Research during 2003 and 2004 sorted out promising elements for management of the white rot that threatens garlic and onions on the West Side of the San Joaquin Valley.

Shannon Mueller, Fresno County farm advisor, concluded after the trials that “a multi-layered approach” will be necessary to manage the fungal disease, Sclerotium cepivorum, which attacks only alliums and has been on the increase for the past several years.

Those layers thus far include use of a biostimulant, fungicides via treated seed and in-furrow applications, and field sanitation. Various combinations are being evaluated.

Mueller said the biostimulant is simply garlic powder, which carries diallele disulfide or “DADS.” Applied to an infected field before planting, the compound's characteristic odor mimics that emitted by roots of growing garlic or onions and falsely triggers germination of white rot sclerotia in the soil before a new crop is planted. Without a host, most of them die off.

“Earlier trials have shown that dads will knock down a white rot population by 85 percent to 90 percent, similar to a methyl bromide treatment,” she told growers and PCAs during a recent vegetable production meeting at Five Points.

However, she added, once a new crop is planted, the surviving poppy-seed sized sclerotia are potent enough to cause significant disease pressure.

Lack residual

Currently registered fungicides applied at planting in the fall do not have adequate residual activity by the time the fruiting bodies activate in crop hosts in the spring. Sclerotia remain viable in the soil for decades, even without a host.

That's why she and collaborators Kurt Hembree, Fresno County farm advisor, and Mike Davis, plant pathologist at the University of California, Davis, began long-term trials with fungicides on garlic in a commercial field infected with white rot for several years. The field has been dedicated for the project.

Mueller said the highest yields, in relation to the untreated check, were with either a seed or preplant soil applications of Folicur, Switch, or Botran. She saw no yield benefit from foliar applications of Switch, Pristine, or Botran.

Rovral, the standard in-furrow fungicide treatment, was better than the untreated check but was less effective than other materials.

Mueller said a foliar application of Pristine, inadvertently made at an excessive rate in the spring, surprisingly turned out to perform on a par with soil treatments with Switch and Botran.

Development of the disease is favored by cool, moist soil conditions typical during the spring in the SJV, she said. One advantage for SJV growers is the disease is inhibited when temperatures rise above 78 degrees.

Treatments start

Foliar treatments began as soon as soil temperatures reached the optimum for infection. Sclerotia germinate at between 59 and 64 degrees, and the range for infection is 50 to 75 degrees, with soil optimum soil temperatures at between 60 to 65 degrees.

The project continues this season with additional fungicides for seed, in-furrow, and foliar treatments, along with applications through drip irrigation. Flooding, soil solarization, deep cultivation, and metam sodium applications also will be investigated.

“We will take the most promising materials and application techniques and start to layer them in an approach to developing a strategy for white rot,” Mueller said.

Meanwhile, she stressed, field sanitation is vital to contain the spread of the pathogen that has already infected 70 SJV fields, totaling more than 10,000 acres.

“Any movement of people or equipment from infected fields can carry the disease. That's pretty obvious when you have onions or garlic in a field, but it is equally important when rotational crops are in the field. The sclerotia are still there and can be easily moved into clean areas.”

TSWV discussed

Another speaker, Scott Stoddard, Merced County farm advisor, described tomato spotted wilt virus (TSWV), a disease that has tomato growers in the SJV and Southern California concerned because no control program is known to be successful.

“The virus has an extremely wide host range and infects more than 800 species, including corn, tomatoes, beans, and weeds such as pigweed, purslane, and nightshade. A lot of times they are symptomless hosts,” he said.

Vectored by adult western flower thrips and able to replicate in the gut of the insects as well as in the host plant, TSWV shows as bronzed leaves and stunted plants that produce deformed fruit with yellow blotches and rings. It reduces yield in processing tomatoes and can make fresh tomatoes unmarketable.

“Control of this virus is very difficult because of its wide host range and that of the thrips,” Stoddard said. “And there are no UC recommendations for managing it on tomatoes, although some success has been reported anecdotally with sprays of Monitor, Warrior, or Success, along with removal of infected plants.”

Control must be early in the season since research in Georgia and Florida has shown that the virus can reduce production by 2.3 percent for each day prior to harvest.

Possible controls

Controls that might work are manipulating planting dates, increasing plant density, multiple insecticide applications, reflective mulches, resistant varieties when they are identified, separation of susceptible crops, or trap crops for the thrips.

Stoddard said his own research with reflective mulches, particularly silver colored, showed some effect in curbing WFS and wilt infection.

“Whatever you do, do it right away after you've determined you have spotted wilt virus. Infected plants are sources of the virus that will spread and spread,” he said.