The availability of active ingredients and commercially available products for use on amenity turf is changing at a rapid pace. Joe Kinder, Technical Manager for Agrovista Amenity, assesses the situation and provides some valuable advice.
Many fungicides have already been revoked and others are expected to disappear with an unknown number of replacements being introduced in coming years. Consequently, it is now more important than ever to understand the mode of action of active ingredients and when to apply them to achieve effective disease management throughout the year. A preventative approach is strongly recommended and remains key to a successful disease control strategy.
How do preventative fungicides (protectants) work?
Essentially a preventative fungicide works by forming a chemical barrier on the leaf but there may also be activity on the surface and in the thatch layer as well. They are not absorbed and will not therefore be translocated from the point of application to other parts of the grass but instead, act to inhibit fungal germination or penetration close to where applied.
There is no ‘kick back’ activity once infection has occurred. A preventative contact fungicide that remains outside of the plant, such Medallion (which contains fludioxinil) will be exposed to environmental factors, such as rain, irrigation and mowing. Therefore, it might only be active for a short period such as 7-10 days. Consequently, this makes fludioxinil, or Medallion, a more appropriate fungicide to use during the winter months. If a contact fungicide is applied after the spores have germinated and the fungus has grown into infected grass tissue, it will be totally ineffective at controlling turf disease.
Strobilurin
The strobilurin class of fungicides will also act to prevent disease. Products such as Heritage (containing azoxystrobin) or Pan Aquarius (containing trifloxystrobin) will stop both spore germination and host penetration but have limited or no effect, once the fungus has successfully penetrated or colonised host plant tissue. However, there are differences within this chemical class on their level of systemic movement and effectiveness relative to the life stage of the disease that is attacking the grass.
Pyraclostrobin is a locally systemic strobilurin that is taken up by the plant but does not move far beyond the point of uptake. In contrast, Heritage (containing azoxystrobin) is also systemic beyond the point of uptake and will inhibit spore germination and development, so it is also effective at the start of infection as well as pre-infection. Its activity can be classed as broad spectrum, systemic, translaminar (movement from the upper sprayed leaf surface to the lower) and therefore, protectant but with some curative ability as well.
Pan Aquarius (containing Trifloxystrobin) is a strobulurin with mesostemic activity (translaminar and vapour movement but lacks systemic movement in the plants transport system). This fungicide is strongly attracted to a plant’s surface and is absorbed by the upper waxy plant layers. It appears to continuously penetrate the leaf surface. However, as it is not translocated in the plant vascular system (xylem or phloem), it is not classed as truly systemic. These fungicides redistribute on the plant surface via localised vapour movement and surface moisture and are therefore, mostly protectant but with some curative ability.
This group of fungicides also work best when applied as a preventive. Moreover, they are not directly exposed to weathering factors like a true contact, such as fludioxinil. They are effectively killing germinating, spores but you may get poor results when applied curatively because of a strong affinity for binding to the cuticle of the leaf and limited systemic movement. Consequently, the dose of active ingredient, present inside the leaf blade, may be too low to suppress the fungi within the leaf and the disease will still express itself.
How do curative fungicides work?
This type of fungicide is considered to have anti-sporulant activity which helps to slow disease development by limiting the reproductive potential of the fungus. A curative fungicide must be able to penetrate, into the turf, move systemically and be able to selectively kill the invading fungi. They are designed to target specific enzymes or proteins made by the fungi.
DMI
A Triazole, such as tebuconazole is classed as a DMI systemic fungicide but tends to be found in combination with other active ingredients, rather than marketed as a single active ingredient.
Tebuconazole when formulated with trifloxystrobin, found in products such as Dedicate and Dualitas, provides systemic and translaminar activity with preventative and curative effects. Tebuconazole’s systematic action is effectively working to prevent and eradicate fungi. This chemical compound specifically eliminates fungi by inhibiting their ability to spread spores, which slows growth.
Localised Systemic (no movement in the xylem or phloem)
Fungicides that fall into this group move only a short distance from the point of penetration of the leaf surface. In addition to pyraclostrobin, the dicarboximide fungicide group and the active ingredient, iprodione (no longer available for managed amenity turf), is a further good example of a chemical with this activity. These fungicides do not enter the xylem or phloem tissue and therefore, this fungicide remains mostly on or near the plant surface.
Disease pressure and timing
If latent disease pressure is high but also if disease is present and visible, it is worth emphasising under most circumstances the visible symptoms may get worse before they get better. This is due to disease and life cycle stages. A fungicide will work at certain stages of disease development but if grass is sprayed too late, or with incorrect fungicide, the disease will have to express itself before it can move into a stage where it can be controlled. Sporulation, for example, onto turf sprayed with a protectant application of fungicide, such as Medallion (containing fludioxynil), would then be able to suppress further spread into unaffected turf.
Environmental factors
Dew and rain, when spraying with any fungicide, or too much moisture on the leaf, can over-wet and cause run off. This will result in severe under-dosing of the product. Applying products before rain can cause the same problem. The product must be dry on the leaf for the manufacturers label to be truly effective. An adjuvant specifically suitable for classes of fungicide can increase efficacy in these conditions.
Adjuvants
Fungicides are formulated with chemical compounds, but they may also be mixed with a supplementary, adjuvant to enhance the performance of the product on the turf. Indeed, adjuvants can be used to target specific sites of application; increasing the speed of uptake by improved spreading effects on the leaf for example or they may have a broad spectrum of activity, such as acting to reduced surface tension, pH buffering and lessening runoff.
Summary
When selecting a fungicide, to maximise the potential of the chemical, it is important to consider the disease stage before application. In this regard, consideration of the ability of the chemical to act as a preventative, curative or to eradicate is strongly advised. Many fungicides claim to possess one or all three abilities but their ability, will also be affected by their mode of action and the disease pressure at the time of application. Avoid fungicides when disease stages are beyond the ability of the active ingredient to make a meaningful impact.