California processing tomato producers gathered in Modesto to hear of a relatively new and devastating virus that has hit fields outside the state’s major growing regions, new techniques being tested to quantify mold, and efforts by breeders to stay ahead of diseases.

Those were among topics discussed by University of California farm advisors, specialists and others during a Northern San Joaquin Valley Processing Tomato Production Meeting that preceded the annual meeting of the California Tomato Growers Association.

The tomato yellow leaf curl virus, “the most damaging of tomato viruses worldwide,” is now in parts of Southern California, said Bob Gilbertson, a UC Davis plant pathology specialist. He said the virus, spread by whiteflies, was introduced into the Imperial Valley in 2007 and has spread into Riverside County.

“Those are not big tomato producing areas, but we are monitoring this closely,” Gilbertson said, adding that the chief tomato growing regions of the state have cold winters “with a host-free period to keep whiteflies out.”

One risk, he said, is that the virus could be spread through infected plants sold at retail stores.

Gilbertson talked of a wide range of viruses and advised growers to try to select tomato varieties that are virus free, to choose high quality seed and to avoid planting near old, established fields with weeds. “It’s best to get rid of old plants after harvest,” he said.

More common viruses in California’s tomato growing regions include tomato spotted wilt virus spread by thrips and other sucking insects. Symptoms include a mosaic or mottling appearance of the leaves.

Gilbertson said there are 100 viruses affecting tomatoes worldwide and 50 that impact California. He said they are best managed through integrated pest management approaches that include multiple controls: biological, cultural, genetic, and chemical.

It’s vital to understand the virus, and it can sometimes be difficult to pinpoint the problem virus by symptoms alone, Gilbertson said. Various techniques are used to identify the virus, including seriological tests and an ELISA test.

Tom Turini, a UC farm advisor for Fresno County, talked of recent research on management of tomato spotted wilt virus on processing tomatoes. He said the problem has increased in the past five to seven years, although the virus has been present in the state for decades.

Researchers have looked at potential sources that included transplant nurseries, almond orchards and plantings of radicchio. Although thrips and the virus were found in radicchio, the other two sources did not show evidence of the virus. The virus was also present in lettuce in the fall.

Turini said effective materials for controlling thrips include Radiant, Surround, Dimetholate, Lannate and Beleaf plus Mustang. He said only larvae can carry the virus.

Turini also said there are some varieties that are resistant and it is best to avoid planting near spots known to have the virus.

David Slaughter, UC Davis biological and agricultural engineering specialist, talked of trying to find more objective methods for quantifying mold in processing tomatoes during grading station inspections.

Use of visual methods to detect mold, he said, are subjective and imprecise. One method, called the Howard mold count, dates from 1917 when it was developed by the U.S Department of Agriculture.

A more recent approach uses an antigen/antibody system with diluted tomato juice to do a test that Slaughter said is akin to a pregnancy test. His research includes trying to determine which fungi are of particular importance in tomatoes, such as the alternia fungi.

The Modesto program also included discussion of plant development by Steve Schroeder, with Nunhems USA, and Rich Ozminkowski, with Heinz Seed.

Both breeders said their companies are working on varieties resistant to the deadly tomato yellow leaf curl virus. They said global breeding programs are helpful in developing plants that can grow in a wide range of conditions.

Schroeder and Ozminkowski talked of the fact that coming up with new varieties with sought-after traits can be a long process, taking several years.

Ozminkowski said varieties have been developed that delay field rotting, relying on Midwestern research where that is more of a problem than in California. He pointed out that California is the world’s leader in processing tomato production with 31 percent of the global total, followed in order by Europe at 27 percent and Asia at 25 percent.

The new varieties make possible an extended harvest season, he said.

Other topics discussed during the production meeting included:

• Fall-timed reduced tillage in processing tomatoes. Gene Miyao, a UC farm advisor in Yolo and Solano counties talked of studies at the UC Davis campus field that showed similar yields for conventional and reduced tillage systems and no significant difference in fruit quality. He wants to expand testing into grower fields.

• Garden centipede sampling and quantification. Jan Mickler, a UC farm advisor for Stanislaus County, said the centipedes feed on roots and organic matter. They prefer warm and moist soil, meaning the better the soil management, the greater the populations of the pest.

Mickler said a potato baiting technique used effectively on the coast of California did not work well as an indicator in the Central Valley. “Maybe we’re a little on the dry side,” she said, adding that she is trying another detection approach called a Berlese Funnel, which uses a light source while collecting the centipedes in a jar with ethanol.

• Powdery mildew. Brenna Aegerter, a UC farm advisor in San Joaquin County, said there are few registered materials for use in preventing powdery mildew in tomatoes. She said trials done at locations that include Dixon, Tracy, Los Banos and Five Points found sulfur dust to be most effective, followed by Quadris alternated with Rally.

She cited significant yield increases in Fresno County where sulfur was used and mildew was a notable problem. But because multiple applications were required, she raised a question, “Is it economically justified?”

• Fertilizer forms and additives. Tim Hartz, with the UC Davis Plant Science Department, said research shows that nitrogen levels in soil and tissue did not vary greatly regardless of whether the form of nitrogen was in ammonia sulfate or calcium nitrate.

He added that it’s difficult to raise calcium levels in fruit because of the way calcium moves through the plant. Hartz said research on the use of humic acid to improve fertilizer performance indicated it did not appear to provide a return that made it economically viable.