SILVERLEAF RETURNS By Elizabeth Sagehorn

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SILVERLEAF RETURNS
By Elizabeth Sagehorn

The Rip Van Winkle of the fungus world, Chondrostereum purpureum or silverleaf, is back in California after a 30-year nap.

This is the first time it has been observed in almonds. So far it has been identified in orchards in Stanislaus, San Joaquin and Merced counties. Lynn Garber, a 4th generation farmer in Modesto, California found his first affected almond trees in a two-year old group last spring. "I was driving along the orchard with my son Darrell. He was looking out the window and said, 'Something doesn't look right out here.' [University of California Cooperative Extension Farm Advisor in Stanislaus County] Roger Duncan visited within 24 hours after we called him. He wanted to see what was going on. He knew it was silverleaf as soon as we told him."

Silverleaf is a more common enemy in Australia, New Zealand and Chile. It attacks almonds, apples, apricots, cherries, pears, peaches, plums, eucalyptus, lilacs, poplars, rhododendrons, roses, silver birches and willows. It is even used as a natural herbicide in parts of Canada and the Netherlands. The disease first showed up in California about 30 years ago. It was found in Stanislaus, San Joaquin and Contra Costa counties and attacked mostly peach trees. Then it disappeared.

Silverleaf seems especially fond of Padre, Butte and Peerless almonds. The first noticeable symptom is usually a silver cast to the leaves, which gives the disease its common name. Mr. Garber remembers, "As we looked across the orchard after the trees were past bloom and the orchard was once again green, there was a very distinctive cast to the infected trees. We had several different degrees of infection. There were highly infected trees that the silver cast could be seen from a distance, and trees that you had to look closely at the leaves to see any infection."

According to Mr. Duncan, the shimmer "is caused by a toxin the fungus produces. It causes the upper epidermis of the leaves to separate from the tissues, so you have an air space. It also causes a staining on the interior of the wood." There is usually another telltale sign. "A lot of the trees that have that kind of silvery look do not have a whole lot of almonds on them. Many times it looks like a reduced set." Some apple trees spontaneously recover, but that is not the case with almonds. The fungus continues to wreak havoc until the tree dies. "The next year those trees that had a little bit of silvering will have dead limbs, dead scaffolds, very small leaves and almost no crop at all."

Microscopic spores spread the disease. Fruit bodies (which resemble velvety exotic mushrooms) grow on the trunks of dead trees wait for the rainy season. They swell in higher humidity and once their water concentration level reaches 75%, they release their spores. The spores attach themselves to the raw wood of pruning wounds and work their way into the tree through the xylem. Initial symptoms are usually noticeable one or two seasons later, and the fruit bodies appear on the tree trunk in the fall of the second or third year of infection. The mushroom-like growths' lifespan is two to three years.

Why the disease returned recently has experts scratching their heads. There are a number of natural hosts. If you're planting near an infected area, you could be setting up a buffet table for silverleaf. The fungus also seems to like lurking in damp areas. Dr. Jim Adaskaveg, a plant pathologist at U.C. Riverside who specializes in these types of fungi explains, "As the almond industry expands and puts into new acreage, a lot of these acres go in near watersheds or rivers and those environments seem to be conducive to the development of this organism." He also notes that, "All of these almonds are grown on peach rootstock, they're closely related and so that's interesting. The fungus probably goes right into the peach, as well." Is there a connection? No one knows, yet.

Whether the most recent infestation occurred naturally or was imported from nursery stock is nearly impossible to establish. Many nurseries are working closely with the university to learn how to practice better hygiene and ensure that their trees are free of the disease. Dr. Adaskaveg advises, "When you're buying nursery plants and you make big heading cuts, if it's a nice and clear yellow, typical woody color, then everything's O.K. But if there is any discolored wood, if you see brown discolorations then that could mean that it's already infected."

While the folks at the University of California search for a magic bullet, farmers like Mr. Garber are being asked to pull out all affected trees, burning the branches, stumps and roots before re-planting. Mr. Garber remembers, "My immediate neighbor was a little bit concerned that we didn't get the infected trees out in time, but you have to have that fruit body for there to be a new infection." Farm advisors are also encouraging growers to use proven preventative measures like pruning in dry, low-humidity weather while the fruit bodies can't release spores. This past autumn Mr. Garber, "sprayed the orchard with zinc to remove the leaves and pruned in late October. We are trying to get 30 days from the time we prune until the first rains."

Dr. Adaskaveg adds, "Don't wait [until the tree is older] to prune. The smallest branches heal much quicker than if you start making big cuts on big branches. We usually say four to six weeks before the wound becomes naturally more resistant to the fungus. The big wounds might take longer." It is also important to make clean cuts and avoid splits or ragged cuts, which make it easier for the spores to permeate the wood. You should also never concoct a homemade wound dressing. Some substances actually hasten spore infection. The good news is that silverleaf does not appear to be transferred via pruning tools.

Some of the most promising research is prophylactic. Mr. Duncan explains, "If you have a large pruning wound and you put the fungicide on it, if a spore should happen to fall on it, it won't be able to germinate and penetrate." In an attempt to protect those pruning wounds, researchers in New Zealand have done a lot of experimentation with wound dressings. Their results have been mixed. Dr. Adaskaveg has also been conducting experiments on orchards at U.C. Davis. "We've been primarily focusing on potential fungicide wound dressings. We've done the chemical stuff and then we're going to look at the biological stuff. The idea is to establish another fungus that might function like an antagonist to the colonization of the wound area to the silverleaf fungus."

At the moment, Dr. Adaskaveg is pouring most of his energy into preventative measures rather than a cure. "It's difficult to save the trees once they're infected. If the silverleaf got in first, it's got a foothold in the tree. Even with the chemical control it's very difficult to eradicate it." Silverleaf effectively kills the tree's circulatory system. An injected fungicide would not be able to travel past the infected areas to protect the rest of the tree.

According to Dr. Adaskaveg, "One of my strategies was to work with registered fungicides on almonds that are used for other diseases. If you start from ground zero with a chemical or biological control there's a lot of effort: manufacturers, labeling, registration, mass production. It's not simple to do. There is already a fungicide active against it [Laredo registered by Dow AgroSciences], it would be a lot easier to amend the label and include silverleaf. Then you could treat the wounds and you wouldn't have to start from square one."

Dr. Adaskaveg is optimistic that this silverleaf outbreak can be contained. "Because it has a wide host range, it's always going to be a threat, but I don't see it as a big threat." The outbreak in the early '70s was quickly eradicated by the same measures that are being recommended now, primarily grower education. · Raise awareness among the farmers who produce these crops. · Teach growers alternative pruning practices to avoid contracting the disease. · Immediately remove and burn sick trees, stumps and roots.

Hopefully in another year or two this disease will have been eliminated and in three more decades your grandkids will have to ask, "What was silverleaf?"