Wood rot fungus threatens San Joaquin Valley almond orchards

Fresno County’s $1.2 billion almond industry has a scary new threat: a mystery fungus that devours trees from the inside out, causing them to snap and fall over.

Making the threat worse, there is no cure for the fungus, researchers say.

Almonds are Fresno County’s top crop and one of the main commodities produced in the central San Joaquin Valley. Statewide, almonds are a $5 billion crop.

Scientists say the wood rot fungus typically attacks older fruit and nut trees but is being found in much younger trees in the Valley.

University of California researchers say the fungus, known as ganoderma, is no stranger to agriculture and probably is the reason why most trees blow over.

But what is puzzling experts is why the fungus seems to be showing up in greater numbers and in younger trees.

The disease slowly eats away at a tree’s heartwood until it becomes unstable and eventually falls over. In many cases it can take years for the fungus to bring an older tree down.

But UC graduate student Bob Johnson, who is studying the fungus as part of his doctoral thesis, found thousands of infected trees in a Hanford almond orchard that were just 9 and 10 years old.

“It was so bad you could push the tree over and it would fall,” Johnson said. “The trunk broke off right at the soil. It was unbelievable.”

Johnson said the grower chose to remove all the trees in the 120-acre orchard. Ganoderma also has been found in numerous trees in two orchards within a mile of each other in west Fresno County. And he has recently received phone calls from growers in Kern, Tulare and Madera counties with concerns about falling trees.

Farmers say they, too, want more answers explaining why the fungus seems to be affecting younger trees.

“It is a little early to say how big of a threat this might be,” said Tom Rogers, a Madera County almond farmer. “But it does raise a lot of questions, like how did it get started, and how was it brought in, is it something in the field?”

Those are all questions that Johnson and others are trying to answer. What scientists do know is that it takes time for the fungus to grow inside a tree and become infectious.

The fungus damages a tree by slowly attacking its root structure. The tree often looks normal and can even produce a crop. The only telltale sign that the fungus has colonized a tree is the appearance of a mushroom-like growth, called a conk, that sprouts at the base of the tree.

The disease spreads to other trees as the conk releases trillions of spores into the air.

Johnson suspects that the size of the state’s booming almond industry – valued at $5.3 billion and taking up more than 1 million acres – combined with stress from five years of drought have created the right conditions for the disease to flourish.

“This could be a huge problem, if we knew how widespread this is,” Johnson said. “But the fact alone that it has taken out 120 acres of 10-year-old trees tells me this is important.”

Mae Culumber, a University of California nut crop adviser in Fresno County, agrees that fungal disease in almonds could become a bigger issue.

Culumber said the fungus has no known cure. And there are few, if any, preventive measures. What she tells growers is to try to keep their trees as free from injury as possible. The spores infect a tree by entering into any opening in the tree’s bark or roots.

So far, Culumber has found infected trees in two west Fresno County orchards. In both cases many of the trees had advanced stages of the disease.

“The grower is losing several trees a month,” Culumber said.

David Doll, a University of California pomology (the science of fruit growing) farm adviser in Merced County, said wood rot could prove to be costly if growers have to replace orchards after only about 10 years.

The normal lifespan of an almond tree is about 20 years. Doll estimates the cost of replacing a tree at about $250.

He is eager to see what Johnson and other researchers find out about ganoderma in the Valley.

“This is an issue that we will need to address as an industry,” Doll said. “Because this is not something that you can run out to a field and apply a fungicide to or easily diagnose.”

This story was originally published February 3, 2017 at 2:29 PM with the headline "Wood rot fungus threatens San Joaquin Valley almond orchards."