Article: Will fungicides be able to control soybean and wheat diseases on their own?

Fungicides are an emergency, quick and efficient control tool for controlling plant diseases. The greatest efficiency in its use is obtained when the Technical Information for the Cultivation of Wheat and Triticale (2013) and Soybean Production Technologies (2013) are observed.

The potential risk of developing fungal resistance to fungicides is related to the extent of the area treated, the number of applications per harvest, the population of spores of the target fungus that is constantly exposed to the chemical agent, the genetic variability of the causal agent, with the fungicide's biochemical mechanism of action whether specific site or multisite, with its specificity, with the concentration of the active ingredient in the fungicide mixture and with the quality of the foliage coverage.

Basic concepts

For a better understanding of this article, the concepts of terms used in the text are presented:

Disease control: What is control? Is it just to reduce the intensity (amount) of the target disease that is present or will cause harm? Then see the following concepts accepted by agronomic science.

Integrated Control (CI): According to the Food and Agriculture Organization of the United Nations (FAO) (1968), IC “is a system of management of harmful organisms that uses all appropriate techniques and methods in the most compatible manner possible to maintain populations of harmful organisms at levels below those that cause economic injury".

Integrated disease management (IDM): A year later, the NAS (National Academy of Science) (1969) of the United States presented the official concept of MID as “the use of all available techniques within a unified program in such a way as to maintain the population of harmful organisms below the Economic Damage Threshold (LDE) and minimize harmful side effects to the environment".

Therefore, the MID addresses the technical and ecological sustainability requirements of agriculture.

Fungicide: Chemical substance that kills fungi in low concentration (e.g. < 1 mg/L of active ingredient).

Fungicide action spectrum: A single fungicide does not control all disease-causing fungi just as all diseases are not controlled by one fungicide. Specificity comes into play, which fungus the chemical is most potent.

Specific site fungicide: It acts to kill the fungus at a single biochemical site in the fungal cell. Examples include the mechanisms of respiration, reproduction, synthesis of proteins, fats, synthesis of membranes, etc.

Multisite fungicide: It is one that acts on more than one metabolic site of those mentioned above. The most frequent cases of development of resistant fungi occur with this group.

Fungicide mixtures: Generally, two site-specific fungicides are mixed, but they act on different metabolic processes, for example, one blocks the fungus' respiration and the other interferes with the synthesis of cell membranes. These mixtures come pre-formulated.

Fungal resistance to fungicides: The use of several wheat crops, making two to three applications per crop cycle, leads to the emergence of strains of the fungus that have reduced sensitivity. In this case, the person who first detects the fact is the producer. In previous harvests, he obtained efficient control and was satisfied, now he observes a failure in control and complains that something wrong is happening.

Why do fungi become resistant to fungicides? To defend themselves from the threat of the fungicide to eliminate their species. It's a self-defense mechanism.

The observed phenomenon is known to science and is called fungal adaptation. Adaptation is defined as the ability of living beings (fungi, bacteria, insects and weeds) to adjust to environmental changes.

Control failure: Situation in which the producer observes that, when compared to previous harvests, the efficiency of the fungicide has changed. He says there was a “failure of control” and starts to complain and seek explanations for the fact.

The resistance of powdery mildew and leaf rust to trizol fungicides has been scientifically proven.

Resistance of wheat pathogenic fungi to fungicides: The resistance of wheat powdery mildew to triadimenol and strobilurins has been proven; from speckled leaf spot to strobilurins, from leaf rust to triazoles and reduced sensitivity of the fungus(ies) that cause yellow spot to triazoles and strobilurins Cases of occurrence of fungi resistant to fungicides have become frequent.

Final considerations

Due to the reduced sensitivity of some fungi that cause diseases in wheat compromising control efficiency, sustainable control should not be based solely on the use of fungicides with the risk of increasing cases of resistance. The MID recommends the use of all available strategies such as: production of healthy seeds, seed treatment with potent products and efficient doses, crop rotation, elimination of ryegrass from the area, elimination of volunteer plants, creating cultivars with durable resistance to powdery mildew and leaf rust, application of fungicides to aerial organs as recommended by the Wheat and Triticale Indications (2013).

Therefore, the entire responsibility for controlling wheat diseases should not fall solely on fungicides, but share the task with other practices that are part of the MID.

References

Arduim, FS; Reis, EM; Barcellos, AL; Turra, C. In vivo sensitivity reduction of Puccinia triticina races, causal agent of wheat leaf rust, to DMI and QoI fungicides. Summa Phytopathologica, Botucatu, v.38, n.4, p.306-311, 2012.

Klix, MB; Verreet, JA; Beyer, M.. Comparison of the declining triazole sensitivity of Gibberella zeae and increased sensitivity achieved by advances in triazole fungicide development. Crop Protection 26 (2007) 683–690.

Reis, EM; Zanatta, T. Efficiency of seed treatment with the fungicide triadimenol on the intensity of wheat leaf rust. Summa Phytopathologica, v.37, n.3, p.145-148, 2011.

Reis, EM; Zanatta, M.; Brustolin, F.; Danelli, ALD Sensitivity reduction of Blumeria graminis f. sp. hordei to triadimenol fungicide applied in barley seed treatment. Summa Phytopathologica, v.39, n.4, p.276-280, 2013.

This article was presented by the author during the SEMINAR ON FUNGI RESISTANCE TO FUNGICIDES IN WINTER CEREALS AND SOY, organized by OR Melhoramento de Sementes Ltda and held on December 16, 2015, at the University of Passo Fundo.