Subbarao, based at Salinas, has spent a decade studying the fungus, also known as Fusarium oxysporum f. sp. apii, with funding from the California Celery Research Advisory Board. The disease stunts or kills susceptible celery and is managed only by resistant varieties.
The board has also supported resistance-breeding programs at UC, Davis, where UC1 and subsequent improved lines have been developed to combat the two known races of the pathogen. The source of resistance is the celery relative, celeriac. Traits of these lines have been bred into commercial varieties currently in use.
Subbarao’s trials in recent years revealed that where the apii species was combined with three other Fusarium species not previously thought to harm celery, the disease severity on susceptible celery was greater than from apii alone. That data on plant height and disease severity is being incorporated into the breeding program at Davis.
"It was an entirely new thing," he told celery growers and PCAs at a recent meeting in Salinas. "Nothing in the textbooks or research papers said this could happen. So it would be wise to test for other Fusarium species before the release of new varieties."
More severe disease
Fusarium yellows is also a disease of commercial celery in Ohio, and Subbarao, with celery board support, collaborated with scientists at Ohio State University in comparative, greenhouse trials in 2000 and 2001 at Ohio State. They learned the Ohio disease isolates were more severe than those from California.
"This means," Subbarao said, "we need to be breeding new celery cultivars against the Ohio Fusarium population. We don’t know yet if this is a third race of the pathogen, but we do know it is more virulent than the California isolates."
The researchers fear that the Ohio isolates could eventually spread to California, although he added, "So far, we have no evidence of the Ohio isolates showing up in California soils."
Fusarium yellows, identified first by slow growth and yellowed foliage and later by blackening of crown interiors, moves from the soil thorough roots and blocks the infected plant’s vascular system. It was reported in Michigan in 1915 and later in California and other celery-producing states.
The celery industry turned in 1952 to Ferry Morse Seed Co.’s Tall Utah 52-70 resistant variety and more recent, improved varieties.
With the advent of resistant varieties, the disease was rare in California until the mid-1970s when it appeared first in Orange and Ventura counties and later on the Central Coast, where it continues to spread in celery fields.
Soil fumigation reduced severity of the disease in trials, but the practice did not prove to be economically viable.
The celery breeding program at UC, Davis was begun in 1977, and its UC1 line, drawing resistance from a landrace selection of celeriac from Turkey, appeared in 1984.
Once resistance to Fusarium yellows is established in a new line, breeders then seek to add multiple resistances to other disease and insect pests.
At the meeting in Salinas, Steve Koike, Monterey County farm advisor, reviewed possible management choices for other diseases of celery and related crops.
Celery and its relatives are infected with bacterial leaf spot caused by different pathogens. "The one on cilantro does not go to celery, so it pays to know what you are dealing with." The same applies for Septoria. Its two pathogenic species, one for celery and one for cilantro, do not cross-infect.
That is likely not the case for powdery mildew, however. Koike said growers and PCAs should be alert for fungus, even though they may be unaccustomed to finding it on production celery.
"Powdery mildew on celery has not really been an issue for us on the coast, but you should watch for it. We saw its white, powdery growth last year in production fields, even though it’s only been seen before on celery grown for seed. There’s some more research to be done to confirm it, but we are pretty sure it is the same pathogen that goes to parsley and chervil," he said.
Two Sclerotinia species, S. minor and S. sclerotiorum, are not specific and cross-infect celery, fennel, and parsley, as well as lettuce and other crops.
Varieties as key
"Having a resistant or tolerant variety," Koike said, "is the key to a disease-management strategy. With one you don’t have to worry about chemicals and other factors. But we don’t have that option for many crops related to celery.
"You just don’t find resistant varieties for minor crops like chervil or fennel. There’s very little incentive for seed companies to develop them because of the lack of return on such a small acreage."
Koike said it’s important to watch for the source and quality of seed and to know whether the pathogen is on the seed. "You see the term ‘pathogen-free’ on a seed label, but we don’t take that seriously. There’s no such thing as a seed lot that is actually free of the organism."
He explained that samples of given seed lots are tested, and if they meet a certain detection threshold those lots are sold. Even if they are cleared for sale, the seed may still contain low levels of a pathogen.
Site selection for celery is particularly important because reservoirs of pathogens accumulate. An example is asters yellows. It is not seed borne like other pathogens but occurs on weeds and other hosts where leafhoppers can carry it to celery.
Koike recommended considering different methods of irrigation. For example, significant control of celery late blight, which depends on splashing water for transmission, has been achieved where sprinklers were replaced by drip irrigation.
Fungicides are the only defense for some foliar blights, and Koike said he and others are screening several new, environmentally-friendly products in hopes of strengthening the existing arsenal.