Disease in winter cereals, May 2019

Winter barley

Awns are peeping in the most advanced crops, with flag leaf peeping in a wider proportion. The dry weather has helped reduce the pressure from rhynchosporium so, where crops look clean, fungicide options at booting to ear emergence will likely be at the lower end of the dose ranges given below. But do keep in mind that stress is one of the major triggers for ramularia and the last thing you want is a flair up of rhynchosporium, net blotch or mildew to tip things off. To manage ramularia, chlorothalonil will be the norm rather than an optional extra for any crop at risk of ramularia, which in reality is pretty much any winter barley crop at a site where ramularia has been noted before. Other fungicides will, of course, have a space to manage any other lingering diseases from T1 and rates should increase to reflect the severity of what you are seeing.
This is the last season where we will have chlorothalonil as an option – the use up period is still unclear but next May is at the long end of the use up estimates, and it could be shorter. With robust varietal resistance still some way off, unfortunately, we are just going to have to carry the additional yield loss and uncertainty that ramularia represents.

Winter wheat

Crop growth stages are a bit variable so some T1s have been on a while and some crops have still to be treated. Yellow rust is out and about but generally not at desperate levels and as crops are now pretty widely protected by spray programmes. Hopefully, a lid can be kept on this risk. Crops should grow rapidly now which shortens the window to the flag leaf (T2) spray.

The message about hanging back and waiting for leaf three to fully emerge on the main tillers has largely been heeded, but on crops where the T1 was a little early keep an eye out for anything sneaking in on final leaves 2 and 3, especially where the interval to flag leaf sprays edges beyond 3-4 weeks. An inserted spray to target leaf 2 (the so-called T1.5) shouldn’t be the game plan, but on a few crops it might be needed if yellow rust gets in, or if septoria levels are very high and conditions conducive. Speak to your adviser if that’s the case to talk over options.

Septoria levels are still quite low for the time of year. The current wet weather will help to splash what is there up the crop but temperatures are still low so lesion development will stay slow for as long as that lasts. Things will warm up and speed up but that slower development rate does help when it comes to thinking about the kick back of flag leaf sprays and their ability to manage septoria levels of the flag leaf and leaf two. In a warm year leaf two is often very exposed as the septoria inside the leaf can be way beyond what the flag leaf fungicides can kick back against.

Managing ear disease

Scotland’s long day lengths aid the high yield potential of wheat crops, and retention of green leaf area is key in maximising crop potential. Fungicides applied to the ears of wheat therefore have a dual purpose as they top up earlier foliar disease management but also help to reduce the risk of ear diseases. The ear diseases themselves split into two camps – those that limit yield and those that can limit yield but also cause quality issues through the potential production of mycotoxins. It is essential that mycotoxin risk assessments are carried out for all wheat entering the food chain. The risk assessment is available online and reproduced below. It captures the main drivers of fusarium risk which are wet weather at flowering and then wet weather over grain flling and ripening.
This should be started before flowering so that you can include the risk in your T3 fungicide plans (a nice example of IPM practice) https://cereals.ahdb.org.uk/mycotoxins
A record should then be kept of rainfall at flowering and over-ripening as this is the main driver of risk and accounts for almost 60% of the maximum risk score that would be possible. We are at inherently lower risk in Scotland because many of the mycotoxin forming Fusarium species like warm temperatures, but many do occur widely in Scotland so we shouldn’t be complacent. Trash is another risk factor so direct drilled crops are at slightly greater risk.

Winter oats

Mildew levels are creeping up in some crops and although mildew is not the most damaging of disease to yield, it is very visible and in some areas can creep up the crop canopy.  Speak to your agricultural or agronomist adviser.