New study may benefit both seed crop growers and carrot growers

Bacterial blight caused by Xanthomonas hortorum pv. carotae (Xhc) is the most important disease of concern to the carrot seed industry in the Pacific Northwest. Seed contaminated with Xhc can initiate disease in carrot root crops across the U.S. Research conducted by plant pathologists in Oregon and Washington may provide the key to suppressing bacterial blight in carrot seed crops. Dr. Bo Ming Wu, of Oregon State University, is working with OSU Plant Pathologist Rhonda Simmons and Drs. Ken Johnson and Lindsey du Toit of Washington State University to find out how to eradicate the disease from infected seeds without causing unwanted side effects.

The bacterial pathogen spreads mainly by water splash (from rain or sprinkler irrigation) in the field. Simmons, along with Fred Crow, another OSU plant pathologist, demonstrated that seeds from fields irrigated with a drip system have lower bacterial population than seeds from fields irrigated with sprinklers, inspiring many carrot-seed growers in the Pacific Northwest to convert from sprinklers or furrows to drip irrigation. However, this change alone does not provide adequate suppression of the pathogen population. Seeds from fields with drip irrigation systems are often contaminated at higher than acceptable levels, as determined by the International Seed Testing Association. If planted without treatment, these infected seeds may cause a 5 to 10 percent yield loss in carrot root crops.

Treatment options

A treatment with hot water at 126 degrees Fahrenheit for 25 minutes can reduce seed bacterial contamination to acceptable levels. However, hot-water treatment is expensive, due to the time involved and precision equipment and infrastructure needed to treat the seed safely. In addition, it can have the undesirable side effects of reducing seed germination and seedling vigor.

Copper-containing products such as ManKocide (copper hydroxide + mancozeb) are applied regularly to carrot seed crops in the Pacific Northwest in an attempt to prevent contamination of seed by Xhc, but the effect of this treatment on bacterial populations on leaves is generally short term, and its efficacy in reducing bacterial populations on seeds is typically poor to fair.

Other alternative chemical strategies, including foliar applications of Actigard, also have shown limited efficacy. A 2009 study showed that a root drench with Actigard, an inducer of systemic acquired resistance (SAR), offered four months of protection against bacterial canker of citrus, whereas a corresponding foliar treatment provided partial protection (~50 percent) for only a week. A preliminary study by Johnson demonstrated that a root drench with Actigard significantly reduced Xhc population on carrot leaves.

Drip irrigation has become a widely adopted cultural management tool to suppress contamination of carrot seed by Xhc in the semi-arid production areas of central Oregon and central Washington. According to Wu, the widespread use of drip irrigation provides an ideal method to introduce SAR inducers to carrot seed crops. He and his colleagues believe that adding SAR inducers to the drip irrigation flow will protect carrot leaves, flowers and umbels against Xhc, and ultimately reduce the pathogen population on the harvested seeds. Via greenhouse and field experiments, Wu and his colleagues have begun to evaluate this approach as an alternative tactic for management of bacterial blight in carrot seed crops.

The scientists’ intentions:
1. Determine the effects of dose and timing of Actigard drench on populations of Xhc on greenhouse and field-grown carrot seed plants grown from stecklings (vernalized carrot roots).

2. In replicated field trails in central Oregon, evaluate Actigard applied through drip irrigation for suppression of Xhc in carrot seed crops.

Although central Oregon is the most important area for carrot seed production in the U.S., the goal of the study is to produce clean seed for root crops in California and other areas in the world.

Varietal immunity

All commercial varieties are susceptible to bacterial blight, but the rapidity with which the disease spreads varies among varieties. Some varieties, like Blaze (NVH803) and Flame (NVH1005), tolerate Xhc, exhibiting a lower incidence of bacterial blight, lower bacterial population and lower yield loss from the disease. No variety is completely immune.

Whether different varieties of carrot seed respond differently to treatment is unknown. Research on tomatoes shows that different varieties respond differently to Actigard treatments. Theoretically, since Actigard functions by inducing resistance in carrot plants (depending on the defense system of plants), it is reasonable to expect some variations among varieties.

Because Wu has seen little variation in resistance against bacterial blight among carrot varieties, he does not expect to see a huge difference in terms of response to Actigard (and inoculation after treatment). He says this is something he wants to investigate further in the future. For this study, he will use an inbred female line from Central Oregon Seed, Inc.

Next steps

Wu expects to have some results by the end of 2010. If the results from the greenhouse experiments and field trials are convincing, he will seek funding to extend and expand the field trial. “We want to see if different varieties respond differently, and how other factors affect the efficacy of Actigard treatment and optimize the application methods,” reports Wu.

He estimates the second stage may take two to five years. All field trials and most laboratory and greenhouse work will be done at the Central Oregon Agricultural Research Center.

The author is a freelance writer based in Massachusetts and a monthly contributor to Growing.