The Fungicides

Tom Allen, Extension Plant Pathologist
By Tom Allen, Extension Plant Pathologist March 19, 2013 14:50 Updated

Throughout this post I’ll focus on the fungicides used in soybean production systems.

Since soybean rust was first detected near Baton Rouge, LA in November 2004 most plant pathologists have focused their extension training sessions on soybean rust and managing potential yield losses with fungicides.  However, we probably haven’t focused as much time in the past few years on the differences between the two main fungicide classes utilized in row crop production systems (corn, peanut, rice, soybean, wheat).  Ultimately there are differences between the two fungicide classes (e.g., strobilurin, triazoles) as to when they are most beneficial and the specific response post-application.  In addition, difficulty arises when attempting to tell the two fungicide classes apart from one another.  The information below will help determine the specific fungicide class for a given product as well as determine the potential benefit of using each product class in particular situations.

The strobilurins (or quinone outside inhibitors (QoIs):

The easiest way to determine if a particular fungicide is a strobilurin is to look at the name of the active ingredient associated with the product.  See attached image for information as it pertains to the fungicide label and specific information included on a fungicide container (active ingredient, trade/product name, FRAC code) Fungicide label.  The trade or product name of the fungicide is normally the most recognizable word (e.g., Headline, Quadris).  However, to determine what particular class of fungicides the product can be classified as you need to consider the active ingredient.  If the word ends in “-strobin” then the fungicide product is a strobilurin fungicide.  For example, Quadris is a commonly used fungicide in several of our production systems (corn, cotton, rice, soybean, wheat).  The active ingredient in Quadris is azoxystrobin.  Since the active ingredient ends in “-strobin” the product is a strobilurin fungicide.

Generally speaking, the products that comprise the strobilurin class of fungicides have broad spectrum activity on numerous disease causing fungi.  Even though some individuals will say that the strobilurin products have systemic activity, the particular activity associated with this class of fungicides should only be considered to be locally systemic.  At best, any movement of the fungicide once it lands on the leaf will be either from the upper leaf surface through the leaf to the underside of the leaf or within a limited area around the droplet’s location.  Fungicides should not be considered to move throughout the plant from the point of entry.

Fungicides in the strobilurin class are best applied when disease is not present and so they would be most beneficial when used on a preventive basis.  In that particular situation the residual expectations of the product are on the order of 21 days.  However, any residual that could be expected of the product will be based on the environment that occurs after the product has been applied and several additional factors.

The associated products that are labeled for application in soybean are included as a pdf strobilurins.

The triazoles

Since the detection of soybean rust in the U.S. we have likely only considered the triazoles as “rust” products.  However, with the recent occurrences of strobilurin-resistant frogeye that have been identified in several states we need to start relying on the triazoles to manage leaf spot diseases in addition to soybean rust.  Much like the strobilurins, the triazoles are easiest to tell apart by looking at the name of the active ingredient that can be found on the label as well as the fungicide container.  If the name of the active ingredient ends in “-azole” then the fungicide is a triazole Triazole fungicide label.  However, there is one exception to this rule.  The active ingredient in Topguard fungicide, is flutriafol.  To my knowledge, this is the only exception to the rule stated above.

In general, the triazole fungicides are narrower in their specific activity against diseases so they are limited in the diseases they can be used to manage.  Much like the strobilurin fungicides the systemic activity of the triazole fungicides is limited to movement within an area around where the droplet lands on a leaf.  Systemic activity should not be considered to be throughout the plant once the application has been made.

Even though plant pathologists hate to hear the triazole fungicides referred to as “curative” they do have the ability of being curative and can be applied in the presence of disease.  But, with that said they perform best when applied prior to the onset of visible disease symptoms.  Once applied, they have a more limited residual period of time along the order of 14 days.  In addition, even though the triazole fungicides can loosely be referred to as “curative” they do not make the spots disappear upon application and in some cases disease symptoms will worsen even after a fungicide has been applied.

The associated triazole products that are labeled for application in soybean are included as a pdf triazoles.

The last group of products, the pre-mix fungicides, includes a strobilurin plus a triazole (or a strobilurin plus a carboximide as in the case of Priaxor) is also included as a pdf pre-mix fungicides.

Diflubenzuron (Dimilin)

Over the years Dimilin has been suggested to have the ability to prevent yield loss as a result of frogeye leaf spot.  Dimilin is a chitinase inhibiting insecticide and should not be regarded as having the ability to prevent yield loss as a result of such diseases as frogeye leaf spot.  In the past numerous people have suggested this particular product might limit yield losses attributed to frogeye leaf spot since the fungus that causes this particular disease has chitin in the cell walls.  However, laboratory data suggests that Dimilin does not reduce the growth of the fungus and therefore we should not consider this product to be used as a fungicide.

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Tom Allen, Extension Plant Pathologist
By Tom Allen, Extension Plant Pathologist March 19, 2013 14:50 Updated
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