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Almond crazy top, hull rot more evident this season

Almond bud failure, often called "crazy top" is much worse this year, according to Merced County farm advisor Lonnie Hendricks.

One reason for the genetically linked problem is that it affects the two most widely planted varieties, Carmel and Nonpareil, according to Mark Freeman, Hendricks counterpart in Fresno County.

Other varieties where the problem has been severe are Jordanolo, Merced and Harvey, according to Hendricks.

"Nonpareil bud failure has been less common in recent years than it was in the 1960s and 70s," said Hendricks "But we are again seeing more in Nonpareil."

Peerless, Price, Thompson and Mission seldom show bud failure. Butte Padre, Sonora, NePlus, Ultra, Fritz and Monterey have shown no BF signs. However, Hendricks said there is no assurance that it will never appear in these varieties.

Bud failure vegetative or leaf buds, not the flower or fruit-setting buds, explained Freeman. Although the problem was reported as severe this year, the impact on tree’s ability to set and mature nuts will likely come next year when there is reduced leaf surface and less tree vigor as result of bud failure this season.

While the root of the bud failure is genetic, it is brought on by hot temperatures.

Last June there were a few days of temperatures of more than 100 degrees in the valley and that probably caused bud failure growers are seeing this year.

"The problem is latent or hidden until environmental conditions are right for it to manifest itself," said Freeman, who said growers "must assume Carmel trees tend to have bud failure" and should not be planted in hot areas.

Hendricks added that research and field experience suggests water stress may increase BF the following year. "This may be due to higher bud temperatures on stressed and partially defoliated trees," he said.

The only solution to crazy top is tree replacement and Freeman said many Carmel producers are seriously looking at variety replacement as well.

If trees show bud failure within the first five or six years, Hendricks recommends replacing them. After that, trees should be allowed to continue producing until yields fall below economic returns. Budding or grafting BF trees with good wood is possible, but may not be economically successful or practical, Hendricks said.

Both Freeman and Hendricks said growers must be inquisitive with nurseries about budwood sources and if there are performance records, especially budwood from hotter areas.

"However, past good budwood performance is only an indication, not a guarantee of good future performance," said Hendricks.

Like bud failure, hull rot can be a nettlesome, long-term problem. Unlike crazy top, hull rot has a cultural solution, according to Brent Holtz, Madera County UC farm advisor.

Holtz said many growers this spring noticed their Nonpareil and Sonoma almond varieties lacked bloom in their lower portions while other varieties, like Carmel, displayed normal bloom.

Growers believe this is a result of shading. However, it is often caused by hull rot.

Hull rot problems start about mid hull split. It is then that dry leaf clusters begin to appear scattered in the tree canopy.

"Individual spurs, small shoots or entire small branches may collapse. The loss of fruiting wood, especially in the lower parts of the tree, can negatively affect yield for years afterward," said Holtz.

Hull rot is caused by one of two fungi. Monilina fructicola is the one of the brown rot fungi and rhizopus stolonifer is called bread mold fungus.

Lesions or dryish rotted areas develop on the hull. Dense masses of Rhizopus spores produce a powdery dark gray to black growth between the hull and the shell. Monilinia spores are buff-colored, said Holtz, and are evident on inner and outer hull surfaces.

These do not damage the nutmeat, but move from the hull to nearby leaves and shoots, killing these tissues and reducing bloom and yield in later crops.

"Once hull split starts, trees are at risk of becoming infected," the farm advisor said.

Hull rot often occurs in vigorous, heavily cropped, well-water and fertilized orchards. Holtz calls hull rot the "good growers disease" since it is often worse in well-maintained orchards.

"The reasons for this are not clear. The association with heavy crops might be simply a matter of numbers: more infected fruit means more toxin produced resulting in more leaf and shoot death," he said.

University of California researchers found that the disease was dramatically reduced with mild water stress during early hull split.

"Cutting water by 50 percent during this early hull split puts the tree into moderate stress…not enough to drop the crop… but enough to decrease hull rot infection by half or more," said Freeman.

The researchers found that the best irrigation regime to reduce hull rot was to irrigate normally until just before and during hull when irrigations were reduced to 50 percent of the evapotranspiration rate.

Holtz noted that this work was done in Kern County and soils can vary throughout the state. "Irrigation management may be difficult in some orchards," he noted, adding that UC is currently testing several approaches to use water stress under different irrigation strategies and soil types.

"In another trial we are experimenting with midday stem water potential readings to monitor deficit irrigation to reduce hull rot without severely stressing trees," added Holtz.

In trials in Stanislaus County, hull rot increased with increasing nitrogen amounts.

"Nitrogen should not be applied in excess of that needed for tree health and productivity," Holtz said, adding that the nitrogen content of irrigation water should be calculated into any fertilizer program.

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