a team of field specialists are trying to convince Washington’s cherry growers just how serious the little cherry disease actually is. They have urged growers to “get ahead of it and learn how to spot it” so as not to suffer the same fate as other unfortunate fruit farmers.

Smith cited Stemilt Growers Inc who last year pulled out 200 acres of cherry trees on Stemilt Hill and Wenatchee Heights, in addition to growers in the Malaga region who have pulled out several patches of infected trees to stop the disease really taking hold.

Smith said he understood how busy growers are but he stressed that little cherry disease should not be underestimated or ignored.

“I witnessed one situation where the grower said they had seen two or three trees on the edge of their orchard that seemed to have a problem but they didn’t identify it as little cherry disease. Three years later I was back there amongst beautiful trees just reaching their full production and loaded with fruit. Sadly, all this fruit was ruined all the way across the orchard, two hectares in total over a three year period,” he said.”

As part of an educational program, Smith is hosting meetings to raise awareness and advise growers on their best course of action. Experts are also going out into the field to help train growers. In addition, the Washington Tree Fruit Research Commission and Oregon Sweet Cherry Commission have agreed to spend almost US$64,000 on a study to discover why the disease is increasing and to develop management strategies.

“We’ve held meetings over the winter that most of the cherry growers have been attending and we’re going out into the field to work with the field advisory people to train them on how to spot this disease. These are key individuals who will continue to raise awareness and help identify the problem,” Smith said.

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