AGRONOMIC SPOTLIGHT
Clubroot
INTRODUCTION
Even the name sounds ominous. Clubroot. And only a mere five years ago, this disease wasn’t even on industry’s
radar. In the last few years, field infestations have exploded. Some pathogists now question whether the disease
was present prior to five years ago, but was misdiagnosed or unrecognized. Growers, agronomists, and
researchers tend to focus on above ground biomass and don’t routinely examine root health without just cause.
Perhaps clubroot was present before we really knew it.
Dr. Stephen Strelkov, Plant Pathologist at the University of Alberta, and expert on clubroot, reports approximately
250 fields in Alberta, or roughly 40,000 acres, were infested with clubroot in 2007.
What is clubroot?
Symptoms
Clubroot is
caused by a
fungal-like
pathogen called
Plasmodiaophora
brassicae and
infects crops in
the Brassicaceae
family. Typically,
clubroot is found
in vegetable crops like cabbage, cauliflower, radish,
bok choy, and rutabaga. But now, clubroot in Western
Canada found a new host – canola.
The key symptom of clubroot is the unsightly galls
attached to the roots. Initially, the galls are hard and
white. Once they age and start to decay, they turn soft
and grey. The galls vary in size and can grow to be 6”.
The clubroot pathogen is an obligate parasite,
meaning it requires a host to survive and reproduce.
Without a host, dormant resting spores can last in the
soil for many years, but in the presence of a
susceptible host, spores are signaled to germinate and
produce zoospores. Zoospores are able to swim in soil
water and infect plant root hairs. Strelkov explains,
“Primary zoospores infect only the root hairs. Then
secondary zoospores are produced and re-infect the
root hairs, invade the root cortex, and form plasmodia
that will eventually give rise to resting spores. It’s this
infection by secondary zoospores that result in the
development of large galls.” The galls restrict nutrient
and water uptake, causing the plant to suffer. The
disease cycle continues when infected roots discharge
secondary zoospores into the soil, or when decaying
galls release resting spores.
DID YOU KNOW… The technical name for
clubroot is ‘Plasmodiaophora brassicae Woronin’.
Woronin is the Russian scientist who discovered
this pathogen in 1878.
The clubroot pathogen can infect seedling canola, but we
don’t usually see symptoms early on. Strelkov explains,
“The pathogen can infect seedlings at a very young age if
the moisture conditions are right and the pathogen is
present. However, because it takes up to six to eight
weeks for full-blown symptoms to develop, it is very hard
to identify infected seedlings.” Initially infected plants
look relatively healthy, but as the disease takes hold,
plants can prematurely bolt, wilt, stunt, yellow, and
prematurely ripen. Depending on when the infection
occurs and the gall size, infected plants usually yield
poorly. Overall yield loss can be significant, with reports
of infested fields yielding only 50% of their potential.
Under extreme pressure, some fields are not even
harvested.
The infestation is usually patchy. According to Strelkov,
once an infestation is confirmed, even in a small patch,
the entire field is considered clubroot infested for
management purposes.
Is my field at risk?
Clubroot is new to canola on the Prairies, and right now
seems to be isolated to fields around Edmonton, and
most recently in fields near Brooks and Viking. But, says
Strelkov, there’s no reason to believe it can’t spread to
other regions in the Prairies.
The disease can spread by wind, water, and soil
transport. The main mechanism of spread is soil
movement. In fact, infestation is usually worse
around field entrances, suggesting equipment traffic
accounts for much of the disease spread. In Alberta,
it’s likely the initial source of canola infection came
from vegetable crops.
Clubroot is not a seed borne disease, so there’s no risk
of planting infected seed. The only link between
disease spread and seed would be clubroot infested
earth-tag on seed. But seed cleaning and planting seed
from clubroot-free areas would negate this risk.
Warm, wet soils are a key element for this disease. Low
pH soils (<6.5) also seem to favour the pathogen but it
has been found in soils ranging from pH 4.8 to 7.6.
Preventing the spread of clubroot
Steps for clubroot prevention include:
• Minimize the spread of infested soil on equipment,
clothing
• Clean all equipment from the infested field.
Although spores can potentially spread in dust
storms, water, and soil, the suspected culprit in
disease spread is contaminated equipment. Take
time to clean soil and crop debris from suspected
fields
• Do not move bales, straw, chaff, or manure from the
infested fields
• Employ good weed control, especially for
cruciferous weeds like wild mustard, shepherd’spurse, and stinkweed. These weeds are hosts for the
disease in the absence of canola
• Practice a one-in-four-year canola rotation. Good
rotations won’t necessarily stop clubroot
introduction, but will help to prevent serious
infection
• Scout canola fields regularly. Check plant health both
above and below ground.
If you suspect clubroot in your field, contact your local
provincial Department of Agriculture representative.
Prevention is key, because once clubroot is confirmed
there are no effective control measures. There are no
fungicides options and cultural control is limited to crop
rotation. Strelkov recommends that in fields with low
levels of infestation, canola should not be grown for at
least four years. In severely infested fields, canola should
be kept out of the rotation for at least seven years.
Monsanto breeding
Did you know some members of the Brassica family
contain naturally occurring clubroot resistance genes?
Species such Polish canola, Chinese cabbage, broccoli,
and radish have a natural resistance to clubroot.
Monsanto has been working hard to breed a clubroot
resistant canola variety. Our goal is to introduce a
Roundup Ready® hybrid with clubroot resistance in five
years.
SUMMARY
Clubroot is a serious economic threat to canola.
Large galls on infected plants restrict nutrient and
water flow to developing pods, and rob yield.
Curative measures aren’t available, so prevention is
key. With clubroot “an ounce of prevention is worth
a pound of cure.” Understanding the disease cycle
and preventative measures is critical for Western
Canadian producers.
This GROWING KNOWLEDGE bulletin is produced by Monsanto's
Western Canada Technology Development Group written by
Trish Meyers, MSc, PAg, CCA. For further information call
Monsanto’s CustomCare® line or email us at
[email protected]
Always read and follow pesticide label directions. Roundup Ready crops contain a gene that confers tolerance to glyphosate, the active ingredient in Roundup
agricultural herbicides. Roundup agricultural herbicides will kill crops that are not tolerant to glyphosate. CustomCare, CustomCare and Design, Growing
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Monsanto Canada, Inc. licensee. ©2008 Monsanto Canada Inc. 0708