Defeat Plant Fungus
June 1, 2012 Modern fungicides have become critical components in the prevention and treatment of plant diseases. Ever since the beginning of agricultural production, plant diseases have had a devastating impact on the human population. In the 1840s, potato blight caused by ooymcete Phytophthora infestans was responsible for the Irish Potato Famine. It is estimated that 1.5 million people starved to death when this critical food staple was wiped out. More recently, in 1970, the Southern Corn Leaf Blight Epidemic in North America caused more than $1 billion in crop losses. In most fields, the crop loss from this blight was between 80% to 100%. Therefore, as the world’s population is expected to increase by more than 1 billion people in the next 15 years, it is critical for farmers to have the needed fungicides in order to maximize crop production and protect against crop losses.
Fungi in a nutshell
In order to simplify things, plant pathologists group all fungal diseases as “fungal pathogens.” Actually, there are two distinct types of fungal pathogens: the true fungi and the fungal-like pathogens know as the Oomycetes. The cells of all true fungi are surrounded by cell walls made up of the polymer chitin. Chitin also is the main structural component of the exoskeletons of insects and arthropods. Most of the true fungal diseases are caused by either ascomycetes or basidiomycetes. Ascomycetes are responsible for common diseases such as southern corn leaf blight, peanut leaf spot, gummy stem blight, and powdery mildew. Basidiomycetes are famous for producing the white button mushroom; however, they are also responsible for a number of devastating plant diseases such as white mold in peanuts, corn and bean rust, southern blight, and rhizoctonia.
Oomycetes, also known as water molds, closely resemble the true fungi, however, they are totally unrelated to them. The cells of the oomycetes like the true fungi are surrounded by a cell wall. However, their cell wall is primarily made up of cellulose, the same polymer found in the cells walls of both plants and algae. Oomycetes are distantly related to brown algae such as kelp. Common diseases caused by oomycetes include downy mildew, late blight, pythium root rot and tobacco black shank. In this article, the terms fungal pathogens and fungal body will refer to both the true fungi and the oomycetes.
Types of Fungicides
Fungicides can be grouped together according to how they interact with fungal pathogens. Protectant fungicides or contact fungicides were the first type of fungicides to be discovered. These fungicides are not absorbed in to the plant tissue and can be washed off by rain or irrigation. They form barriers around the plant cells. These barriers kill the fungal spores and hyphae, the thread-like strands of the fungal body. Since protectant fungicides are not absorbed into the plant tissue, they can only prevent fungal infections. They cannot treat fungal diseases that have already penetrated into the plant tissue. As a result, to achieve good disease control, these fungicides must be applied before the onset of disease. Sulfur and copper were some of the first protectant fungicides discovered. Bordeaux mixture, for example, was accidentally discovered by grape farmers in the Bordeaux region of France. The farmers sprayed the grapes near the roadside with a mixture of copper sulfate and hydrated lime to deter passersby from helping themselves to the ripe grapes. The farmers then observed that the vines which had been sprayed with the mixture remained free of downy mildew. Other common contact fungicides discovered in the 1930s and 1940s include, chlorothalonil, mancozeb, thriam and captan.
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Protectant fungicides are an effective tool in combating fungal diseases. However, what are your options if a disease outbreak catches you by surprise? You will need a fungicide that can move inside plant tissue. The first penetrant fungicide family the Carboxamides, were discovered in 1966. Penetrant fungicides are classified as either curatives or eradicants. Curative fungicides are absorbed into the plant tissue and are effective at treating existing fungal infections in plants that are not showing any disease symptoms. Eradicant fungicides also have the ability to penetrate inside plant tissue and are effective in treating infections that are showing symptoms. Always use good disease management techniques. If conditions are favorable for disease development do not wait until crops are showing symptoms before spraying, even when using an eradicant fungicide. Penetrant fungicides are always more effective when applied in preventative manner.
Penetrant fungicides are also known as systemic fungicides. However, the term “systemic” can be misleading. Penetrant fungicides have different routes in which they move inside plants. Local penetrants move through the leaf from one side to the opposite side. The passage of the fungicide through the leaf tissue is known as translaminar movement. The fungicide stays within the leaf in which it was applied and does not move in the xylem and phloem cells. In contrast, acroptetal penetrants only travel in an upward direction from the point of application towards the tip of the plant. This is due to the fact the acropetal penetrants move only in the xylem tissue of the plant. (Remember, “xylem up, phloem down”).
Most fungicide families have the same phytomobility. For example, all of the DMI (demethylation inhibitors) fungicides are all acropetal penetrants. Common DMI fungicides include propiconazole, tebuconazole, and myclobutanil. The main exception are the strobilurin fungicides. Azoxyoburlin and fluoxastrobin are acropetal penetrants while the rest of the strobilurins are local penetrants. In order for it to be possible for a fungicide to move from the leaves down into the roots in would need to be able to travel in the phloem tissue. Systemic penetrants travel both in the xylem and phloem tissue and are known as ambimobile. As of right now, only the phosphonic acid fungicides are systemic penetrants.
Fungicide Resistance
As stated in the beginning of the article, fungicides are an important tool in the struggle to produce more food on the same amount of land. If many of our important fungicides were to lose their effectiveness, the results to agriculture could be devastating. Unfortunately, this has already happened in certain areas and is likely to continue unless steps are taken to prevent or at least slow down the process of fungicide resistance. How does fungicide resistance happen? Just like antibiotic resistance, insecticide resistance and herbicide resistance, repeated use of fungicides with the same mode of action leads to resistance. As fungi reproduce, they produce millions upon millions of spores. The majority of the time, the fungicide kills the fungus. However, through natural genetic mutation, one or two of the spores develop resistance to the fungicide. When the same fungicide is sprayed again, all the other normal spores are killed and the only resistant spores can reproduce and multiply. If the same fungicide mode of action is sprayed again, all the normal spores are killed and the resistant spores continue to reproduce. In a matter of time, there is a population shift and only the resistant spores are left.
The Fungicide Resistance Action Committee (FRAC) classifies fungicides by the mode of action. The multisite fungicides will all have a FRAC group starting with M. Common multi-site fungicides include copper (M1), sulfur (M2), mancozeb (M3), captan (M4) and chlorothalonil (M5). These fungicides target numerous cellular proteins found in the fungus making it very unlikely for resistance to development. The penetrant fungicides target the fungus at a single molecular target site. Therefore, it is much easier for the fungus to modify the target site through a random mutation that can allow it to defeat the fungicide. Because of the concern of fungicide resistance, most fungicide manufactures’ labels require that the fungicide be rotated with a fungicide with a different mode of action. That can be difficult if you are just going by the fungicide name. Kresoxim-methyl (Sovran, Cheminova) and pyraclostrobin (Cabrio, BASF) may sound different, but they are both strobilurin fungicides (FRAC group 11).
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In order to make things easier, most new fungicide labels place the FRAC group on the first page of the label. In addition, some manufactures’ labels require that the fungicide be tank mixed with a multi-site fungicide to further reduce the chances of resistance. A spray schedule for gummy stem blight, for example, calls for a DMI fungicide (group 3), tank mixed with chlorothalonil (M5), followed by then thiophanate-methyl (Topsin, UPI Inc.) (group 1) or a strobilurin fungicide (group 11).
Choosing The Right Fungicide
Due to the wide variety of fungicides, selecting the correct product can be a difficult task. Some fungicides are very broad spectrum while others have a more limited range on what they can treat. Therefore, it important to have the correct identification of the disease before choosing which fungicide to spray. As a rule, the multisite contact fungicides such as chlorothalonil have activity against both the true fungi and the oomycetes. Some of the strobilurin fungicides like azoxyoburlin have also have activity against both the true fungi and the oomycetes. However, strobilurin fungicides have greater efficacy on the true fungi and should not be used as a first line of attack against the oomycetes whenever possible. The DMI fungicides are effective against a wide range of diseases caused by the true fungi, but have no activity against the oomycetes. Finally, there is a diverse group of fungicides including, but not limited to, mefenoxam (Ridomil, Syngenta Crop Protection), fluopicolide (Presidio, Valent U.S.A.), and mandipropamid (Revus, Syngenta Crop Protection) that are only effective against oomycetes pathogens.
The Fungicide Label
The restrictions that the fungicide manufactures place on the label such as “make no more than one application before switching to a fungicide with a different mode of action” or “tank mix with a fungicide with another mode of action” are not suggestions or guidelines but requirements in order to prevent to the development of fungicide resistance. In addition, it is a violation of federal law to use a fungicide in any manner that is inconsistent with its labeling. Once a fungal pathogen develops resistance to a fungicide, that fungicide, and in some cases the entire fungicide group, will be lost forever against that pathogen. Therefore, it is crucial that all aspects of the fungicide label are followed at all times.
Author Note: To simplify information available in this CEU series, it is sometimes necessary to use trade names of fungicides. No endorsement of these fungicides is intended, nor is criticism implied of similar fungicides not mentioned.
References
Agrios, G.N. 2005. Plant Pathology. Elsevier Academic Press.
Latin, R. 2011. A Practical Guide to Turfgrass Fungicides. American Phytopathological Society Press.
Paret, M., N. Dufault, and S. Olson. 2011. Management of Gummy Stem Blight (Black Rot) on Cucurbits in Florida. http://edis.ifas.ufl.edu/pp280
Schoelz, J. 2003. Learning from Past and Current Plant Epidemics. extension.missouri.edu/eden/Resources/L1_Learning.ppt
Ware, G.W. and D.M. Whitacre. 2004. The Pesticide Book. W.H. Freeman and Company.
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