Top Crop Manager

Features Fungicides Seed & Chemical
Mounting resistance management strategies

Protect the effectiveness of valuable new fungicides.

November 14, 2007  By Top Crop Manager

Potato growers know the value of effective fungicides against early and late
blight. Without them, these diseases would clobber the potato industry. Just
remember the Irish potato famine!

In recent years, several valuable fungicides have been introduced to help manage
these two diseases. Unfortunately, most of them belong to the Group 11 family
of chemistry that targets the same single site. They inhibit respiration of
the mitochondria, the energy cells of living organisms, including blight fungi.
Products registered for use in Canada in this chemical family include Headline,
Quadris, Tanos and Reason.

Margaret Tuttle McGrath, associate professor in the department of plant pathology
at the Long Island Horticultural Research and Extension Center at Cornell University,
has worked with this group of fungicides. Researchers had initially thought
the Group 11 mode of action would be difficult for fungi to overcome. It did
not take them long to change their minds. They determined the resistance risk
with these fungicides was much higher and more difficult to predict than expected.


Resistance developed quickly. Because these fungicides have a single-site mode
of action, a single genetic change in fungi is all that is required to cause
resistance. "It's critical to manage resistance before it develops,"
says McGrath. "Apply Group 11 products alternately with other systemic
fungicides that have a different mode of action. Ideally, the fungicide program
will also include multi-site contact fungicides. No more than half of the applications
in a season should include Group 11 products."

"The resistance problem in the US was recognized early on and application
recommendations shifted immediately," says Blair Geisel of Gaia Consulting,
an independent crop consulting firm out of Portage La Prairie in Manitoba. "Initially,
they were using sequential applications of strobilurins over and over again.
Now they're tank-mixing and alternating with other modes of action."

Table 1. Early and late blight: how to
tell them apart.
Early blight Late blight
Lesions are angular because they do not grow
past leaf veins.
Lesions are not angular at all, as they grow
past leaf veins.
Lesions are on the leaves only. Lesions may be on the leaves, stems or petioles.
No white mycelial growth during periods of high
A white mycelial growth will appear on the under
side of the leaf during periods of high humidity.
Lesions typically appear after flowering. Lesions will show up during any growth stage
of the potato plant.
Lesions grow out in rings creating a 'bull's eye'
No 'bull's eye' appearance within lesions.
Tuber lesions are dark and circular. Tuber lesions are irregular shape and brown to
purple coloured.
Source: Bayer CropScience.

Resistance management right in the tank
So far in Canada, there are no documented cases of early and late blight organisms
developing resistance to Group 11 products. Now is the time to do everything
possible to deter that development.

Reason, the newest one to be registered in Canada, is the only Group 11 fungicide
registered as a tank-mix with either Bravo or Dithane. Research has shown that
when Reason is added to a tank-mix with either of these standard contact fungicides,
protection against disease is enhanced. "When these products work together,
they're additive," says Andrew Dornan, horticulture field development representative
with Bayer CropScience.

Dornan looked at full rates and the lowest labelled rates of Dithane and Bravo.
"The research data shows a tank-mix of even the lowest rate of the contact
fungicides with Reason provides better activity than the full rate of the contact
product alone," says Dornan. "And the crop receives all the benefits
of Reason's antisporulant activity, local systemic and translaminar movement
plus built in resistance management."

Geisel has seen the product synergy in his client's potato crops too. "I
look at how effective a treatment is and compare it to standards," says
Geisel. "The tank-mix of Reason with either Bravo or Dithane is very effective,
especially against early blight. In Manitoba, most of the growers are using
a Group 11 for early blight and a standard contact product or protectant for
late blight."

Geisel observes that most growers in his area are following best management
practices when it comes to Group 11 fungicide use. "Generally, these products
are more expensive than other fungicides, so they use them sparingly,"
he says.

A tank-mix of Dithane at the low rate with Reason costs about $2 more per acre
than a full rate of Bravo alone. This tank-mix helps manage resistance and turns
a regular contact program into a premium performance program without the premium

Managing fungicides at risk
The Fungicide Resistance Action Committee (FRAC), a technical group of CropLife
International, was set up to provide fungicide resistance management guidelines
to prolong the effectiveness of 'at risk' fungicides and to limit crop losses
should resistance occur.

Resistance develops through the survival and spread of initially rare mutants
during exposure to fungicide treatments. In some cases, resistance can result
quickly from a single gene mutation. In others, development is gradual, involving
a number of genetic mutations. The mechanisms of resistance vary but all involve
the primary site of action of the fungicide within the fungus. Fungicide treatments
with a single mode of action are most vulnerable to resistance development.

FRAC evaluates the risk of resistance development based on the presence of
cross resistance with existing fungicides, repetitive use of products with the
same mode of action, large populations of target fungi and extensive geographic
areas of use. For high risk groups, the committee established specific resistance
management guidelines. In Canada, fungicide groups considered at high risk of
developing resistance include Group 1 (seed treatments Mertect and Senator PSPT),
Group 4 (Ridomil) and the newest entries, the Group 11 family (Reason, Headline,
Tanos and Quadris).

Best use guidelines set
Based on its understanding of the risk of resistance development, the FRAC developed
best use guidelines specific to Group 11 fungicides, including:

  • Never make sequential applications of Group 11 fungicides. Always rotate
    to a product or tank-mix with a different mode of action.
  • If applying a Group 11 fungicide by itself and more than five fungicide
    applications will be made through the season, no more than one-third of the
    fungicide applications should be Group 11 fungicides.
  • If applying a tank-mixture of a Group 11 with Bravo or Dithane (multi-site
    modes of action), no more than half of the total season fungicide applications
    should be Group 11 fungicides.

Bayer CropScience developed Reason with these guidelines in mind. As a result,
Reason will only be recommended as a tank-mix with Bravo or Dithane. This tank-mix
will provide excellent blight activity with built-in resistance management.



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