Clubroot and Aster Yellows

In this past growing season, canola growers were hearing lots of discussion about Clubroot and Aster Yellows. It is important that producers take the time to learn about these two diseases as each is different in their symptomology and in their economic impact.

Here’s a summary of the facts we know today.

Clubroot

Clubroot has found canola to be a hospitable host; however, it isn’t a new disease. It’s been around for centuries and was first identified in Europe in the thirteenth century. It’s a serious soil-borne disease of cruciferous crops and is a major pest in cole crops such as cabbage, broccoli, cauliflower and others in areas of British Columbia, Ontario, Quebec and the Atlantic provinces.

Canola is a member of the same plant family as cole vegetables. There have been two previous reports of clubroot in cole crops in Alberta prior to 2007. For the first time, though, 2007 saw clubroot infecting canola fields near Edmonton. Since then, it has spread into nearby counties around, including Sturgeon, Parkland, Leduc and Strathcona.

Clubroot was added as a declared pest to Alberta’s Agricultural Pests Act in April 2007. That means municipal pest inspectors have the power to enter land without consent to inspect for clubroot and to collect samples. If clubroot is found, it’s up to the land owner or occupant to take measures to prevent its establishment and spread.

The causal organism doesn’t fit neatly into normal disease classifications. While it used to be called a slime mould fungus, it is now considered a protist which is an organism with plant, animal and fungal characteristics. Resting spores can survive in soils for up to 20 years. Their longevity is a key factor in the seriousness of this disease.

The disease is not airborne, but resting spores are capable of moving with soil transported by wind, water and machinery. This is important for farmers but even more important, to limit the spread of the disease, custom operators and oil and gas construction operators must be diligent in sanitizing their equipment. They travel hundreds of miles and could become the major transporter of infections.

Symptoms

Symptoms vary depending on what stage of crop growth infection occurs. Early infection results in wilting, stunting and yellowing of canola plants in the late rosette to early podding stages. These symptoms are easily mistaken for heat stress or other diseases such as blackleg or Fusarium wilt. To properly diagnose, dig up wilted plants and check for gall formation on roots. Galls are a nutrient sink that ties up nutrients and prevents the transport of both water and nutrients to above-ground plant parts, essentially starving the crop.

At later stages, infected plants will ripen prematurely and seeds will shrivel, reducing both yield and quality.

Prevention and management

There is no clubroot resistance in current Canadian canola varieties and there are no registered fungicides for control or suppression in canola. Prevention is critical.

If you have an infestation, long rotations out of canola is the only strategy at present. Canola, mustard, kale and other cole crops should not be grown for at least four years in slightly infested fields and seven years in severely infested fields.

“Ultimately, rotation out to five years will help reduce clubroot spore load in the soil,” says Chris Anderson, vice-president, crop production at the Canola Council of Canada. “It takes 17 to 20 years for spores to die off to below detectable levels, though. Farmer’s best practices are to clean equipment carefully. Think about it before you move equipment from field to field.”

Recommended preventative measures include:

• Long rotations – canola only every four years.
• Sanitation – clean soil and crop debris from equipment before leaving or entering all fields. Knock off or scrape off soil clumps and sweep off loose soil and crop debris.
• If you are farming in a high risk area, consider power washing equipment with hot water or steam.
• Mist equipment with a weak disinfectant solution such as 1% to 2% household bleach.
• Direct seeding and other soil conservation practices will reduce spores moving with through wind or water erosion.
• Scout canola fields and identify causes of wilting, stunting, yellowing, and premature ripening.
• Avoid spreading straw or manure from suspicious areas. Researchers suspect clubroot can survive through the digestive tracts of livestock.
• Use certified seed. Soil-tags on seed can introduce resting spores to clean fields.

Aster Yellows

Aster Yellows can stand out in a canola crop, but it has little effect on yields. A survey of Saskatchewan canola showed 86% if fields infected, ranged from trace infection levels to 15% infection. Incidence rarely exceeds 5% of plants infected.

Infected plants produce distorted, sterile inflorescences with pods being replaced by blue-green, hollow bladder-like structures. Affected plants are highly noticeable because infected plants stand up taller than the rest of the crop because of empty pods.

Yield losses are usually much less significant than they appear. To estimate yield loss, collect 100 random plants and determine the level of infestation. The percent yield loss is suspected to be closely related to the percent of infected plants. But a plant that is only showing symptoms on one branch may well produce seed on other branches so there would still be yield from that plant.

What is it and where did it come from?

Aster Yellows was first reported in 1953 and can infect a wide variety of crops, including canola, alfalfa, flax, sunflower, echinacea, caraway, coriander, celery, carrot, pea, ornamental plants, weeds and, to a lesser extent, cereals.

Aster Yellows is caused by a phytoplasma that inhabits the phloem or nutrient-carrying vessels of plants. It is spread from plant to plant by the six-spotted leafhopper. Not all leafhoppers are infected with the pathogen and leafhopper feeding, in itself, is not considered an economic threat to crops.

The majority of leafhoppers arrive on wind currents from the U.S. usually in June. Some leafhoppers overwinter in Canada either as eggs in leaf tissues or as phytoplasmas in perennial plant hosts. The leafhoppers become infected by feeding on infected perennial crops and weeds.

Aster Yellows aren’t variety specific when it comes to infection. But stress can affect the crop’s response to the disease, so be sure to choose canola hybrids that withstand stress well and perform in a wide variety of growing conditions.

Dr. Christel Olivier, a research scientist with Agriculture and Agri-Food Canada in Saskatoon has been studying Aster Yellows. Specifically, she is trying to understand why the infection rates appeared higher in the 2007 growing season but at this point has no conclusion.

Prevention and management

Not much can be done to prevent or manage the disease nor is there a great need as the disease rarely results in significant economic loss. While insecticide applications can control leafhoppers in canola, it is not considered economical. And you may do more harm than good by inadvertently affecting beneficial insects.

A few things can be done to prevent infection and limit the negative impact of Aster Yellows, including:

• Seed canola as early as possible in the spring so the crop has a chance to mature making it less palatable to migrating leafhoppers.
• Control perennial weeds in and around canola fields which can host the phytoplasma.
• Avoid planting canola near perennial forages, herbs or spices known to be infected with Aster Yellows
• Rotation

1 Source: www.canola-council.org/hybrids.html
2 Source: http://www1.agric.gov.ab.ca/$department/deptdocs.nsf/all/agdex8593?opendocument
3 Source: http://www.canola-council.org/slides/pests/diseases/diseases.html