Apply your FUNGICIDES WISELY

MARYKE CRAVEN, ARC-GRAIN CROPS INSTITUTE, POTCHEFSTROOM

Much has been written in the past on the various diseases that impact negatively on crops and the importance to correctly identify the problem pathogen before initiating spraying programmes.

Bacterial leaf streak and even sunburn damage have proven to be very deceptive in this regard, and care should be taken not to attempt to control them with fungicides.

Once the pathogen has, however, been correctly identified and is indeed a fungus, fungicides should be administered with great care. The effective control of plant pathogens within a plant production system is a critical component in the objective of achieving optimal yields.

It is therefore required that fungicides are administered to such an extent that the optimal yield is obtained. Optimal control is, however, not only required to ensure that financial expenses are justified, but also to ensure that fungicide resistant fungal populations do not occur in the process.

A lack of knowledge on fungicides and how they work, could therefore not only result in financial loss through ineffective sprays, but could also create a situation in a production system which will be extremely difficult to control.

Fungicides differ from each other in various ways. Knowledge about the strong, but also weak points is essential for effective control and also to ensure that resistance build-up against the available fungicides does not occur.

The cornerstone of fungicides is their active ingredients

Fungicides are classed with regard to their mode of action, which in turn is determined by the active ingredient. A very simple explanation as to how fungicides work is that fungicides disrupt the metabolism of the plant pathogen. This disruption impedes the development of the pathogen or results in the death thereof.

Various fungicides are available, each with different active ingredients. These active ingredients work in at different points of the metabolism pathways of fungi. Some active ingredients are very specific in their mode of action and will for example block the activity of a specific enzyme or groups of enzymes. Others will inhibit various metabolic steps, which mean that they are less selective with regard to the fungi they influence.

Contact versus systemic fungicides

As contact fungicides cannot penetrate plant tissue to reach existing fungal structure, it can only protect the plant where it has been applied.

Time of application is critical as such chemicals must already be on the plant surface before the pathogen has infected. In addition it is necessary that the fungicide spray is administered as a uniform layer on the plant surface to ensure effective protection.

Contact fungicides normally have a broad spectrum activity which minimises the risk of a pathogen developing resistance to the fungicide. The negative aspect of this fungicide is that it will only be effective against the first phase of the development of the pathogen. Once the pathogen has been successful in infecting the plant tissue, the fungicide will have no effect on the pathogen, and the pathogen will continue to cause disease.

New plant growth that develops after the initial spray, will not be protected and more regular follow-up sprays will be required (compared to systemic fungicides). Climatic conditions as well as the plant growth stage are important factors when follow-up sprays are considered. Contact fungicides administered to leaves; provide protection for between seven to ten days. Maneb, chlorothalonil and mancozeb are examples of contact fungicides.

Unlike contact fungicides, systemic fungicides have the capability to penetrate plant tissue and have some degree of mobility as they are able to move within the leaves. Some of the systemic fungicides, such as azoxystrobin, benomyl, carbendazim, etc also move via the xylem to the upper plant parts.

They are capable to express their activity within the plant tissue and are selective in their mode of action. Only a specific group of related fungi are affected by these fungicides. Triazoles are less specific systemic fungicides and have a wider activity spectrum.

Triazoles, such as difenoconazole, cyproconazole and propiconazole, demonstrate translaminar as well as upward movement, but the degree and speed of the movement differ. Depending on factors such as climatic conditions as well as the developmental phase of the plant, an average of 10 to 21 days of protection can be expected.

Corrective versus preventative

Fungicides are accordingly also classed based on the stage at which they affect the pathogen. Preventative fungicides prevent infection and the establishment of the pathogen. All fungicides have some form of preventative action. As contact fungicides are unable to penetrate the plant, they are viewed as preventive fungicides.

Some systemic fungicides have corrective qualities, which imply that they have the capability to stop infection that occurred a few hours or days before the spray was administered. The fungicide will not be able to control the disease if the fungicide was administered too long after infection occurred and if the degree of infection is too high.

Resistance management

Various reasons exist for the ineffectiveness of fungicides, of which the too early or too late application is very common. Other possibilities include wrong application concentration, irregular application as well as the uneven covering of leaf surfaces. When everything has, however, been done correctly and insufficient control is still obtained, consideration should be given to the possibility that a fungicide resistant fungal population is present.

Continuous use of the same class of active ingredients can result in resistance build-up of the pathogen. This resistance can occur as a result of genetic change that occurs in the cells of the pathogen. Such genetic mutations will initially be present in only a small fraction of the fungal population in a specific region. When fungicide is applied, this small population of resistant fungi will survive and multiply. Resistant populations will therefore always start out small and will take years before their presence is detected.

Various degrees of resistance against fungicides exist. A fungus can for example only show resistance against an active ingredient of a specific fungicide. Cross-resistance refers to a greater degree of resistance that implies that the fungus demonstrates resistance to all active ingredients belonging to the same family (e.g. triazole or storbiluringroup). Multiple resistance refers to resistance against various fungicidal families. An example of such a resistance is when a fungus is not effectively controlled by either the triazole or storbilurin families.

Care should therefore be taken with the use of fungicides to ensure that a situation is not created that result in an unmanageable disease. Spraying programmes should be structured as such that they consist of a variety of active ingredients with different modes of action. (e.g. the use of both systemic and contact fungicides).

Unnecessary applications at insufficient dosage rates should be avoided. Not only will this result in financial losses, but it will also contribute to the generation of fungicide resistant populations. Other agronomical approaches such as the implementation of crop rotation with nonhost crops as well as the use of resistant genotypes will also assist with the management of the disease and ensure that resistance buildup does not occur.