Chilling effect of winter disease fears
The challenge of winter disease control is the greatest concern of greenkeepers and turf managers. In the latest survey by ICL, turf disease control was the top named issue for greenkeepers, with the loss of chemical controls coming second on the list.
New developments seek to find more reliable ways to get the most from the fungicides and programmes remaining available for turf managers and agronomists, reports Syngenta Technical Manager, Glenn Kirby.
Recent research into fungicide longevity has focused on identifying now much of an active is required to counter a specific pathogen threat. That can be influenced by how much is left from the previous application, along with the intrinsic potency of the fungicide applied.
As temperatures cool into the winter there are two factors that help greenkeepers in their disease control.
Firstly, most pathogens life cycle slows down at lower temperatures, which is the case for the most problematic infection for the time of year, Microdochium nivalle.
When M. nivale has been isolated and grown in laboratory conditions, the radial growth rate indicating rate of disease development was measured at 1.3 mm to 1.6 mm (av. 1.4 mm) per day at 2°C. But when incubated at 20°C it grew at 7.5 mm to 8.5 mm per day.
Furthermore, there are many different isolates of M. nivale, and constant evolution of the pathogen through sexual and asexual reproduction. With significant variability in the aggressiveness and impacts of different races, some have the ability to develop and go through its reproductive cycle faster at lower temperatures or with less humidity, for example.
Secondly, fungicides all naturally degrade after application and gradually reduce in efficacy. However, as a general rule the degradation is slower at lower temperatures, so they tend to go on working for longer. Photodegradation, which can be significant in the summer, is less on shorter, cloudy days. Microbial breakdown of chemicals that is faster at warmer temperatures is particularly relevant in soil applications, but would also appear to occur, to a lesser extent, to foliar applications.
Assuming the correct fungicide technology has been selected, a treatment that may be expected to last two weeks in a high-pressure October scenario, for example, could maintain high levels of protection for over five weeks in a typical January or February.
In temperature-controlled studies in the US, the concentration of a turf fungicide actives on the leaf had been shown to fall by 50-75% in 14 days a 30⁰C, compared to an almost linear 40-50% reduction at 20⁰C and 15-20% reduction at 10⁰C. After 28 days, the concentration of active remaining was just 15-20% at both 30 and 20⁰C, but 40-80% at 10⁰C.
The research has shown the rate of decline would vary for different fungicide actives and at different temperatures, but broadly follow the similar pattern.
Further studies have also suggested that the complex way disease pathogens interact and are affected by fungicides, both in and on the leaf, can also be influenced by temperatures and would have an impact on the efficacy of control.
And if we are looking for a third factor, which is a double-edged sword, whilst slower turf growth at lower temperatures means less fungicide would be removed with clippings, lower vigour means it is also more susceptible to the damaging effects of disease, which makes control even more crucial.
In addition to temperature, disease activity is indicated by the number of infective spores present and the prevailing conditions for its development. There is immense benefit in reducing the level of initial infection, through actions such as removing thatch where spores reside or lowering the pathogen loading through effective fungicide control earlier in the season.
Furthermore, integrated measures to make conditions inhospitable for the pathogen, including removing dew or improving air flow to dry surfaces, will slow its development and spread.
New research into turf stress management has also proven that mitigating the effects of stresses – through nutrition, irrigation and biostimulants, for example, can lead to healthier plants. Healthy plants appear better able to withstand disease pressure before any impacts are seen.
But the sheer number of disease spores present in the environment, combined with the speed at which it can proliferate, means that when long periods of leaf moisture coincide with mild conditions, infection can overwhelm the plants natural defences.
That has been exacerbated in recent years, with the trend towards at least periods of mild weather over the winter months. Last year, for example, the month of December overall was unseasonably warmer than March and April across parts of the UK.
Against this backdrop of increasing disease pressure and a reduction in the fungicide chemistry available for an extended window, it is essential to get the very best possible performance from the tools available.
Since one of the main factors for any fungicide performance is how much active is on or in the leaf, there is clearly benefit in starting with as high a loading as possible. With a winter height of cut of 4mm, for example, the leaf surface on a green offers a tiny target to hit, and retain, applied product.
For contact application that is the mainstay of winter disease control when the plant is not growing, the aim is to also coat all around the leaf. That requires the optimum droplet spectrum to cover the leaf, but also to avoid over-wetting the leaf where spray could run off and be wasted.
The Syngenta XC 025 nozzle, which has been specifically designed for foliar application, typically operated at 2 bar delivers 200 l/ha at 5 km/hr, witha droplet spectrum optimised for retention on the leaf (below, left).
The red XC 04 nozzle would apply a higher water volume to still give good leaf coverage and retention, as well as moving spray down to the crown where any systemic activity will be moved through the plant too (above, centre). The larger white XC 08 is designed for higer water volume and moving spray droplets down through the canopy, which is ideal fopr some products and targets, but typically less retention on the leaf.
The angle of spray trajectory from the rearward facing XC nozzles have also been shown to be especially effective, to counter the forward movement of the sprayer and target all around the leaves.
Avoiding drift, through nozzle selection and operation, is also crucial to maintain consistent coverage and ensure the maximum amount reaches the intended target.
Winter fungicide applications often have to be scheduled around other treatments, including dew dispersants. Their action, to remove moisture from the leaf, clearly has an implication for fungicide retention, particularly with a contact activity application. The advice is to avoid mixing the two products and, where possible, to apply the fungicide and let it fully dry and bond onto the leaf, before a subsequent application of a dew dispersant, or to make more use of switching to remove surface wet.
A well planned programme is key to making this strategy work and good disease forecasting and application window tools on the GreenCast website can be especially useful to schedule spray programmes, to ensure application at the most appropriate time to counter risks and threats.
Visit the page and click on the 'Disease risk index' tab below the weather information. Also view Live disease maps of risk areas
It is also worth noting that the fungicide’s formulation can have a significant influence on its physical performance. Firstly the blend of built-in surfactants and spreaders can dictate how much of the spray actually sticks and stays on the leaf, then as the spray dries the composition of the active will influence how smooth and complete the surface coverage remains.
Growing Degree Days (GDD) are increasingly being used as a guide for better timing of many turf management actions. The GreenCast GDD Calculator, to create a course’s individual intervals, has been consistently top of the new turf management tools on the website over the summer.
The use of GDD has been well proven with new research for Primo Maxx II application intervals on growing turf. Increasingly research is looking at evaluating GDD for fungicide timings, as a decision support coupled to environmental conditions.
But as temperatures cool, intervals can be artificially extended, particularly when a base of 6⁰C is used for the calculation. The base temp of 6⁰C is typically used in summer to reflect the onset of turf growth, yet in this instance we are looking at a different objective, to assess the longevity of a fungicide. Switching to a base temp of 0⁰C could potentially be a step to improve its effectiveness for winter fungicide interval support.
For example, reviewing last winter’s GDD with the GreenCast calculator, for Dartmouth in the south west to reach a target 130 GDD using a base of 6⁰C would have taken around 52 days at the end of November, but at base 0⁰C would be 17 days. The same period in Darlington, north Yorkshire, would show 113 days to reach 130 GDD using a base of 6⁰C. compared to 21 days at base 0⁰C.
As a methodology it has it flaws, but is still an improvement on a calendar-based approach and an approach that warrants further investigation.
Furthermore, since much of the international research is based on GDD with a base of 0⁰C, it becomes possible to make far better assessment and comparisons of how other trial work could be interpreted for UK implications.
There is a real value in tracking GDD for your course this winter, to gain experience in how the figures specially relate to turf growth, disease incidence and fungicide performance over the season.