Watch out spring barley growers. Ramularia is not confined to sugar beet. David Millar reports
RAMULARIA just didnt feature in the UK barley growers list of potential disease threats until 1998. Now it is right at the forefront for those whose crops were mauled by the fungus.
It is now becoming clear that this disease, previously thought to impact mainly on sugar beet, has been around in the complex of fungi affecting cereals a lot longer than most people thought. Certainly it is now being blamed for extensive yield losses in Scottish and Irish spring barley crops in 1998, and implicated in problem crops in Lincolnshire and other parts of England.
Keith Dawson, technical director of CSC Cropcare, is more familiar than most with ramularias effect on spring cereals although, like everyone else until last year, he was in the dark about what exactly had caused the pollen scorch or stress first seen by CSC in trials in Angus six years ago. An ELISA test at the time on samples from Chariot and Derkado came up positive for Septoria tritici – a member of the same Mycosphaerella group as ramularia – but the effect had diminished in the following year.
It reappeared in the late summer of 1997 as late leaf tissue senescence but the brown leaf blotching came back with a real bang in 1998 in Scottish commercial crops. "It carries a triple whammy of yield loss, screening increases and grain nitrogen increases to create a multiple effect on gross margins," says Dr Dawson.
The blame was finally pinned on ramularia during an international conference for plant pathologists held in Edinburgh during the summer. A group, including experts from New Zealand and Germany, were invited to look at CSC trials and immediately thought they recognised the symptoms of ramularia from experience in their own countries.
Under UK conditions it appears that the coincidence of wet weather with pollen release helps create a microclimate in the spring barley crop which favours the disease and provides a food source in the shape of pollen landing on leaves. The disease tends to like cooler, moister climates so Scotland and Ireland are ideal.
However, CSC had noticed from its 1997 trials that the then mystery disease had a varietal effect, and that there now appeared to be an effect from strobilurin application missing from the previous affected trials in 1992 when only a triazole plus morpholine mixture had been available.
"On three sites we had a problem with ramularia to varying degrees and it became evident that the strobs were having an effect on green leaf retention," confirms Dr Dawson. "It wasnt perfect but we were getting 90% plus control with azoxystrobin while both famoxate and kresoxim-methyl were also giving good results."
Timing and dose rate appear important, he adds, and azoxystrobin (Amistar) and famoxate appear most promising of the new fungicides. Enhanced control was seen with azoxystrobin used with the adjuvant Arma. However, there is no eradicant effect from these fungicides so it is important to spray for protection before GS50 when the symptoms are most likely to appear.
Plant populations appear to have an influence on ramularia infection because of the differing microclimates that will be created. However, there appear to be quite strong varietal responses to ramularia establishment. Derkado and Chariot were particularly affected in 1998 and responded strongly to treatment with a strobilurin.
Prisma and Maud, which respond well to strobilurin treatment against other diseases, did not do so on the ramularia sites and are probably more resistant to the infection. Treatment brought responses (see table) in newer varieties such as Landlord, Chalice and Optic.
Dr Dawson believes growers will have to consider the possibility of a further ramularia infection when choosing spring barley varieties this year to make up the shortfall in winter barley drillings last autumn. In a very competitive end market for grain this autumn, quality could be paramount.
With pressure on seed sales for spring sowings there will inevitably be many growers who can only obtain varieties susceptible to ramularia infection. "Hopefully it will be a dry year with less of a problem but ramularia overwinters on stubble and straw and is spread by rain splash," points out Dr Dawson. "There is potential for a widespread inoculum source."
Dr Dawson believes ramularia is a major component of the leaf blotching symptoms but not necessarily the total cause. Further trials this year could help cast more light on the complex, as well role of pollen production and flowering habit in producing the varietal differences seen so far.