Think Before Pulling Out Trees With Almond Leaf Scorch Disease
May 1, 2008 
When faced with almond leaf scorch disease, many almond growers opt to pull out the infected trees and replant. On the face of it, the decision makes perfect sense. After all, the disease is incurable, and can severely diminish yields. But in studying the effects of almond leaf scorch (ALS) disease on yield and tree vitality over the past several years, USDA-Agricultural Research Service entomologist Mark Sisterson says growers should consider the decision carefully.
First of all, while the disease is prevalent in orchards throughout California, says Sisterson, it usually only affects a small proportion of trees. In addition, it is highly unusual to see large increases in the number of infected trees within orchards from year to year. While some people maintain that the disease can spread from tree to tree, and in theory an insect could spread ALS from an infected tree to a “clean” tree, he says that doesn’t appear to be the case. In other words, it doesn’t make sense to remove trees to limit the chance that the disease will spread, because there’s no evidence it will.
Another reason growers will remove diseased trees is because ALS is incurable. But just because it is incurable, says Sisterson, does not mean it is deadly. “It’s dogma that infected trees will die. It will lower yields, and you can see some death, but tree death appears to be much rarer than previously thought,” he says.
Age, Variety Are Keys
High: Sonora, Solano, and Peerless.
Medium: Nonpareil, Neplus, Price, and Mission.
Low: Butte, Carmel, Padre, and Thompson.
Still another reason that growers will remove and replace trees that are infected with ALS is because of declining yields. Some varieties are more susceptible than others, and Sisterson has found that some particularly susceptible cultivars, such as Sonora, can sustain average losses of 40%. But even with losses that great, growers should consider some other factors before deciding to replant. First, the cost of replanting is more than many growers might think, says Sisterson, who has run economic models that take into account numerous factors, including the age of the trees. It’s not the actual replacement of the tree that costs the most, because trees don’t cost that much and most growers have the workers and the equipment on hand to do the job. The real cost of replanting is in the yields lost until the newly planted tree matures. Because of that, it would rarely make sense to replace a variety that does not display dramatic yield reductions when infected. For instance, even in a tree that averages 20% yield losses, such as the industry mainstay, Nonpareil, it would be highly unusual if replacement penciled out. The only exception would be a very young orchard.
In sum, before replanting, growers should consider the variety. (See “Choose Varieties Carefully.”) If it’s very susceptible to ALS, then consider the age of the orchard. “If it’s a 20-year-old orchard, the risk of leaving a diseased tree is low,” says Sisterson, who pegged the lifetime of a commercially productive orchard at 25 years. “If it’s five years old, it might be worthwhile to pull out the diseased tree and plant a new one.”
Choose Varieties Carefully
If you’ve had problems with almond leaf scorch (ALS) disease, or you’re concerned that your trees will contract it, you should pay special attention to variety selection for a couple of reasons. First of all, the disease is incurable. (It is caused by the bacterium Xylella fastidiosa, and causes Pierce’s disease in grapes, which is also incurable.) Second, there’s no way to protect your trees from contracting ALS, says USDA-Agricultural Research Service entomologist Mark Sisterson. “There’s no silver bullet at this point,” he says. “The easiest thing to do is be careful in your choice of cultivars.”
Some varieties are much more susceptible to ALS than others, says Sisterson. Here’s a brief look at some of the major varieties, from highly susceptible to low. But before deciding, growers should work with nurseries, and consult University of California Cooperative Extension experts if they have questions.
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