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Anthracnose Update

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An ancient disease that had been lying low (or mistaken for something else) for many years, has emerged as a potentially serious affliction of many California almond orchards. The disease is anthracnose, the subject of an increasing amount of attention on the part of growers, agricultural researchers, and the farm press.

It was also the subject of a cultural practices seminar (by popular request) at the Blue Diamond® Growers annual meeting in December of 1996. Dr. Jim Adaskaveg, Department of Plant Pathology, UC Davis, presented his findings in a recent California Almond Board study of the disease.

"Anthracnose has been with us for many years," he noted, "but the flagging symptoms associated with it were probably mistaken for brown rot or some other problem. Now, however, it is spreading and causing economic damage."

"In the 1980s, Joe Connell (Butte County farm adviser) and Joe Ogawa (USDA researcher at UC Davis) identified the disease and found that it was causing economic loss."

In a study funded by the Almond Board, Adaskaveg and his associates found several species of collectotrichum, the organism that causes anthracnose. One, C. accetatum, is the culprit damaging California almonds. Other species attack local fruit crops and almond orchards elsewhere in the world.

C. accetatum infects blossoms, fruit, and shoots. The symptoms are blossom blight, gummy fruit, and leaf yellowing. It damages crops, causes branch dieback, reduces tree vigor.

Anthracnose damage differs from brown rot damage in that the disease does not stop with the blossom or restrict itself to the early season. Late season rains can trigger an outbreak of anthracnose that infects mature fruit, terminal buds and laterals.

Anthracnose, which produces a tell-tale sunken lesion on infected fruit, penetrates the hull and destroys the developing nut. It then travels into the spur. By late May or June, characteristic dieback of spurs and branches begins to appear. Mummified fruit hang on the tree. Adaskaveg's study found that 10 to 15 percent of the mummies contain viable cultures that can produce next season's infection.

Widespread Occurrence

Adaskaveg's team surveyed orchards in all of the major almond producing areas. They found anthracnose in every district, from Kern County to Butte County, not necessarily in economic-loss concentrations, but the disease was clearly present. Major outbreaks, he noted, have occurred in orchards from Butte to Merced County.

It was found that all currently-grown almond varieties are susceptible, although to varying degrees. The most susceptible are Merced, Thompson, Price, and Butte. The least susceptible is Nonpareil. Staking out the middle ground are Harvey, NePlus, Butte, Fritz, Carmel, Peerless, Padre, and Mission.

Disease Control

A variety of chemical controls were tested in the lab and in the field, but none of the currently available materials were able to eradicate it, although some provided a useful degree of control. Whenever rain occurred after achieving control, the disease bounced back, Adaskaveg said.

A field test in Carmels in Merced County achieved good control with Abound, Rally in combination with Captan, and Rally with Manex. A field test in Butte County achieved similar results. None of the materials eliminated the disease because none of them has long residual efficacy.

In both cases, Elite and Orbit, materials that, unfortunately, are not registered for this use, achieved the best results. However, a vigorous effort is underway to get federal EPA approval for their use in the primary area of occurrence this spring, said Adaskaveg.

In either case, with or without Orbit, control is protective only, he cautions. "Any control effort has to be made before rain occurs for the material to have an effect."

Mix Your Punches

First make sure of your target, Adaskaveg urged. "Don't start spraying without making certain of the disease that is causing the problem. Get your PCA or farm advisor to help you with the diagnosis. If it is anthracnose, and you have a history of it, use a strategy similar to what you would use for brown rot. Begin with a pink bud application and continue as long as weather threatens."

"Repeated treatments with a single fungicide will likely lead to resistance," Adaskaveg warns. "It's like using Benomyl only for brown rot. It was good at first, but resistance soon builds up." It is best to treat with a variety of materials use different combinations of Captan, Maneb and Rally, and rotate them to reduce the chance of the disease developing resistance, he suggested.

Helpful Hints

In response to questions from the audience, Adaskaveg said:

  • Copper is not considered an effective treatment for anthracnose.
  • All tests were conducted with today's standard treatment airblast with 100 gallons per acre as a concentrate spray.
  • Elite and Orbit were very effective against brown rot by themselves, compared to Rovral and oil.
  • Pruning out dead wood can be useful. Cut ten to twelve inches into living wood to make certain any anthracnose toxins that might be present are removed.
  • Assume spores are always present. Spores are moved around by splashing water, not wind. Tests showed that any rain event, even in May and late June could cause an outbreak.
  • Elite and Orbit provide surface protection only. They are not systemic. They have only limited penetration.