With the winter barley harvest in Scotland almost complete, planning for the 2019 crop begins. Selecting seed of a good health status has always been important but emerging issues with fungicide resistance in the seed-borne disease loose smut illustrate why an over reliance on chemistry is sometimes not sustainable.
Loose smut is highly visual in a field as grain sites fail to form and are replaced by black fungal spores. It is typical of many seed borne diseases in that a few infected heads in one season can lead to high infection levels on surrounding ears and hence to massively increased infection levels if the daughter crop has not had an effective seed treatment applied. Loose smut is carried deep in the seed so is harder to target than some of the surface borne pathogens but until this season treatments such as Raxil Star with efficacy against loose smut on the product label have given good control. Disease control failures this season are in the process of being investigated by researchers and agrochemical companies but is appears that resistance to azoles and possibly SDHIs has emerged. How widespread the problem is is not yet known. It seems to be linked at the moment to one or two varieties but that is typical of the spread expected for fungicide resistance in a seed borne disease – resistance will have occurred as a spontaneous mutation and then spread outwards with the seed lot in which it started and from there will infect surrounding crops and may well become more widely entrenched.
The advice on seed health has always been to either buy certified seed which guarantees low levels of disease or to test seed that is for home-saving. But in reality because seed treatments were very effective it was tempting to treat crops with high levels of infection. This exerts a strong selection pressure on the fungicide. This year we would strongly advise testing for loose smut and not relying on chemistry to pull a poor seed lot up to full health. Testing for germination is a basic tool which lets seed rates be geared for optimal plant counts.
Product losses due to resistance development are becoming sadly familiar in barley, as are product losses due to legislative withdrawals. This is of course the last season where we will be able to use the neonicotinoid, chlothianidin. Seed treated with chlothianidin will reduce the risk of BYDV and also of wireworm damage. Treated seed can be sold up until the 19th September and must be drilled before 19th December – which would be well after good establishment would be expected anyway.
Fiona Burnett for the Farm Advisory Service
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