Rhynchosporium leaf scald in barley
Rhynchosporium is a disease that is so common in our barley crops that it is probably assumed that we know everything there is to know about it. But it is a highly variable fungal pathogen and it is only fairly recently that it was recognised as a mix of five fungal species, so the form that infects barley has now been renamed Rhynchosporium communae, rather than Rhynchosporium secalis as it was previously. R. secalis is now recognised as the species specialised to rye and tritcale. We still don’t fully understand how it is transferred and spread but it is really well adapted to our barley cropping and carries on seed and in trash and there are suspicions of a sexual borne stage. Young plants are infected from seed or trash and the disease then splashes upwards through the foliage causing the classic scaled type symptoms. That association with rain and wet conditions explains why it is worse in wetter areas of the country and in seasons of high rainfall.
Yield losses can be severe – sometimes up to 40% of yield potential. In winter crops Rhynchosporium can establish in autumn and already be present in the crop when spring sprays are applied. In spring barley crops there is always a quandary about how to treat when they appear clean at stem extension but where there is a risk that Rhynchosporium will appear before the T2 timing when it will be too hard to control. Putting together a management strategy should involve thinking about your past experience of the disease – if it has been common then selecting varieties suited to your planned market but with good Rhynchosporium resistance is a really sensible starting point. Examples of recent breeding improvements for Rhynchosporium resistance would be the ‘7’ ratings for winter barley feed varieties like Volume or Surge which compare favourably to the ‘4’ ratings for others such as KWS Cassia or KWS Glacier. For winter barley malting varieties then a variety like Craft carries a ‘6’ rating. There have been similar improvements in the spring crop with a variety like Laureate rated ‘6’ which compares well to Concerto at a ‘4’ rating.
Fungicides can give some protection but more limited eradication so on the winter crop a T0 at mid tillering may be needed before the main spray at T1, the start of stem extension. The T0 should be on a case by case basis and only applied in high risk situations and where the disease is already noted. Fungicides with efficacy include azoles, SDHIs, cyprodinil, chlorothalonil, morpholines and the strobilurins so there is opportunity to mix and match fungicides in the programme, providing stewardship against fungicide resistance development. Similarly it is possible to alternate actives and to different actives between spray timings. This is really important when thinking about Rhynchosporium as, in common with many other cereal diseases, there are pressing issues with resistance. Older azoles have a history of declining in efficacy against Rhynchosporium and prothioconazole is demonstrably better in SRUC and other independent trials than older azoles such as tebuconazole and epoxiconazole. There are also concerns over the strobilurins. Mutations conferring resistance are noted sporadically but over the last two seasons have been reported more consistently.
Fiona Burnett (SRUC) for the Farm Advisory Service