Can common sowthistle in coastal lettuce fields be reduced by simple cultural practices such as minimal bed-shaping? A Salinas-based, University of California weed specialist wants to find out.
In a mid-season report on his work for the California Lettuce Research Board at Seaside recently, Steve Fennimore said since the weed is of the same Compositae family as lettuce, it is difficult to manage with herbicides.
He seeks multiple means to control the weed to use before it sets seed and perpetuates an infestation. In one approach, he learned sowthistle sets seed an average of eight weeks after emergence. Burning nettle, by comparison, sets seed after four weeks.
Sowthistle is shallow germinating, so burying the seed deeper than one inch may prevent emergence. He buried fresh seed packets and will remove and check them for viability after 6, 12, and 24 months in the soil.
Preirrigation of fields to be planted to lettuce in the mid- to late-season is a common practice on the coast for a firm seedbed and may be a way to cut down on the weed.
Fennimore said improved control may be possible by forming a bed at listing, as close as possible to its final contour but without a final shaping pass, after the preirrigation. He said he has observed fewer weed seeds on partially shaped beds rather than those peaked.
“The idea is to knock down the bed as close as possible to final form, even though it's hard to do without some clods,” he said. The final, minimal shaping work creates a firm seedbed and kills some shallow, germinating weed seeds at the same time. It stimulates an early flush of sowthistle that can be handled by shallow tillage, flaming, or a Roundup application.
Pending more research on the cultural approach, Fennimore's project for the board earlier this year on herbicides showed that the combination of Kerb plus Prefar did not eradicate the weed but gave significantly more control than Kerb alone during the first four to six weeks after planting. After six weeks, he added, Prefar's effect on sowthistle diminishes.
About another study due to be completed this fall, Fennimore said, “it looks like Prefar provides early-season control that is better than Kerb alone but does not persist for the full 60 to 70 days of a lettuce cycle.”
The other major part of his project is rating herbicide efficiency on 80-inch beds. His trials in the spring monitored six seed-lines per bed at Arroyo Seco and Gonzales and five lines per bed at Salinas. This year's trials were for mid-season plantings, adding to research from previous studies for later plantings.
At Arroyo Seco, weed pressure was too slight for conclusive data. At Gonzales the predominant weeds were purslane and shepherd's purse, while at the Salinas site nightshade and shepherd's purse were most numerous.
The density of weeds at Gonzales was relatively low and weeds were best managed with a banded treatment. A Kerb application across the bed at the Salinas trial was most efficient, although Fennimore learned that either Kerb or Prefar banded also gave very good control.
“Given the light weed pressures in most central coast vegetable fields, herbicides applied as bands over the rows should manage weeds on most 80-inch lettuce plantings,” he said.
“Of course,” he added, “we still recommend bed-top applications in summer lettuce plantings where common purslane is expected, between April and September in the Salinas Valley.”
Fennimore suggested the board investigate the potential for use on lettuce of DuPont's Muster, registered for canola in Canada. A sulfonylurea herbicide for broadleaf weeds, it has shown good crop safety in trials on broccoli in California and on lettuce in Florida.
Herbicide-tolerant lettuce is also on the horizon, he said, and scientists at the University of Idaho produced germplasm through a classical breeding cross of Lactuca serriola and Lactuca sativa. Patents and other restrictions aside, he pointed out, the advantage to such plants is that they skirt controversies of gene-insertion biotechnology.
In his progress report to the board, Frank Martin, USDA plant pathologist at Salinas, said the cause of bottom rot on lettuce in desert production areas of California and Arizona remains a mystery.
The disease, characterized by brown lesions, first appearing where mid-rib portions of leaves contact the soil and later advancing into the head, occurs elsewhere in North America and in Europe and Japan.
It is associated with species of rhizoctonia and pythium, which tend to make the disease more severe. Either may be prevalent, according to location, and a certain combination of the two appears to be most common to bottom rot of lettuce.
Hits dozen crops
The combination also attacks a dozen crops grown in rotation with lettuce, including cotton, cantaloupe, alfalfa, beans, potatoes, sugar beets, and onions.
“We've been trying to collect samples of the disease from all the production areas around the state to get a better feel for the types of pathogen variation,” he said.
His next step will be looking at how cropping practices and rotations affect incidence of the disease.
Martin is using growth chambers to observe which conditions are more conducive to the pathogens. R. solani, for example, is detected at 23 degrees Centigrade but infection from it becomes greater at 28 degrees C.
Temperature and soil moisture field data loggers have been set out at Huron, Calif., and readings will be correlated to disease incidence.
Another part of his project is finding germplasm having resistance to the pathogens associated with bottom rot. Initial trials showed no type is completely resistant. Iceberg, butterhead, and latin types were highly susceptible to R. solani, but romaine and leaf types showed less disease. Field trials with 36 different cultivars having a range of sensitivity to the disease are being evaluated this fall.
Martin and others are also monitoring fungicide effectiveness on lettuce in the Huron area, using materials registered for lettuce, registered for other crops, or still in the experimental stage.