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Fungicides: Clarifying the Terminology

July 11, 2001

Throughout the year I receive many questions about fungicides: Is a systemic fungicide my best bet? Which ones are systemic? How often do I need to apply this product? These are all good questions, and for the most part they are addressed on the pesticide label. Pesticide labels simply tell us to do things a certain way, with little or no explanation. This article is for those of you who, like me, like to know “why.”

In the world of pesticides, every category has its own specialized terminology. Some of these terms can be used interchangeably and our colleagues still know what we mean. For example, the term “contact” pesticide works pretty well whether we are referring to a herbicide, insecticide, or fungicide; when the pesticide comes in contact with the pest, some or all of the pest is killed or inhibited. In the first case, the insect that comes in contact (for example, while eating or walking on leaves) with the insecticide is killed. In the second case, the herbicide enters and kills only the parts of the plant that are directly exposed to it. Finally, in the third case, a fungus that attempts to infect a plant sprayed with a contact fungicide is not successful. These pest-control processes are quite different; and, although we can and do get by with the simple terminology, contact fungicides are more appropriately referred to as protective–contact.

Unlike a contact herbicide, a protective–contact fungicide does not enter the plant at all but rather acts as an exterior shield that protects the plant or seed from certain fungi for some period of time. Uniform spray coverage is vital (how well does a shield work if it is filled with holes?). The length of protection that the protective fungicide provides depends on many factors. As with any pesticide, rainfall or irrigation within a couple of hours after application washes away much of the pesticide residue and greatly reduces the protective value. Even after drying on the plant surface, residues of a protective fungicide may continue to be eroded via rain, dew, vaporization, sunlight, etc., thus reducing the protection. Furthermore, as the plant tissues expand or are replaced, the new tissue is left unprotected. For these reasons, protective–contact fungicides need to be reapplied more often (have a shorter application interval) than systemic fungicides.

The addition of a spreader–sticker adjuvant to the spray mix may help improve coverage and slow residue loss. One may be led to believe that a spreader–sticker should be added to the spray mix in all situations; however, there are some other factors to consider:

    1. Always read the directions (checking the fine print) for both the fungicide and adjuvant label.
    2. The fungicide formulation you selected may already have certain adjuvants included by the formulator. If you include additional adjuvants, it may increase runoff and decrease pesticide deposit or cause other problems. Adding adjuvants to the fungicide during the formulation process is becoming increasingly popular. The problem is, the label does not always make this addition obvious to the user (in these cases, adjuvants are considered secret inert ingredients—at least to the public and to the competition). With products such as Daconil Weather Stik, it is quite obvious from the trade name that a sticking agent has been added to the formulation. My point is, you need to read the label to find out what you may or may not add to the spray mix.
    3. Is the plant you need to treat difficult to “wet” (the tissues especially waxy or hairy)? Have you exper-ienced poor coverage (due to poor wetting) in the past?
    4. Adjuvants may increase penetration of systemic fungicides and may cause phytotoxicity. If you can’t find the answers to your questions, test the mix on a few plants and observe them for signs of injury over several days to weeks.

While reading fungicide labels and literature, you have likely noticed the many terms used to describe systemic fungicides. Examples include systemic, translocated, eradicant, and curative. On the practical side, some of the terms are picky and may seem irrelevant. Let’s take a closer look at this terminology compared to protective–contact fungicides just described.

Systemic fungicides are absorbed and translocated in the plant. They serve to prevent the development of disease at the site of uptake as well as in other plant regions. Translocation is simply a term used to describe the movement of any compound within the plant from the site of application to distant tissues.

Local penetrant (also known as local systemic) fungicides are absorbed into the immediate area of application but are not translocated far from the site of uptake. They serve to prevent the development of disease at (and in a small zone surrounding) the site of uptake. The term local systemic is often used but is not the best description of these fungicides.

You may also see the terms curative and eradicant used to further describe certain systemic and local penetrant fungicides. Curative or eradicative fungicides have the unique ability to stop the progress of infections that may have occurred a few hours or days before the application. Using this “kick-back” or “reach-back” effect can reduce the number of fungicide applications because it allows an applicator to react to an infection period rather than spray at predetermined intervals. However, such a program should be used only by applicators who are able to accurately monitor (and rapidly respond to) infection periods. Use caution with these terms as some clients may be led to believe that their dead leaves or their half-dead plant will regain perfect health and appearance with the use of this fungicide—that is not the case. Make no mistake, fungicides are most effective when they are applied before infection.

As you can see, the choice of terminology depends on whether the fungicide is (1) absorbed into the plant and (2) translocated in the plant. Most people tend to group the last two terms together and simply call them “systemics.” If a fungicide is considered to be a systemic, does that mean it travels throughout the entire plant? Not necessarily. In fact, among the fungicides on the market today, almost none translocate throughout the entire plant (that is, via the xylem and phloem). Most systemics are only translocated upward in the plant’s xylem (water-conducting) vessels. This type of translocation is termed apoplastic (or acropetal) translocation; whereas adequate distribution of a fungicide in the phloem (food-conducting) tissues is termed symplastic (or basipetal) translocation—which would include translocation down into the roots.

Because virtually no systemic fungicides have basipetal movement, several turf fungicide labels recommend watering-in the fungicide after application so that it can be taken up by the roots and crown to protect against certain diseases. Can this technique be used to control tree leaf diseases such as crabapple scab? I asked this question of several chemical company researchers recently, and the answer is “no.” First of all, this technique, used for this purpose, is not recommended on the label—thus, it is illegal. Second, the researchers have tried this technique with several systemic fungicides against several diseases and found it to be ineffective.

What are the advantages to using systemic fungicides versus protectant fungicides? First, systemics provide longer residual activity because they are absorbed by the plant and protected from washoff and weathering. Second, they may protect plant tissues (for example, crowns, roots, newly formed tissues) that we cannot effectively spray. Third, they may control fungi that have already entered the plant. What’s the downside? Systemics are relatively new to the market, and (as you know) new is generally synonymous with more expensive. Systemics tend to be much more specific (targeting only a few types of fungi) than the older protectant fungicides (this can be good or bad). However, some of the most recent systemics to enter the market are fairly broad- spectrum. Perhaps the biggest disadvantage of systemics has been the development of pathogen resistance or tolerance to these fungicides. Without getting into the specifics, suffice it to say that protectant fungicides tend to affect fungi in more complex ways, compared to systemics. This makes protectant fungicides more difficult for the fungi to “get around.”

The take-home message from this summary is that all fungicides, even within the systemic group, do not act the same way after application. Thus, our application procedures and expectations should reflect these differences. You can learn more about mobility, site or mode of action, risk of resistance, and effectiveness for specific fungicides on page 18 (applies mainly to turf fungicides) of the 2001 revision of the Commercial Landscape and Turfgrass Pest Management Handbook. In addition, page 103 of the Home, Yard, and Garden Pest Guide (U of I Extension Circular 1374; March 2001) lists the basic characteristics of fungicides readily available to nonprofessionals.

Author: Bruce Paulsrud


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