Fungicide use in the turf industry - prevention or cure?

The Impact of Climate Change on Weed Management

There is a direct link between Climate Change and Turf Weeds which have serious implications on turf management in Australia. Climate Change is going to affect herbicide efficacy and weed distribution.

Weed distribution will be affected due to the impact it will have on many plant functions of turf weeds. Being highly adaptable weeds will undoubtedly adapt to these changes, and this will raise a completely new set of management issues.

Increasing CO₂, together with higher temperatures and variable moisture conditions, will affect the growth and geographical presence of turf weeds, and alsothe efficacy of herbicides that you use to control them.

C3 vs C4 plants.

C3 turf weeds will benefit from rising CO₂ levels, but studies suggest, that rising temperatures will negate the beneficial effects of CO₂ on photosynthesis. Once temperature go over 25°C, this negatively impacts C3 plants because their photorespiration rate increases.

Photosynthesis of C4 plants is more effective at higher temperatures, but C4 plants are not usually not affected by increases in CO₂.

As CO₂ levels increase:

• C3 weeds will get bigger more quickly
• C4 weeds will grow at the same rate.

As CO₂ and Temperatures Increase:

• Growth of C3 and C4 plants becomes limited.
• Some C3 plants have a smaller treatment window.

Water Use Efficiency.

Drought favours C4 weeds because C3 plants have a lower water use efficiency.

Because C4 the water-use efficiency is better than C3 plants, C4 turf weeds will be more competitive than C3 plants under conditions of drought and increased temperature.

Table of C3 and C4 weeds

Survival Mechanisms.

Turf weeds can adopt three survival strategies to survive environmental change. They can migrate, acclimatise or adapt to the change in conditions.

Migration: As the climate changes weeds move from one place to another where the conditions better suit it.

We have already seen this occur in Sydney. Floods in 2023 lead to Buchan Weed moving onto playing fields that had never seen this weed before.

Acclimation: A change in climate leads to weeds modifying their phenotype, that do not pass from generation to generation.

Adaptation: Traits pass from generation to generation. A good example is ragweed which now produces greater amounts of pollen.

winter grass/annual bluegrass is also likely to become more of an issue in the future, as it increases in seed production, under increasing CO₂.

Increased CO₂ Levels and Herbicide Efficacy.

As CO₂ and temperature increases it will stimulate overall plant growth.

Increases in top and root growth

More rapid top growth means that the amount of foliar herbicide taken up by weeds is diluted, and so there is less herbicide to kill the plant9.

Bigger plants and increased leaf area results in less control with glyphosate in Rhodes grass and Paspalum at higher CO₂ levels.

CO₂ increases root growth, and root deeper into the soil profile at high CO₂ levels. This limits the uptake of pre-emergent herbicides, which stay in the top layers.

In Canada thistle increased CO₂ cause greater root and shoot growth and the failure of herbicide to kill roots and regrowth of the whole plant.

Physiological Effects.

As CO2 levels increase, it can reduce the diffusion of CO₂, water vapor, and O₂, through the stomata of a plant by up to half.

This means that foliar applied herbicides may give poorer results in the future.

High CO₂ levels also cause leaves to thicken, and this may reduce the efficacy of the post-emergents taken up by the plants..

Higher CO₂ levels stimulates weed growth, and reduces the time when many weeds are susceptible to herbicides. This reduces the window for herbicide application.

Increasing CO₂ stimulates roots more than the shoots. This means that weeds that spread underground vegetatively become more difficult to control.

Temperature and Herbicide Efficacy.

Temperature affects herbicide efficacy and impacts plant growth and development, and in turn the translocation and penetration of herbicides.

Herbicide movement and the viscosity of cuticle waxes are also directly affected by temperature, and so affect herbicide efficacy.

Higher temperatures increase the breakdown of herbicides, and so reduces herbicide performance on weeds.

This increase in the speed of breakdown, is because microbial activity increases as the soil temperature increases.

It’s not all bad.

In some cases, increases in temperature increase the absorption and translocation of herbicides.  Work shows that this may well be the case with the sulfonylurea herbicides like Tribute Selective, Duke Herbicide and Recondo Herbicide in the future.

Excessive Rainfall Events.

All pre-emergent herbicides require optimum soil moisture for movement within the soil and active absorption by plant roots. While dry soil conditions increase herbicide adsorption to soil particles, heavy rainfall immediately after the application may result in herbicide loss due to leaching.

For example, US work with the pre-emergent herbicide, Indaziflam that this tends to leach more with increasing rainfall.