Alternaria leaf blight probed in almonds Research on alternaria leaf blight of almonds during 2000 centered on evaluation of fungicides and tree architecture as controls.

Beth Teviotdale, University of California Extension plant pathologist, and Mario Viveros, Kern County farm advisor, talked about their work during the recent 28th Annual Almond Industry Conference in Modesto.

Alternaria was detected in Kern County in 1980 but remained dormant until 1993 when it caused premature defoliation and poor nut removal on large almond acreages. Infected orchards have been found in Delano, McFarland, Wasco, Shafter, and Rosedale.

The disease is worsened by dews and high humidity and is often more severe on trees having spreading, rather than upright, canopies.

In the projects, supported by the Almond Board of California, Teviotdale continued study of the epidemiology and control of the fungus, while Viveros began a multi-year investigation of tree architecture.

"Alternaria leaf blight causes leaf lesions and defoliation and can be found in orchards throughout the state," said Teviotdale.

"In most instances, noticeable defoliation occurs only after harvest and causes no apparent damage. Where severe and repeated preharvest defoliation occurs, most commonly in southern valley areas, trees are weakened and yield losses can exceed 50 percent."

In her trials on a commercial block of Sonora trees in Kern County, Teviotdale compared treatments with several experimental fungicides.

She reported that only strobilurin products, which have a chemistry similar to Abound, were successful. She said the addition of Rovral plus Ziram early in the year may improve the performance of Abound.

Complex of forms Teviotdale noted the disease is actually a complex of several forms of the fungus. Three species, Alternaria alternata, A. arborescens, and A. tenuissima, have been collected from infected almond leaves.

Further studies are in progress, but Teviotdale said A. tenuissima germinated spores more rapidly at lower temperatures than A. alternata. That may explain why high infection rates of A. tenuissima occurred earlier in the year in inoculation tests comparing the two.

Viveros set up a trial on a three-year-old orchard of Sonora, Nonpareil, and Butte. In addition to a control with a standard tree configuration, three treatments were made: hedging, topping, and a combination of the two.

"The tree architecture due to different pruning methods didn't improve the control of alternaria," he said. "However, this is not unexpected since it takes more than one year to position scaffolds and limbs in an upright position."

Viveros said orchards infected with alternaria have two things in common: First, the disease occurs first on the outside and mid-section of the canopy. Second, the worst infections are in trees having an opened canopy center.