Integrated Clubroot Control Strategies for PNW Brassica Producers

View the project final report

• 2016 annual report

Commodities

• Vegetables: broccoli, brussel sprouts, cabbages, cauliflower, rutabagas, turnips

Practices

• Crop Production: crop improvement and selection, crop rotation, food product quality/safety, nutrient management, organic fertilizers, tissue analysis, varieties and cultivars, water management
• Education and Training: decision support system, extension, farmer to farmer, networking, on-farm/ranch research, participatory research, workshop
• Farm Business Management: risk management
• Pest Management: chemical control, cultural control, economic threshold, genetic resistance, integrated pest management, sanitation, trap crops
• Production Systems: agroecosystems, holistic management, integrated crop and livestock systems, organic agriculture
• Soil Management: soil analysis, soil chemistry, soil quality/health
• Sustainable Communities: local and regional food systems, social networks, sustainability measures, urban agriculture

Clubroot is a major disease of brassica crops; it causes significant crop losses to Pacific Northwest farmers. The overall goal of this project was to provide farmers, university Extension faculty, and agricultural professionals with the information and resources necessary to implement an effective and economically sustainable integrated clubroot management program.

A major component of the project was identifying commercially available cultivars combining clubroot resistance with desired horticultural characteristics. Using the European Clubroot Differential (ECD) set, only the pathotype 16/2/30 was identified in western Oregon. Through collaborative research, our farmer-scientist team screened twenty-one vegetable cultivars from nine crops (broccoli, cabbage, napa cabbage, etc.) with purported resistance to clubroot for disease incidence and severity in field and greenhouse studies to pathotype 16/2/30. Compared to a crop-specific susceptible check, 17 of 21 cultivars had some resistance to clubroot, and of those, 15 were highly resistant (≤15% incidence with low disease severity). Of the resistant cultivars identified, farmers provided input about their suitability to grower farming systems and whether these cultivars would meet market demands and consumer expectations.

This project focused on soil liming as a principle clubroot management strategy because it is a practice that farmers can easily implement, it is economical, and it has shown to consistently and effectively control clubroot when done correctly. Here are the key findings from multiple collaborative, on-farm research projects:

• Liming did not completely suppress clubroot in the field, but it can significantly reduce infection rate and disease severity when done correctly.
• Liming had little or no effect on incidence and disease severity when a minimum target pH of 7.0 was not reached at planting,
• Soil’s pH response to lime is non-linear above a pH of approximately 6.5. As a result, routine liming recommendations for crop production are not adequate to reach a target at or above pH 7.0. Routine recommendations rarely address pH targets more than 6.5.
• Lime should be applied and incorporated at least one month prior to planting; more time gives better results.
• Thorough lime incorporation is critical to achieve a uniform soil pH, which eliminates localized pH zones below 7.0, where infection can occur. 

The results and knowledge gained from these collaborative research projects have been incorporated into a journal article and a peer-reviewed, catalogued Extension publication that is unique from any other Extension clubroot publication currently available in the US. These results have been communicated to clientele through presentations, informational booths, a nationally broadcast eXtension webinar, and electronic media. We have documented that farmers have adopted the clubroot control recommendations generated from this project.

The goal of this project is to provide farmers with the information and resources necessary to implement an effective and economical integrated clubroot management program. The farmer-scientist team will conduct collaborative research and Extension activities:

Objective 1: Conduct on-farm research investigating cultivar resistance to clubroot, and the effectiveness of cultural practices for clubroot suppression. 

Performance Target:  Based on the findings of variety screening, cultural control, and farm rotation studies, we will develop integrated recommendations for clubroot disease management.  

Objective 2: Provide Extension outreach to growers to assist them in developing an integrated approach for clubroot disease control. 

Performance Target:  During the final year of the project, we will assess changes in farmer understanding, intentions, and practices. Farmers will be asked to identify specific practices they have adopted or intend to adopt as the result of this project. For each strategy, farmers will describe 1) whether or not they adopted it or if they intend to adopt it, 2) if it appears to be effective (if they adopted it), and 3) if it is cost effective. Farmers will also be asked how much damage and lost income they experienced due to clubroot before the project, and how much damage and lost income they anticipate after adopting project findings.

Introduction

Clubroot (causal organism, Plasmodiophora brassicae) is a major disease of brassica crops (broccoli, cabbage, cauliflower, Chinese cabbage, rutabaga, etc.), and causes significant crop losses worldwide. Brassica farmers in the PNW are increasingly concerned about clubroot. Extension personnel, agricultural professionals, and farmers have indicated that the incidence and severity of the disease is increasing. This may be due to

• increased acreage producing forage radish and turnip cover crop seed to meet Midwest demand;
• the increasing number of vegetable farmers with a 15-year or more history of intensive, short-rotation production;
• an increase in the proportion of land planted to brassicas on vegetable farms due to increased demand; and
• increasing acreage planted to overwintering brassica crops as winter markets increase.

Managing clubroot is a challenge. Thick-walled resting spores can remain viable in soil for 15 years or more in the absence of a host, making it very difficult to eliminate the pathogen from an infested field. Therefore, the goal is to manage rather than eradicate the disease once pathogen populations have reached levels that cause economic damage. Rotating out of brassicas for a minimum of five years will often reduce disease severity enough to avoid significant crop loss. However, many smaller, diversified vegetable farms lack sufficient acreage to rotate out of brassicas for this period; nor do they have alternative rotational crops as profitable as brassicas. Although some farmers can lease clubroot-free ground, this may be impossible for organic growers due to a limited supply of organically certified fields. To remain competitive and profitable, they must actively manage clubroot.

A significant body of research has explored strategies to minimize crop loss from clubroot. This work has been compiled into multiple review papers (e.g., Hwang et al., 2014; Donald and Porter, 2009). Some of the management strategies reviewed include the use of bait crops to stimulate spore germination, boron applications, use of biological control agents, production of resistant cultivars, water management to avoid wet areas in a field, and liming to increase soil pH. Many of these strategies are not consistently effective, do not provide sufficient disease suppression, require changes to farming practices, may not be compatible with a farm’s production system, may be prohibitively expensive, or are not allowed in organic systems.

Of the management strategies mentioned, production of resistant cultivars is an attractive option that requires little or no change in farming practices or equipment, gives the farmer flexibility in when and where to plant, and may be significantly less expensive than other options. Besides the in-season benefit of growing resistant cultivars, there is a long-term benefit. Resistant cultivars may act as a bait crop by stimulating resting spore germination, but few or no viable spores are produced. This can decrease spore concentration and ultimately disease incidence and severity in future crops (Murakami et al., 2000 and 2001; Hwuang et al., 2011b). New cultivars have become commercially available in the last decade, but they have not been rigorously screened for resistance to the dominant pathotypes present in the Pacific Northwest. There are many clubroot pathotypes worldwide; some cultivars with putative resistance may not be effective in our region.

Growing resistant cultivars alone will not control clubroot. There are a limited number of cultivars, many of which may not meet the needs of farmers in our region or consumer preferences. In addition, there are no cultivars available for some high-demand crops such as kale (B. oleracea var. acephala), mustard greens (B. juncea), or arugula (Eruca sativa). Too-frequent production of a resistant cultivar in the same field may select for a clubroot population able to overcome resistance.

Another practical, effective, and economical control strategy is liming to raise the soil pH to 7.0 or greater. This approach has been so effective at controlling clubroot in areas such as the Salinas Valley of California, a major brassica production area that the plant pathologist for the region no longer works on the disease (Steve Koike, UC Cooperative Extension plant pathologist, personal communication). Liming does not kill the pathogen but reduces spore germination, thus reducing infection and clubbing (Dixon, 2009). By reducing clubbing, fewer spores are released into the soil. Future infection rates and severity are reduced.

Although liming has been an effective strategy in California, farmers in western Oregon have had mixed success. In 2012, a group of 37 conventional and organic fresh market and processing vegetable farmers in western Oregon who grow significant quantities of brassicas responded to a survey. Eighty-three percent had used lime in an attempt to control clubroot, yet only 21% of these aimed for a pH of at least 6.8, the minimum pH level shown to control the disease. Of those who had used lime, 38% did not determine whether the target pH was reached. Only 26% indicated that liming contributed to clubroot control. These survey results indicate that while there is a general acknowledgement that pH manipulation can control clubroot, Extension faculty, agricultural professionals, and farmers lack the specific information necessary to implement a successful liming program.