Spinach germplasm having resistance to leafminers, the main insect pests of the crop, have been discovered in USDA breeding research at Salinas, Calif., and released to seed companies for use in improved commercial varieties.

Geneticist Beiquan Mou, who led the breeding project, reported on his findings in this and five other assignments during the California Leafy Greens Research Board’s annual meeting at Coalinga.

Over the past three years, Mou screened more than 800 spinach germplasm accessions from gene banks around the world to identify sources of the resistance.

Adult leafminer flies, including Liriomyza huidobrensis, L. sativae, and L. trifoli, create “stings,” or feeding sites that disfigure tender young spinach leaves. Females deposit eggs in the leaves, and larvae create whitish “mines” inside the leaves, making them unmarketable and weakening the plants.

Chemical sprays are available to control the adults, but the required frequency of applications makes them uneconomical. The pests are present throughout California throughout the year, except for December and January. In severe cases, infested fields must be abandoned.

Initially, only partial resistance to leafminers was found, but Mou said later selections showed that resistance could be improved and combined with yield traits for use in developing new cultivars.

The germplasm lines are 03-04-9, an oriental type spinach, and 03-04-63, a semi-flat type having dark green, semi-erect leaves.

Both, Mou said, “showed significantly fewer mines per plant and per unit plant weight than check cultivars in four field trials during three years.”

In a project on resistance to Stemphylium leaf spot, Mou evaluated the entire USDA spinach germplasm collection, consisting of 338 accessions, plus 22 commercial cultivars. In a pair of attempts, none was found to be completely resistant, although there were significant differences in incidence and severity of the disease.

Two accessions from Turkey consistently rated low in both those characteristics, and they are in an additional selection process to raise the level of resistance.

Mou also screened the USDA spinach collection, plus several wild relatives and 11 commercial cultivars in search of oxalate concentration.

Excessive oxalic acid content in spinach is a nutritional issue because it interferes with the absorption of calcium and iron in human diets. That is why breeders are searching for low-oxalate cultivars.

Trials at the Salinas Station showed that two accessions from Syria, Shami and Baladi, had low levels of the compound. “With the genetic variation and sources of low oxalate concentration found in these, genetic improvement for low oxalate seems feasible,” Mou said.

Several commercial spinach cultivars were found to be infected with impatiens necrotic spot virus (INSV) in an experimental field at Salinas in the fall of 2008. Tests by Mou and USDA colleagues for tomato spotted wilt virus, cucumber mosaic virus, and tomato mosaic virus were all negative.

Although numerous lettuce fields have been found to have INSV in recent years and spinach is a host of it when inoculated in the laboratory, Mou said this is the first report of a natural occurrence of the virus on spinach plants in California. “It’s something we need to keep an eye on,” he noted.

The USDA collection of spinach seed was also used for an investigation of Verticillium wilt, which revealed that 45 percent of the 390 accessions in the collection plus five commercial cultivars carried the disease.

Mou and others are collaborating with the curator of the USDA collection in Ames, Iowa, to identify the source of the pathogen. Meanwhile, in a search for resistance to it, Mou has a greenhouse trial with 60 USDA accessions and nine commercial cultivars inoculated with the disease.

In a separate project for the board, Lindsey J. Du Toit, plant pathologist at Washington State University at Mount Vernon, sorted through nine conventional and 11 organic seed treatments to control Verticillium dahliae in spinach.

The series of trials demonstrated the internal nature of spinach seed infection.

Du Toit pointed out that the seed borne nature of Verticillium on spinach means that no symptoms are visible on young spinach leaves for the fresh or processed markets. The symptoms do appear when the plants reach the bolting stage — when the disease affects seed production.

Among her evaluations of seed treatments to remove the pathogen was a hot water and chlorine treatment. “It can be quite effective,” she said, “but the more the seed lot is infected, the harder it is to clean it up.”

She tested several fungicides to reduce the pathogen. The top three conventional treatments are with Topsin M 70 WP, Mertect (with or without Farmore D300) and BAS 595 XG F.

The three leading organic treatments are Seedgard, Seed Support II, and Seed Support I.

“Some treatments limited Verticillium from developing on the outer pericarp, but the fungus was observed on the embryo through the split end of the pericarp, where the radicle emerges, or the funiculus, the site of attachment to the plant. This illustrates the systemic nature of infection of spinach seed from the mother plant,” she said.

She stressed that few of the treatments are currently registered for use on spinach seed in the U.S., although Washington State University recently obtained a Special Local Needs 24c seed treatment registration for Topsin M70 WP.

Thirteen seed treatments showing the best results in her seed health assays are being tested for efficacy at reducing seed transmission and soil infestation by the pathogen.

Steve Koike, Monterey County farm advisor, reported on his investigations into the outbreaks of INSV, which causes stunting with brownish spots and dead areas resembling chemical burns in lettuce.

INSV, which was not known to occur in the crop in California, showed up in lettuce first in September 2006 and in the following two seasons in several Salinas Valley fields. While most fields had less than 5 percent damage, some has more than 50 percent. INSV was formerly associated with ornamental plants in greenhouses.

According to Koike, initially a new strain of INSV was thought to be involved, but evaluations showed it is the same.

Surveys showed it has been prevalent on the coast but it has not been found in Fresno County, where lettuce is infected by tomato spotted wilt virus, which has similar symptoms as INSV.

INSV and the related tospoviruses are acquired by immature thrips, mostly Western flower thrips, which transmit them as adults. INSV has no other insect vector.

For the past two years, Koike has been surveying weeds as a potential reservoir for INSV. Although hairy nightshade, shepherdspurse, London rocket, and annual sowthistle have occasionally tested positive, surveys have failed to confirm a suspect weed or plant reservoir for INSV.