Grey Leaf Spot - Learnings from across the pond
Grey leaf spot has now been identified in stadium turf situations in a number of instances in the UK. As one of the new disease pathogens attacking turf, that doesn't make it an epidemic or public enemy number 1, reports Syngenta Technical Manager, Glenn Kirby. However, it is now highly important to be aware and fully understand the pathogen and the risks. Learning from previous experiences of the US could help UK turf managers to be better prepared to tackle its threat.
The rise of grey leaf spot issues in the US coincided with a transition from Kentucky blue grass, to the growing of finer perennial ryegrass species. Within a few years, the pathogen developed rapidly so, by the mid-90s, whole fairways across the mid-west were reported as being lost within a few days of infection onset. Treatments for pythium, at the time, proved largely ineffective.
Timing of infection on golf courses has been associated with late summer and early autumn, particularly through periods of heat stress and drought. However, on perennial ryegrass in particular, it can continue to linger on right through to the first significant frosts and manifest as really damaging situations.
The implication in UK stadium turf, where frosts are, for the large part, non-existent is that it could result in prolonged attacks over the autumn and into winter, with limited opportunity to recover.
Research into grey leaf spot development in the US highlights the microclimate created within stadium conditions could prove near perfect for the pathogen.
High risk conditions typically occur with continuous leaf wetness for nine hours at a relatively high temperature of 28 to 32⁰C, but even at a cooler 20 to 23⁰C it will develop if leaves are continuously wet for 21 to 36 hours, for example. At pitch level in stadiums, such temperatures can occur at almost any time.
Where infection has previously occurred, even at relatively low or non-damaging levels, the pathogen can remain dormant as mycelium in infected leaves and on plant debris - waiting for conditions conducive to development.
Early identification is important, as it may give the chance to control the pathogen before it spreads too far (see panel: Grey leaf spot symptoms).
Disease lesions produce conidia, or mitospores, during periods of relative high humidity and moist leaf surfaces. Research in the US has also shown that alternating cycles of wet and dry leaves over time are ideal for spore production and new tissue attack from existing infections.
Trials and observations have established the pathogen is more prevalent on ryegrass managed with high levels of nitrogen fertiliser, as well as turf mown at a relatively high cut level. Newly established turf is also far more susceptible to attack, compared to mature swards.
Unfortunately, all those agronomic risk factors are inherent with modern stadium management strategies and repeatedly resown stadium turf in the UK.
Some mitigating management practices that could help reduce risk would be to water thoroughly, but as infrequently as possible, to allow the leaf surface to dry. Using controlled or slow-release nitrogen nutrition could limit more susceptible growth. When disease is active, mowing when the canopy is dry and removing clippings could also help to limit infection spread.
The challenge would be to integrate those agronomy regimes within the intensively managed stadium environment, or at least to be super aware of drivers for disease and to protect turf with a well-timed fungicide strategy.
In the US, turf managers have a range of fungicide solutions that researchers there have highlighted show excellent activity on grey leaf spot. They utilise programmes with fludioxonil (Medallion TL), azoxystrobin (Heritage) and difenoconazole + Solatenol (Ascernity).
For susceptible turf, the best practice recommendation is to use systemic fungicides on a preventative basis. In curative control situations, however, advice is to tank-mix an effective contact and systemic fungicide, applied at the shortest interval permitted on the label.
Ascernity contains both the systemic and contact activity. It also has effects on disease in organic matter, which has been seen as a real positive for control of grey leaf spot in the US, as well as other turf diseases such as brown patch, along with the key microdochium patch.
Research in the US, undertaken with Rutgers University, on ryegrass showed natural infection of grey leaf spot that hit 95% of untreated turf, was held at just 7% leaf infected with a Heritage programme on 14-day interval.
Furthermore, the same trials team showed that Solatenol in Ascernity was even more effective as an active. Trials in 2020, on a recently established perennial ryegrass sward, overcame an extremely high-pressure late summer situation, to see infection on the treated area checked to less than 4% by early-October, compared to 90% affected in the untreated area. By the end of the study, Ascernity was reported as the top performing fungicide in the trial.
With disease pressure building in ever more intensively managed stadium turf situations, along with ever higher expectations for turf quality throughout the season and shorter periods from renovation to play, learning to manage grey leaf spot could be an essential new skill.
Grey leaf spot symptoms
Disease first exhibits as grey lesions on the margins of the leaf. Those lesions may show a yellow halo effect, along with twisting of the leaf tip that turns brown. Furthermore, multiple small dark-brown lesions may also be present.
Within days, turf may take on a purple hue, as a multitude of spores are released - particularly in humid or early morning conditions.
Rapid turf die back may be more typically associated with drought conditions, only faster and non-recoverable. In a mixed grass sward, the non-ryegrass species may remain green and healthy.