Leaf wetness & disease – Its no fairy tale

Leaf wetness has a huge part to play in disease occurrence, it and temperature are two of the biggest driving factors when it comes to likelihood of disease outbreak in the sward, so as we look to remain in this eternal autumn its important to have a plan in mind.

Leaf wetness for golf courses

I’m conscious I’ve been talking about Leatherjackets a lot lately, so to anyone who isn’t interested or who hasn’t had problems, I’m sorry for that.

Here I’m going to try more generally to talk about leaf wetness. It being November, leaf wetness for most will bring up thoughts of Microdochium patch. But more generally the presence of moisture has an important role in the spread of many fungal diseases turf and other.

Think athletes foot – you wouldn’t easily get ‘Athletes forehead’ as, whilst you still shower your head and your head may sweat when exercising, the moisture doesn’t sit for long on the forehead, so its not a very hospitable environment for the fungus to develop. The feet on the other hand remain moist and sweaty for much more time in the day allowing the pathogen to thrive.

For anyone NOT eating (its pretty graphic) here’s the link to the NHS information page on Athletes foot.

Moving back to the safety of turf management diseases swiftly! All of the major turf diseases we deal with in the UK and Ireland are fungal, and fungal pathogens need moisture.

To read up on the most common turf diseases the Turf Disease Identification Guide is free to download (or request a hard copy if that’s your bag) and a great training aid for any staff just starting out in the industry.

The major factors which are going to drive fungal turf disease are as follows:

  • Temperature
  • Leaf wetness
  • Plant health
  • Presence of pathogen

 

Not all of these we can easily effect, in fact three of the four are pretty hard to control in an outdoor environment. The last one is virtually impossible to meaningfully impact.

If you had the perfect fungicide that eradicated all of a fungal pathogen in an area (please note this product doesn’t exist) if you waited until the product broke down you would get reintroduction of the diseases we all know and love again. The spores are so small and robust they can be carried by wind, on the feet of birds, golfers, seed, water droplets…. the list goes on. So the aim is generally to control not eradicate.

For some diseases we may have the perfect growing conditions already, and we just don’t have the pathogen yet (not many in this camp – but Gray Leaf spot – pathogen of Ryegrass in stadia environments is one).

But for most, the pathogens are in the wider environment (unmanaged surrounding areas) and throughout the soil profile so if your target is complete eradication, then insanity may be your destination. You can manage spore populations with the right preventative program.

I’ll touch on temperature and plant health as those are important factors to think about at another time, plant health is where I believe much of our focus needs to be – healthy plants are much less susceptible to disease.

Leaf wetness

Fungi don’t need any additional water to spread within an infected plant, once they are in a plant their is enough of everything they need to develop and thrive. For some diseases like Anthracnose the pathogen can be in a grass plant for a long time laying dormant, waiting for the right conditions to develop.

Leaf wetness becomes more important when the fungus wants to breed and spread, generally speaking fungi are not as robust as plants in the outside world so they need optimal conditions to develop. If everything else is just right in ‘the formula for fungal growth‘ temperature is where it wants to be, pathogen is present, and the turf is worn out and stressed; then disease may start to take hold, generally this will start with those legacy infected plants and perhaps spread to a few neighbors, you probably wouldn’t see any change at this stage. But this really kicks on when leaf moisture enters the game.

As we can see with those dew drops above, the water on the leaf offers ideal conditions for fungal pathogens to spread.

The life of a spore is an odd one. Pretty robust as a spore (think tiny version of an oaks acorn, takes a squirrel or stamp from a boot to do damage to it) but once the spore lands on a leaf it starts to germinate. The browny-gray thing you can see above is the spore and the thinner section coming out to the right is it searching for an entry into the plant. That thinner section is much more susceptible to drying out than the spore pre-germination, and so not making it into the host. So in the absence of adequate moisture (can be high humidity in the sward canopy, not necessarily physical dew drops) it may dry out before it spreads.

Water can also physically move spores to neighboring plants when it runs, and help spores stick to new targets and allow them time to get inside.

It’s important to have an integrated plan (ITM) for disease pressure this forever Autumn season and strategies for managing leaf wetness should be an integral part of that to reduce disease pressure.

The table below gives a flavour of the leaf wetness and temperature changes between October and November over the UK & Ireland, remember that these are averages and site specific differences – like aspect, prevailing wind direction and microclimate will mean that each site will be different.

For more information on leaf wetness and disease see the Weather to spray article.

And don’t forget to have a listen to the latest On the Horizon podcast which also covers leaf wetness.

 

 

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