Manage fungicides for long-term effectiveness

Knowing fungicide characteristics and making wise selection prevents fungal disease resistance problems.

Modern agricultural systems are typically monocultures with large acreages that have necessitated the use of pesticides. An increase in pesticide use has modified the evolution of insects, fungi, bacteria and viruses, and caused an increase in pesticide resistance. Although resistance is an issue in insects and weeds, many pathogens develop resistance even faster because of their high reproduction in a short amount of time (think of bacteria or spores produced by a mold). This issue has been aggravated by the transition of fungicides with broad-spectrum contact activity to narrow spectrum, single-site action that is more prone to resistance development.

Though resistance is inevitable with narrow-spectrum fungicides, knowing a fungicide’s characteristics can help you slow resistance development with careful management. Anti-resistance strategies include:

  • using non-chemical fungal disease control options
  • applying fungicides preventively to prevent population buildup
  • using multi-site compounds as the first line of defense
  • limiting use of site-specific fungicides
  • using multi-site mixing partners with site specific fungicides
  • rotating or tank mixing (if compatible) site-specific classes of fungicides

To employ the above suggestions, you have to do more than just read the label. Know if you’re using a contact or penetrant fungicide.

Contact fungicides have to be applied prior to the fungus invading the plant tissue and typically target the spore itself preventing germination. Contact fungicides provide no activity once tissues have already been infected and colonized. As weather washes the fungicide off or the plant outgrows the fungicide coverage, tissues will be susceptible to pathogens and require further fungicide application for protection. The first fungicides many of us are familiar with, such as copper sulfate (used in Bordeaux mix) and sulfur are contact fungicides. They have broad activity against a range of fungi and they are not site specific in the fungal cell. Copper and sulfur remain effective fungicides today, in part due to their broad-spectrum mode of action that has not allowed pathogens to develop resistance. Some of the early non-site specific fungicides such as the mercury and arsenic containing products were found to have serious impact on more than just fungi. They have since been removed from the market.

New, narrow-spectrum products are safer for non-target organisms. Recent emphasis has also been focused on the development of fungicides which are penetrants, meaning the fungicide not only is on the surface but also kills within the plant. There are three types of penetrants: local, xylem mobile and systemic penetrants. The penetrants are less susceptible to rain washoff and may provide control despite the lack of thorough coverage. None of the fungicides are effective after disease is well established and the fungus is reproducing.

Local penetrants move a short distance into the plant. Some only move a few cells deep. Others are attracted to waxes so they may move from one side of the leaf through the tissue to the other side of the leaf known as translaminar.

The xylem mobile penetrants move through the stem or leaf surface into the xylem which is the vascular system of the plant that conducts water and minerals up the plant. Xylem mobile penetrants tend to accumulate in mature leaves. Since immature leaves do not have operating stomata providing the pull (through transpiration) of liquids up the xylem, they do not receive the fungicide and remain vulnerable to infections that have occurred there. The advantage of xylem mobile fungicides is that they provide more control of the fungal attacker after infection and distribute more widely from the point of application.

The most mobile of all the fungicides are the systemic penetrants which move in both the xylem and phloem. These fungicides can move up in the xylem or down in the phloem; they predominantly move in the phloem which transports the carbohydrates made by the plant. Systemic penetrants travel based on the sugar transport gradient in the plant.

When spores of a rapidly reproducing fungus are challenged by a site specific fungicide, most of the spores or disease from those spores will be killed. However, with fungi that produce many spores, there will be a higher chance that some spores will be unaffected (fungicide tolerant or resistant) based purely on genetic diversity present in nature. With continued use, the susceptible genetic representatives are killed by the fungicide and do not reproduce while resistant isolates are not controlled, continue to reproduce and become a greater part of the population eventually causing a lack of disease control in the area.

Thus not only is the mobility of a fungicide important but also the mode of action of the fungicide. Fungicides that kill due to a specific single chemical action or cell activity are site specific. They may inhibit production of just one protein needed for cell wall formation compared to broad spectrum fungicides which act on many sites and whole processes of the fungus. Site specific or narrow spectrum fungicides are commonly penetrants. Perhaps an easier way to think of it is that if the mode of action of the fungicide targets only one gene in a pathogen then the pathogen can overcome the fungicide when one small point mutation occurs making the target gene different and affecting the fungicide’s action at the site. If multiple genes are affected by the fungicide then the fungi must simultaneously have multiple gene mutations to overcome this fungicide. This occurs less often so resistance buildup is slower.

The benzimidazole, phenylamide and strobilurin fungicides target a single gene and are at high risk for resistance development. Fungicides such as Mertect, Topsin M, Cleary’s 3336, thiophanate methyl are examples of benzimidazoles. Metalaxyl M(Subdue), metalaxyl (Allegiance, MetaStar, Apron), and mefenoxam (Ridomil Gold, Apron XL) are phenylamide fungicides and Abound, Protégé, Quadris, Cygnus, Sovran, Flint, Stratego are examples of stobilurins, also known as the quinone outside inhibitors or QoI.

Other site specific fungicides include the dicarboximides and sterol demethylation inhibitors but they involve multiple genes so resistance development is slower. Iprodione (Chipco26GT,Rovral) and vinclozolin are local penetrants and are dicarboximides. They affect the cell wall and cell membrane components. Resistance to these has been found in some fungal populations. Fenarimol (Rubigan), cyproconazole (Also, Quadris Xtra), imazalil (Flo-Pro IMZ, etc.) and difenconazole (Dividend, Revus Top, Adament) are examples of sterol demethylation inhibitors and resistance to some of these has also been found.

The most mobile of the penetrants, the systemic penetrants are the phosphonates. They include fosetyl-Al (Aliette, Chipco Signature, Legion), phosphorous acid (AgriFos, Phostrol) and potassium phosphite (Fosphite, Prophyt). They control the fungal-like organisms known as the oomycetes which includes Pythium and Phytophthora. Unlike the other penetrants the mode of action involves multiple sites and thus they are at lower risk for resistance development. Resistance however has been created after repeated exposures over multiple generations in laboratory studies.

Fungicides that have multiple modes of action are not susceptible to resistance development. They include common fungicides like captan, chlorothalonil (Daconil, Bravo), mancozeb (Fore, Dithane, etc.), maneb, metiram (Polyram) and thiram (Spotrete,Defiant); all are contact fungicides. Like copper and sulfur compounds they have multi-site activity and provide non-selective inhibition of a number of cellular functions. Resistance to these has not been shown.

For more information on fungicides and fungal disease control, contact an MSU Extension educator working with your crop. For more information on turfgrass fungicides, see A Practical Guide to Turfgrass Fungicides.

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