Mark Hunt weather corner; 116 - With humidity comes increased risk of disease
As the atmosphere warms, its ability to hold moisture increases. It is an oft-quoted fact that for every 1°C increase in air temperature, air can hold 7% more moisture.
As humidity is a measure of atmospheric moisture content, it figures that we will see higher humidity air more often going forward if climate projections are accurate.
I have some Night Scented Stock (Matthiola longipetala) in my garden and they brighten up my summer border every year. They are, however, very sensitive to Downy Mildew and every year (when humidity starts torise), I see the familiar blemishes on the leaf. Now, I treat it with a foliar spray of micronutrients including manganese, zinc and iron that I know work well on foliar pathogens, but this year it has required repeated sprays to keep it clean.
A more humid environment means potentially longer periods of leaf wetness. During this summer, using data from a Davis Leaf Wetness sensor (pictured above), we have already recorded successive days of >6 hours of overnight leaf wetness in conjunction with temperatures in the high teens and low twenties.
This is a formula for fungal pathogen development and not just the usual suspects of Microdochium, Red Thread (Laetisaria fuciformis) and Dollar Spot (Clarireedia homeocarpa). Some of the more less common turfgrass pathogens in the U.K & Ireland have a clear line relationship with periods of high overnight temperature, humidity and leaf wetness. Rhizoctonia diseases like Brown and Yellow Patch to name but two.
The march of Dollar Spot as a pathogen across continental Europe into Scandinavia and northwards, and westwards into Scotland and Ireland from the south of England, is I think clearly associated with a changing climate. Changing with respect to increases in overnight humidity, temperature and leaf wetness as warm humid air is pushed up from southern Europe on peaks in the jet stream. Low pressure systems that track up from the south (rather than the south west), as I have commented upon before, are another likely culprit.
One other factor independent of climate is the respective length of day and nights as we move past the summer solstice in June. Once we get to the end of August, the duration of night-time increases significantly and, with it, the probability of extended periods of plant leaf wetness / dew. So be on your guard!
Consider the Smith Kerns Dollar Spot Probability model that utilises a 5-day average of temperature and humidity to predict the probability of Dollar Spot (and other foliar pathogens).
Below is a graph of the 2024 Smith Kerns Dollar Spot Probability from a golf course in Oxfordshire. As a rule,once the probability increases over 20%, we can expect to see more activity from foliar pathogens, although in my experience, the point where you actually see Dollar Spot as a pathogen for example may not be to 35 or 45% dependent on a lot of local factors, including local topography (shade), grass species, cultural practices, etc.
Applying a management strategy is a matter of knowing when your probability is increasing (rate of increase of Smith Kerns rather than just the % probability in isolation), and then implementing specific cultural practices like rolling, applying dew dispersants and manual dew removal (dragging hoses across outfields for example). In some cases, applying preventative fungicides or turf hardening sprays is also warranted. Choice of grass species / cultivars in the case of Dollar Spot will, I believe, become more important in the future in terms of a IPM strategy.
One thing is a for certain, judging by the state of my Night Scented Stocks, it is going to be a long summer and autumn!
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