Great Lakes Growers Hear
Timely Carrot Research Reports
Carrot Country
Spring 2007
On muck soil, carrots do not appear to
require nitrogen fertilizer to produce
optimal yield and quality.
So reported Sean Westerveld, speaking
during the Great Lakes Fruit, Vegetable & Farm Market Expo held Dec. 6
at the DeVos Place Convention Center in Grand
Rapids, Mich.
Westerveld is with the Muck Crops
Research Station, University of Guelph,
Ontario, Canada, and was one of four
speakers making presentations during the
session. Others were: Mary Hausbeck,
Department of Plant Pathology, Bernard
Zandstra, Department of Horticulture, and
Beth Bishop, Department of Entomology,
all with Michigan State
University.
It should be noted,
however, that leaf
blight severity can be
increased if nitrogen
application is reduced,
Westerveld warned,
stressing that it is
unknown how these
results would translate to muck soils with
lower organic matter content than the
60-plus percent found at the sites where
the research was conducted.
“On mineral soil, carrot yields are
unaffected by nitrogen when there is sufficient nitrogen remaining in the
soil from
a previous season, but do require nitrogen
if there is low levels of organic matter or
nitrogen has been removed from the soil by
a previous deep-rooted crop,” the speaker
said. “Nitrogen application is necessary to
minimize leaf blight on mineral soil.”
The risk of nitrogen defi ciency is larger
on mineral soil, he cautioned. However,
over-application of nitrogen can cause
seedling death and poor stand establishment.“
There is no effect of nitrogen on quality
or storability of carrot roots,” he told
the group. “Nitrogen should be included in
the integrated crop management program
for the control of carrot leaf blights.”
Carrot Disease Update
In her presentation, Mary Hausbeck
acknowledged that high humidity and
frequent rainfall or irrigation are common
during the Michigan growing season and
yield-threatening foliar blights are a recurring
problem.
“Each year, foliar blights caused by fungi
(Alternaria dauci, Cercospora carotae)
and/or bacteria (Xanthomonas campestris
pv. Carotae) reduce photosynthetic area
and weaken leaves and petioles,” she said.
And since area growers harvest carrots
mechanically, weakened foliage can be disruptive
at harvest time. Carrot tops break
off during lifting. In situations where foliar
disease is severe and uncontrolled, the tops
may be compromised to the extent that the
crop cannot be harvested.
Bacterial blight popped up in several
carrot fi elds in 2006, Hausbeck reported.
Initial symptoms of the disease are “somewhat
similar” to those of the fungal blights,
especially Alternaria. However, bacterial
blight is not controlled by the fungicide
sprays used to manage Alternaria and
Cercospora.
“While copper sprays may be helpful,
the bacteria may become resistant,” the
researcher warned. “To maximize the
efficacy of copper, sprays would need to be
applied early and frequently and initiated
prior to disease symptoms.”
Hausbeck reported on a project
launched to help growers better predict
Alternaria and Cercospora outbreaks.
Known as the Carrot Weather Forecasting
Project, its purpose, she explained, is
to compare the TOM-CAST system using
either on-site weather data or weather data
received via a satellite.
The project took place at two locations,
the fi rst on the MSU Muck Farm, located
near Laingsburg, and second on a grower
cooperator’s farm in Oceana County, near
the town of Hart.
Forecasting systems at both sites provided
significantly better disease control
than the untreated checks. The plots treated
with fungicides applied according to onsite
weather data had significantly better
ratings for percent infected foliage and
plants than the plot treated with fungicides
scheduled by the satellite weather data,
Hausbeck said.
New Weed Control Products Coming
Speaking on weed control in carrots,
Bernard Zandstra told growers that Lorox
is effective preemergence and postemergence
on carrots for a wide spectrum of
grasses and broadleaves. Other herbicides
labeled for carrots include Dual Magnum
and Treflan, preemergence, and Sencor,
postemergence.
“The postemergence grass herbicides,
Fusilade, Poast and Select, also are registered,” Zandstra reported,
adding that the search for other herbicides with suffi cient
safety on carrots continues.
Several herbicides for use on carrots
are in the process of being registered, the
researcher announced. Oxyfl uorfen (Goal)
has fair to good selectivity postemergence
on carrots and would improve control
of common purslane, herbicide-resistant
pigweeds, common groundsel, marestail
and ragweed, he said. The new formulation
of oxyfl uorfen (GoalTender) should
improve crop safety for use on carrots.
Goal is currently an IR4 project.
“
Nortron recently received a carrot
tolerance and will be registered for carrots
in Washington and Oregon,” Zandstra
said, adding that he hopes to see the label
expanded in the future to include Michigan.
Norton has shown improved control
of yellow nutsedge, common chickweed,
mustards and nightshades.
An IR4 label for Caparol (prometryn)
should be issued soon, the speaker promised.
The IR4 labeling is in process. Caparol
is similar in activity to Lorox and has
the same mode of action. It causes slightly
more crop stunting than Lorox.
“The Dual Magnum Section 3 (federal)
label has been expanded to include
carrots,” Zandstra
reported. “Growers
need to indemnify
the company from
any liability from
this use. To agree to
the indemnification
language and obtain a
copy of the label, access
the Syngenta website at:
www.farassist.com.”
Dual Magnum may
be used on carrots on
any soil type, he said,
noting that, in the past,
it was limited to use on
high organic soils.
Good News on Aster Yellows
Monitoring
In her presentation, “Aster Leafhopper
Infectivity Sentinel Plots,” Beth Bishop
pointed out the seriousness of aster yellows
disease of carrots and other vegetable
crops.
“Affected plants have distorted,
discolored foliage and hairy, bitter-tasting
roots,” she said. “Plants become infected
with aster yellows after being fed upon
by an aster leafhopper that carries the
disease.”
Nothing can be done once plants are
infected, the speaker warned, adding that
the only way to prevent aster yellows is to
control the aster leafhoppers that spread the
disease.
The risk of aster yellows to a crop
depends on the inherent susceptibility of
the crop to the disease and on the proportion
of leafhoppers that carry aster yellows
(infectivity rate), Bishop said. Some
years ago, researcher at the University of
Wisconsin developed an “Aster Yellows
Index,” which is a number that measures
crop susceptibility as a means of determine
if treatment for aster leafhoppers was
required to prevent aster yellows infection.
It works, but knowing the infectivity rate
is crucial.
“Prior to 2000, methods to assess
leafhopper infectivity rates were costly and
slow,” Bishop noted. “A limited number
of samples could be processed each year,
so, in general, a single treatment threshold
was used for each crop through the entire
growing season. More recent methods used
by diagnostic Services at MSU involve
molecular tests and assess leafhoppers
directly.
“These tests are much more reasonable
in cost and results are available faster,” the
speaker continued. “Consequently, we can
determine infectivity rate (and resulting
treatment thresholds) for a variety of
different sites throughout Michigan. We
also have tested leafhoppers at these sites
multiple times throughout the growing
season. We have found that both location
and time of year affects infectivity rates
of leafhopper populations, and, thus, we
are able to provide growers with treatment
thresholds that are specifi c to their local
area and particular time of the season.”
During 2006, Bishop said her team
was able to establish a testing program
of several different “sentinel areas” in
west central Michigan. Aster leafhoppers
were collected by crop consultants and
others on a biweekly basis and were sent
to Diagnostic Services for testing. Results
were then sent to Vegetable Entomology
where they were translated into treatment
thresholds and disseminated to growers,
consultants, extension educators and
others.
“As in previous years, there was a
great deal of variability in infective rate
(and treatment threshold), both between
locations and over the growing season,”
Bishop observed. “Similar to 2005, aster
leafhoppers collected from the carrot
growing regions in west central Michigan
(Oceana, Mason and Newago counties)
generally had higher infectivity rates than
aster leafhoppers collected from celery
fields in more southern western Michigan
counties.”
© 2007
Columbia Publishing