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Research explores cover crop role in reducing tulip-bulb weeds, diseases

Posted by CAHNRS.Web Team | September 26, 2016
Carol Miles
Ph.D. student Yushan “Sherry” Duan (pictured here with emerging tulips from a WSU Mount Vernon greenhouse trial) is completing a study this year on how cover crops impact weed development and bulb diseases in tulip plantings. (Photo by Kim Binczewski)

As WSU weed scientist Tim Miller puts it, tulips “go hand in hand with weeds” this time of year in the Skagit Valley, where the annual Tulip Festival during the month of April attracts hundreds of thousands of visitors from around the world to this small farming community approximately one hour’s drive north of Seattle.

Yet weeds are only half the battle for the commercial and small-scale growers here who hope to reap a share of the profits from the state’s $12-million-per-year ornamental bulb industry during the short growing and harvest season for specialty cut-flower and bulb crops.

The rest of the fight is underground, where soil-borne diseases like tulip fire (caused by the fungus Botrytis tulipae) and tulip-gray bulb rot (caused by the fungus Rhizoctonia tuliparum) can spread through the soil and infect the bulbs of their host plants, resulting in significant losses in flowers and bulb yield.

Promising tool for growers

Northwest growers may someday have an added weapon in their agricultural arsenal against tulip bulb weeds and diseases, as a result of cover crop research being conducted by Miller’s WSU Mount Vernon weed science research team, led byPh.D. student Yushan “Sherry” Duan.

Duan was enlisted in 2012 to study the impacts of plow-down and cover crops on field production of tulip — one of 25 research projects funded under the Farm Bill through Washington State Department of Agriculture Specialty Crop Block Grants totaling $3 million.

“Ornamental bulb farmers are limited to fumigation and a small assortment of highly regulated fungicides and pesticides, which can be costly to their bottom line if multiple applications are needed each year,” said Duan. “For the small-scale growers, fumigation is not an option due to restrictions on many of the chemicals that were previously available for agricultural purposes.”

“Our research has been twofold,” Miller said. “First, we wanted to see whether cover crops or green manure plow-down crops can reduce competition from weeds and soil-borne pathogens, and perhaps reduce the need for pesticide applications. Second, we wanted to find out whether growing these crops immediately before tulips negatively affects flower or bulb production by creating field conditions that are unfavorable for tulip.”

Cut-flower tulip grower Roger Knutson helped Duan establish her half-acre plots on his land near Sumner, Washington, where she planted two cover crops – a wheat-and-pea mixture, and a two-variety blend of mustard. Cover crops were planted in mid-summer, grown for four weeks, then plowed into the soil about a month prior to planting tulip bulbs. “We planted the cover crops, and he planted the tulips,” Duan said. “It was a real team effort.”

Potential for crop rotation

In the experiments on Knutson’s farm, half the cover crops were killed with glyphosate, a systemic herbicide, prior to incorporating them into the soil. The other half were not treated.

A concern regarding cover crops is that they can persist as weeds in the following crop, leaving what the researchers refer to as a “trashy seedbed” prior to bulb planting.  “After the first year, we switched to a cereal rye-pea mix, because we found the winter wheat we planted in late summer was quite weedy in the tulip crop the following spring,” Duan said.

In this third year of Duan’s research, results have been surprising and encouraging.

“On the one hand, we didn’t see much difference in bulb yield or flower quality with any of the cover crops on the commercial grower’s field, which are treated with herbicides and fungicides and so do not incur much weed or disease pressure,” Duan said.  “On the other hand, these cover crops didn’t hurt bulb yield or flower quality, so it appears they should fit nicely into a tulip rotation.”

Pre-empting disease

Miller and Duan are also conducting companion trials in tulip at the WSU Mount Vernon research station.  In a controlled experiment, Duan is infesting soil with the fungi that cause tulip fire and tulip-gray bulb rot prior to planting cover crops to determine if the cover crops can help reduce the incidence of these diseases in the tulip crop.  She is working closely with WSU Puyallup Plant Pathologist Gary Chastagner.

“We planted cover crops either in July or in August to see how planting date affects cover crop growth or their ability to control soil-borne diseases,” said Duan.  “We are also evaluating how quickly cover crops break down in the soil before planting tulip. Cover crop residue left on the soil surface should help reduce weed seed germination in the tulip crop.”

In the first round of testing, the level of tulip fire was cut by growing a rye-pea mix in August before planting tulip bulbs in October. If second-year results are similar, Duan may have identified a strategy producers can use to reduce the risk of tulip fire in field-grown tulips.

“It is encouraging that we are seeing a reduction in Botrytis in the rye-pea cover crop, because tulip fire is host-specific to tulips,” Duan said. “The break cover crops provide may be enough to get rid of the disease in the soil. If we can manage this disease with cover crops, then tulip growers could potentially continue to plant the same crop each year — and not have to alternate between tulips and daffodils.”

Unexpected results

Both researchers thought they would see a reduction in the fungi which cause tulip fire and tulip-gray bulb rot in the soil plots into which mustard cover crops were tilled prior to tulip planting, because of mustard’s ability to suppress soil-borne diseases.

“We expected the mustard to eliminate soil-borne diseases, but we haven’t seen that in our trials,” Duan said. “We’re still waiting on soil-test results regarding soil microbial activity, so the jury’s still out as to why that might be the case.”

This June marks Duan’s final harvest season with this cover crop project, as she expects to complete her dissertation with three years’ of field data, two years’ of station research, and several greenhouse trials.

We hope to gain good information on how effective cover crops will be in potentially reducing weed and disease pressure in our Pacific Northwest tulip crops,” Miller said.  “Ultimately, the knowledge gained may lead to reductions in the quantity of herbicides and fungicides that are needed to produce the high-quality cut flowers and bulbs for which the region is famous.”