Field trials in 2005 by the University of Arizona (UA) showed an 81 percent reduction in the effects of fusarium wilt, Fusarium oxysporum, in lettuce by utilizing soil solarization, according to Mike Matheron, UA extension plant pathologist and plant sciences professor.

“For long-term control, soil solarization is something we need to pursue and may be quite valuable,” Matheron said. Soil flooding also shows promising results but solarization was the most effective of the two-tested UA methods.

The first symptoms of fusarium wilt can occur as early as thinning when some seedlings wilt and die. Matheron said infected plants display a characteristic red-brown streak extending from the upper taproot into the cortex of the crown. Yellowing of leaves and a brown to black streaking of the foliar vascular tissue is often present. Infected plants may be stunted or fail to form a head. The cortex of the crown turns a reddish-brown in color, and vascular darkening extends into the root tissue on the affected side of the plant. The only way for Yuma lettuce growers to learn if fusarium wilt is the culprit, said Matheson, is to bring the plant to the extension office for a culture.

Soil solarization involves covering soil with clear plastic for several months during a hot time of the year when the soil receives maximum direct sunlight. The top six inches of soil can heat up to as high as 125 degrees – hot enough to kill a wide range of soil inhabiting pests.

“When the disease was first seen in the Yuma Valley in 2001, plants were brought in from October through December and were almost always head lettuce. This suggested that the problems might be the planting date and type of lettuce,” said Matheron.

Matheron’s plant trials in 2002 and 2003 involved micro plots in five gallon buckets containing soil with the known pathogen. The trials included a control with nothing to reduce the wilt’s impact - while others utilized soil flooding and soil solarization. Eight different cultivars of head lettuce were planted in September, mid-October and December. The results showed early plantings had the most disease, mid-October plantings had less disease and December even less yet. There was also a link to soil temperature. The mid-80s was the optimal temp for the disease. In the plantings, crisp lettuce experienced the worse damage and Romaine was the lowest.

In 2005, the UA conducted a field trial. Plastic was laid for 60 days. Non-solarized areas showed many skips while the solarized lettuce looked healthy. At crop maturity, unsolarized areas featured fewer marketable heads while head growth in solarized areas was quite good.

For the short-term management of fusarium wilt in lettuce, Matheron said the disease can be managed by avoiding the early planting of head lettuce in infested fields. Romaine does better with the wilt in early planting.

Long-term control strategies include resistant cultivars. “We don’t have crisp head cultivars now,” he noted. “The seed companies are working to develop these and hopefully we’ll have them in the future.”

Fusarium wilt first appeared in U.S. lettuce in Huron, Calif. in 1995. The disease was found in Yuma County, Ariz. lettuce fields in 2001. While few fields currently have fusarium wilt, Matheron expects more to be found this year. One of the wilt’s spores, he said, allows the fungus to remain in the soil for an estimated 10 to 15 years.