Almond non-infectious bud failure (BF) is like a year-old hangover: It can start one season and end the next.
It is a genetic disorder exacerbated by too much heat in the summer before “crazy top” causes significant yield loss the following season.
In severe cases, BF can cause a yield loss of 60 percent or more on individual trees. It became so severe 15 years ago in the now-popular Carmel variety that the variety was almost abandoned not long after it was introduced because of major BF losses, says Butte County, Calif. University of California Cooperative Extension Farm Advisor Joe Connell.
This near-loss of Carmel was due to high demand for the variety and unwise selection of wood to propagate it
At the recent 30th annual Nickels Almond Field Day near Arbuckle, Calif., he told growers the key to avoiding major yield losses from BF is early detection of trees with severe cases of the disorder and replacement within the second to fourth leaf.
If you wait longer than that, it is almost too late to economically replace the trees.
Since high summer heat brings out the worst of BF, this season is expected to be a bad one for bud failure as a result of a prolonged, 100 degree-plus hot spell last July. Water stress, premature defoliation, mites, scab, rust, and anything else that puts a tree under stress one year can also result in bud failure the next season.
“You will have different degrees of severity from year to year, based on the previous year's weather and growing conditions,” says Connell.
The problem is more prevalent in the hotter almond-producing areas in the Southern San Joaquin Valley, but it can occur anywhere in the state on trees with Nonpareil parentage, such as Carmel, Merced, Peerless, and Harvey. It has even been found in Mission in the Southern San Joaquin.
BF occurs in vegetative buds — not flower buds — that fail due to high summer temperatures. One symptom of BF-infected trees is that they bloom four to seven days after non-affected trees.
Ironically, Connell says, this could cause trees with mild crazy top to actually out-yield non-BF trees if the delayed bloom hits better pollination weather than the non-crazy top trees.
But in severe BF situations, yield losses are due to less fruit wood and possible reduced leaf surface that reduces carbohydrate production.
Buds set in the early and later portions of the year are not normally affected by BF because the weather is cooler in those months.
Pruning out BF wood will not solve the problem because it is impossible to know what the weather conditions will be after pruning, says Connell.
There are three options available to growers.
One is to do nothing, which is often the best option when BF is identified in older trees. “Mild bud failure is not all that bad, particularly in Nonpareil,” Connell says.
Top-working BF trees in the second to fifth leaf, using wood with lower BF, is another potential solution. This, he says, can gain a producer a year of production over replanting BF trees. But it requires considerable work and attention to detail. If a high percentage of the grafts fail, the effort would be for naught.
Just like top-working, replanting should be done as early as possible for identifiable severe BF trees.
It is critical to determine the severity of the genetic disorder, says Connell. In UC studies of mildly-affected Nonpareil trees, yields were 91 percent of that for clean trees.
Mildly-affected BF is characterized by dead buds on several secondary branches.
Severe BF trees have bud failures on one major scaffold, with other symptoms throughout the tree. In the university studies, these trees yielded just 64 percent of normal, and were candidates for early replacement.
Kernel weight and numbers were also reduced in these trees and they had a tendency to produce more double kernels.
Replacing trees must be based on economics, even for young trees. Even at a 40 percent yield loss potential, it takes 14 years to reach a break-even point for the cost of replacing the tree. At a 60 percent yield loss, it takes nine years to recover the cost.
“Generally, the orchard must have more than 10 years of life remaining to justify replacing BF trees,” says Connell.
Early detection is critical in reducing the time to a break-even yield. The first opportunity to identify BF is in the spring of the second leaf. Later in the year, new growth could mask BF.
Connell recommends replacing trees five to six years old only if the main framework of the tree is affected by BF.
“If you find trees that old, it means you probably overlooked them when they first showed subtle signs of the disorder in their second or third leaf,” he says. “Mild BF, affecting only the upper canopy, may not seriously affect yield.”
Since BF is genetic, it is important to determine the origin of the trees. Nurseries do progeny testing to evaluate stock for crazy top. From these trees, mother blocks are established from trees with low BF potential, and those nursery blocks are used to develop commercial trees.
However, as clean as a mother block may be, with each generation away from the mother block comes more BF susceptibility. It is unavoidable, says Connell.
“Where we got into trouble with Carmel was when the industry was expanding by 30,000 to 40,000 acres a year and there wasn't enough wood in mother blocks to meet the demand for Carmel,” he says.
Nurseries and others went to other orchards with low BF symptoms and gathered wood for new orchards.
“You had people getting wood from neighbors' orchards that had no BF, but all of a sudden, the new orchard from that wood had failures far greater than the orchard from which the wood was taken.”
Nurseries cannot completely eliminate BF, even in mother blocks, says Connell. Each step removed from that mother block significantly increases the likelihood of expanding BF.
“Carmel is a great variety, but we almost lost it because of this issue. The industry has gone back to that original prodigy block and has brought BF down to the level where we can try to live with it.
“It'is a constant challenge,” Connell says, “to keep a lid on BF and not let it blow up. If you replace BF trees, make sure you know where the budwood came from.”