Brassica cover crops are known to reduce runoff and preserve nitrogen, and now University of California, Davis researchers want to know how these relatives of broccoli and cauliflower might also suppress weeds, nematodes and pathogens in tomato fields.
Brassicas, including numerous mustard species, provide not only biomass to the soil but when incorporated they release chemicals known as glucosinolates, which further break down into isothiocyanates having fumigant properties similar to Vapam.
In a presentation during a recent fresh market tomato field day at LeGrand, Tim Hartz, Cooperative Extension vegetable specialist at UC, Davis, said brassica cover crops are getting attention in California and elsewhere.
“The biofumigation achieved by disking brassica residues into the soil can significantly suppress weeds, nematodes, and soil-borne pathogens,” he said.
The most conspicuous example of commercial success of this approach, Hartz added, has been in the suppression of lettuce drop, a disease caused by Sclerotinia minor, after the incorporation of broccoli residue prior to planting lettuce. Krishna Subbarao, a UC plant pathologist at Salinas, has done several trials with the concept.
In similar fashion, potato growers in the Pacific Northwest report considerable success in reducing Verticillium with winter cover crops of brassicas.
Various trials showed that the practice reduced weeds such as pigweed, lambsquarter, and barnyardgrass; several nematode species; and Fusarium in following crops.
The idea is appealing for the tomato industry, both fresh market and processed, because it suggests a solution to the frequent decline in production after two or three consecutive years on the same ground, particularly in fields under drip irrigation.
In the 2002-2003 trial at UC, Davis, three brassicas, oriental or brown mustard, winter rape, and yellow or white mustard, were planted in a drip-irrigated field in the winter after the 2002 crop was harvested.
These were compared with a conventional cover mix of vetch-pea-oat and fallow plots. The covers were seeded last November and mowed and incorporated in March of this year.
Hartz said each of the brassicas produced more biomass than the conventional combination, the yellow mustard variety (Ishi 20) producing the most, four tons to the acre, dry weight, vs. the 1.73 tons of the vetch-pea-oat combination.
He added, however, the brassicas were very tall and had a tendency to lodge, making it difficult to incorporate them into the bed without leaving considerable material in the furrows. They also flowered relatively early and consequently presented more of a threat to set seed before termination.
The oriental mustard, known as commercially as Pacific Gold, gave the best combination of traits: medium height and biomass production, high glucosinolate level, and late flowering.
Hartz observed the environmental benefits of reduced runoff from winter rains and the amount of sediment, in comparison with bare soil plots. Ishi 20 cut volume of runoff 50 percent and sediment 46 percent. The vetch-pea-oat combination reduced runoff 43 percent and sediment 37 percent.
“The brassica cover,” Hartz said, “was at least the equivalent of the vetch mixture. The brassica cover also removed approximately 100 pounds of nitrogen per acre from the soil profile, nitrogen that would otherwise have been at risk of leaching.”
Halley variety processing tomatoes in the plots will be harvested in September and the process of covers followed by tomatoes will be repeated for the 2003-2004 season. Hartz will monitor the effects on tomato yield and quality and root pathogens in the third consecutive year of tomatoes on the plots.
In another presentation for tomato growers at the LeGrand field day, Bob Mullen, San Joaquin County farm advisor, said garden centipede, also known as garden symphylid or symphylans, showed up with high populations this spring.
Feeds on roots
Preferring moist soils of medium to heavy texture, the centipede feeds on roots of seedlings of tomatoes, beans, lettuce, and other vegetables.
The whitish or yellowish arthropod is a quarter to a half-inch long with 12 pairs of legs. It lives for several years, and once it appears in a field it tends to recur.
Mullen said the centipede struck new tomato fields from Merced County to Southern Yolo County. “Some fields were damaged severely and some were totally knocked out by it,” he said.
Diazanon is the only material registered for the pest on tomatoes, but Mullen and his colleague Benny Fouche said it did not perform well. They tried several unregistered materials, of which Force gave the best control. The product would face at least two years of residue sampling before it could be registered.
The pest has also been a concern on the Central Coast, and Bill Chaney, Monterey County farm advisor, says “management of symphylans has been difficult at best and largely depended on the use of soil insecticides. As the available soil insecticides are diminishing, symphylans is becoming an increasing problem.”
The effect of the materials against it is limited because of its mobility in the soil, although insecticides may help in giving plants a chance to establish in a protected zone. Careful soil tillage and moisture management may help reduce damage, he said.
Symphylans does have natural enemies, including true centipedes, predatory mites, predaceous ground beetles and various fungi, but Chaney said little is known about their effect on populations of the pest.