When a joint public-private partnership launched the Yuma Center of Excellence for Desert Agriculture (YCEDA) 2.5 years ago, collaborating growers and the University of Arizona (UA) decided it’s No. 1 priority was to identify more management techniques for Fusarium wilt disease of lettuce (FWOL).

FWOL is a soil borne pathogen-caused disease which can render crisphead (iceberg) and romaine lettuces unmarketable. According to one estimate, Arizona lettuce growers lose about $9 million a year in Fusarium wilt-related losses, yet these losses are not as high as other soil borne diseases.

“The difference is that other soil borne diseases are treatable. Fusarium wilt of lettuce is not,” says YCEDA Director Paul Brierley, based at the Yuma Agricultural Center at Yuma, Ariz.

With more and more acreage infected with the FWOL pathogen - Fusarium oxysporum f.sp. lactucae, growers are increasingly losing pathogen-free acres to grow these core vegetables for consumers.

Brierley says, “Fusarium wilt is boxing growers into a corner, keeping them from meeting market demand at certain times of the year.”

In California, FWOL is found primarily in Fresno County, and also in coastal Monterey and Santa Cruz counties, plus southeastern Imperial County. It’s also found in Arizona’s vegetable-growing areas including Yuma County.

FWOL = unmarketable lettuce

Fusarium wilt causes infected plants to wilt, never form a head, or possibly die. An infected plant can simply collapse as if someone stomped it to the ground killing it.

The main disease symptoms before collapse can include yellowing (chlorosis) of one or more leaves, and a reddish-brown discoloration in the cortex of the crown and upper root.

“When the infected tissue becomes necrotic the lettuce plant is severely compromised and there will be little to no future growth from that plant,” says Barry Pryor, one of two UA plant pathologists working diligently with YCEDA on the FWOL issue in winter vegetable fields in low desert areas. This leaves the head commercially unmarketable.

Pryor and fellow UA pathologist Mike Matheron are the chief scientists looking for improved FWOL management techniques, and perhaps one day a solution to the disease. Research has been underway for years yet swung into high gear under the YCEDA.

A global problem

Globally, the disease pathogen was first found in Japan in the mid-1950s. It was first identified in the U.S. in California in 1993, followed by Arizona in 2001. Since then it’s been found in Italy, Portugal, Brazil, and Argentina with the most recent find in the Netherlands.

“It’s marching around the world,” Pryor noted.

FWOL can have three races - 1, 2, and 3. All three are found in Japan yet only Race 1 is found in the states.

Fusarium wilt of lettuce is different from Verticillium wilt of lettuce found around Salinas, Calif. as Verticillium infection causes a black color taproot. Management techniques for these wilt diseases are different.

Examine the tap root

While some believe FWOL can be diagnosed from a pickup while driving by a field, Pryor says the disease can be easily mistaken for other lettuce soil borne diseases caused by Pythium, Sclerotinia, Thielaviopsis, and Verticillium.

“You really need to get out of the truck, walk over and pick up the plant, cut open the tap root, and look for the vascular discoloration to separate Fusarium and Verticillium from other soil borne pathogens.”

Integrated management

Since there is no single silver bullet cure for FWOL yet, Matheron and Pryor agree that an integrated approach provides the best disease management.

These strategies include:

· Using pathogen-free seed;

· Knowing the history of the field;

· Planting the most resistant varieties available;

· Avoid planting the most susceptible varieties in warm planting windows;

· Remembering that soil inoculum can increase on all varieties and in other non-lettuce crops;

· Broccoli and cauliflower are good rotation crops but more research is needed;

· Inoculum can be moved with the soil on equipment; and

· Use good field sanitation practices.

“We are putting together a program using a lot of different strategies and tactics to successfully address this problem,” Pryor and Matheron said during a YCEDA-sponsored FWOL workshop held in April to a packed room of vegetable growers and pest control advisors.

Matheron has the most ‘boots on the ground’ experience on the FWOL issue, having spent several decades studying the disease.

Cultivars, crop protection trials

Over the last two years, the pathologists have conducted grower field trials, analyzing improved cultivars and evaluating crop protection materials to reduce the incidence and severity of Fusarium wilt.

On the cultivar side, grower field trials conducted in 2015 and 2016 determined that current romaine lettuce varieties on the market are generally more resistant than their sister crisphead varieties. In romaine, the Del Sol and Duquesne varieties scored higher in FWOL resistance while the Oracle and Meridian cultivars scored higher resistance in crisphead.

Matheron and Brierley have received specialty block grant funding for expanded field trials to evaluate the latest cultivars from seed companies this fall and next spring, plus crop protection products and cultural practices. Pryor is working with YCEDA on DNA and spectral methods for early FWOL detection.

On field trials weighing the effectiveness of current crop protection products on the disease, Matheron noted, “We really haven’t found a single product that is a stand out stellar performer. Perhaps we can improve on efficacy by finding a better product delivery method.”

Soil solarization

For growers, the management of soil borne inoculum is critical, including a step called soil solarization, in other words placing clear plastic over field rows in July and August.

Matheron says this process can increase the soil temperature to 150-155 degrees Fahrenheit; an effective kill step against pathogens. The pathologist noted four solarization field trials yielded pathogen kill rates ranging from 42 percent to 98 percent.

Matheron wants to test solarization with a drip-irrigation system, supplying moisture to the soil as the solarization process occurs.

Drying fallowed fields to reduce soil inoculum levels can also be effective, according to field trial results from University of California plant pathologist Thomas Gordon, shared by Matheron.

Soil health and other variables are also being studied to mitigate FWOL by avoiding disease triggers.

Talk to the specialists

For more information on fusarium wilt of lettuce, contact Mike Matheron - Matheron@ag.arizona.edu and (928) 941-2094; Barry Pryor - bmpryor@email.arizona.edu and (520) 626-5312; Thomas Gordon - trgordon@ucdavis.edu and (530) 752-4269; and Paul Brierley - PaulBrierley@email.arizona.edu and (928) 782-5873.