by Katie Navarra
Brassica crops are susceptible to Clubroot, a soil borne disease scientifically known as Plasmodiophora. The disease affects crops worldwide and can cause complete crop failure. Two Oregon State University experts, Alex Stone, a vegetable cropping system specialist, and Aaron Heinrich, faculty research assistant, are studying the lifecycle of the disease while exploring strategies for control.
Broccoli, cabbage, cauliflower, turnip, rutabaga and kale are the most widely recognized brassica crops, but all brassica including arugula, mustard greens, radish and others are susceptible to the disease. In western Oregon, the incidence and severity of the disease has been increasing on organic farms. One mid-acreage farmer in Oregon told Stone and Heinrich that in three years (2009-2012) they sustained 25 percent loss of brassica crops due to Clubroot and that resulted in a $60,000 to $80,000 loss. Not only has the disease inflicted significant loss, the farm is also running out of land that is not infected.
During a webinar hosted by eOrganic, Stone and Heinrich share what they have learned about the disease thus far and offer long-term strategies for minimizing the incidence of Clubroot on any farm.
Determining if a crop or a field is inflicted with Clubroot is a two-step process. In an established crop, above ground symptoms include stunted plant growth and yellowing leaves. “The plants look like they are wilting even though they have ample water,” Stone said.
In order to determine that Clubroot is the cause, the plant roots must be dug up. This is the only disease that causes severe “clubbing” or deformed nodules to grow on a plant’s root system.
“It’s important to know what healthy roots and severely and mildly infected roots look like so you can better diagnose,” she said.
Knowing how the Plasmodiophora pathogen cycle functions is critical to control. Stone explains that the clubs on the end of the roots are filled with thousands of resting spores. When the plant disintegrates or is mulched into the soil, the resting spores are released into the soil. “The resting spore can survive in the soil without a host (roots) for a long time, as many as three to eight years,” she said.
High soil moisture content at the time of germination or transplanting contributes to the pathogen’s spread. Stone says that spores swim through high water levels and attacks the young plant’s root system.
Currently, organic farmers can’t kill the pathogen, so diligent, integrated management strategies are the best option for limiting the spread. Stone and Heinrich offer several management methods that can help reduce the incidence of Clubroot on your farm.
Prevention is the number one best method for control. “Soil movement transfers resting spores from infected to uninfected fields,” Stone said.
Tractors, equipment, storage containers, a farmer’s boots and livestock all contribute to the soil movement. Sanitation can limit contamination for these sources. Removing soil from tires and power washing with a 1 percent bleach solution can reduce soil transfer.
Soil can also move with water, specifically during flooding. “You can’t control floodwater, but you have to be aware of it,” she said.
In Canada, canola crops are susceptible to Clubroot and Stone says the website www.canadawatch.org provides a lot of good information relating to prevention and sanitation.
Scouting and record keeping are the second line of defense against Clubroot. “Map where it has occurred in the field, the crop and the year,” she said. “Scout the wet areas first because that’s where you’ll most likely find clubs and be sure to scout all fields, even those without above ground symptoms.”
This type of data allows you to manage rotation to avoid replanting brassica in an infected field and minimizes the spread of infested soil to clean fields.
Since high soil moisture content contributes to the spread of Clubroot, knowing where wet spots exist in the field means you can fine tune irrigation to avoid oversaturation. Adding organic residues, reducing tillage and alleviating competition improve soil infiltration rates.
“Soil has a given ability to move water out of the root zone. Improve the soil’s physical properties to increase infiltration,” she suggested.
Crop rotation is critical to mitigating Clubroot in brassicas. “A five to six year crop rotation should reduce crop failure due to Clubroot,” Stone said.
But, she cautions the rotation planning must account for all types of brassica plants. “A lot of farmers only think of large acre brassica crops such as broccoli, kale, cabbage and cauliflower. Arugula, mustard greens, radish and leafy brassica greens are all part of the same family,” she said.
Stone recommends the book, Crop Rotation on Organic Farms, by Charles L. Mohler and Sue Ellen Johnson, as a guide. The book highlights the rotation of four crop families in four years and calls for two years soil building. That means for every year the soil is planted in brassica crops, it has five years of crops from diverse plant families.
“If you have the luxury of land for this rotation you’re likely not to have crop loss,” she said.
Stone is hesitant to recommend a four-year rotation — one year in brassica crops — three years out, but has observed one farm able to make this work because of a commitment to a diligent and completely integrated management approach.
Liming and resistant varieties
Reports dating back to the 1750s in England show that liming has been an effective control for Clubroot. “It doesn’t kill the pathogen. It inhibits spore germination,” Heinrich explained.
To be effective, the soil pH must be at least a 6.8 to control the disease. However, liming for Clubroot is not equivalent to liming for crop production. Crops can tolerate a wide range of soil pH and an incomplete incorporation of lime and still be productive. That’s not the case with Clubroot, infections can occur in low pH microsites that are the result of incomplete mixing.
Heinrich recommends using the 7 Step Clubroot Liming program, Extension Bulleting EM9148. The bulletin is a thorough guide that provides tips for how to effectively use lime to manage this pathogen and includes guidelines for determining application rates.
Clubroot resistant cultivars are another piece of a larger integrated management strategy. “It is one of the best and most economical methods for control,” he said.
Choosing resistant cultivars requires little or no changes to the current farming system and offer the most flexibility for where and when to plant brassicas. In the long run in may even reduce disease pressure. These varieties are not a one-size-fits-all approach.
“The resistance may be pathogen specific and there are few available options,” Heinrich said.
For example, there are no current options for kale or arugula.
“There’s also the risk that if they are used too frequently, that may result in loss of resistance,” he said.
The cultivar suitability also depends on the farm and the end use. For example, the variety may not be suitable for the processing industry. One farmer reported the core in the cabbage variety created too much waste for processing. On the other hand, the lack of uniformity in the crop at maturity may pose issues for the fresh market outlets. And, the days to maturity may be too long. It’s one of the easiest strategies, but may be limiting.
In summary, Stone and Heinrich believe that no one strategy will cure any farm’s Clubroot infection. “You must have an integrated approach to be effective,” they agreed.
To listen to the complete webinar visit http://articles.extension.org .
Integrated management strategies for Clubroot in brassica crops
by Katie Navarra