Development of a rationale to identify the causal agent of necrotic lesions in spring barley and to identify control mechanisms

Abstract

In 1998, spring barley crops in the north of Britain died back before they had reached their potential yield. The reason for the problem was associated with barley leaf spots. Barley leaf spots are caused by a complex of fungi which include Ramularia collo cygni and Leptosphaeria nodorum  (biotic spots) and oxidation damage (abiotic spots), which may be associated with the mildew resistance gene (Mlo gene).

Yield losses and increased screenings are caused by early loss in green leaf area retention on the top two leaves once the crop is starting to flower. Yield losses of 0.28 tonnes/ha were recorded in 2001, but additional losses in quality, in particular screenings can result in poor grain prices for affected crops.

Green leaf area loss is a better measure of damage than a high level of spots, since spot levels may be low if Rhynchosporium has already attacked the top leaves. Late development of spots on green leaves are also unlikely to affect yield.

Varieties vary in their susceptibility to biotic and abiotic spots and the variety Chariot remains the weakest for biotic and abiotic spots. The variety Pewter is more prone to biotic spots than abiotic spots, but no variety is fully resistant to both types of spots. Varieties do show better resistance to abiotic spots than biotic spots however.

Spots develop a few days later in late maturing varieties, so comparing early and late varieties on the same day may give a misleading picture of their susceptibility.

Areas in northern Europe where spots have occurred is on the increase and once established there has not been a season where spots were not important. Currently the problem is more severe in the north of Britain than in East Anglia, but levels of abiotic spots were higher in East Anglia in 2001 than in previous seasons.

Dull wet weather during tillering followed by normal sunny weather at head emergence can result in both biotic and abiotic spots. Rainfall during the tillering and head emergence stages may be more important for biotic spots.

The application of a protectant fungicide comprising any strobilurin fungicide + Opus (epoxiconazole from BASF) at boot stage (Gs45-49) can provide effective protection from spots and maintain green leaf area.  Fungicides applied before flag leaf fully emerged (Gs39) will not provide effective protection against leaf spots, and fungicides applied after spots have appeared will not be effective.

Higher fungicide doses will provide longer periods of protection, but the equivalent of 0.4 - 0.5 dose of strobilurin plus 0.4 l/ha Opus is recommended. Unix 0.4-0.5 kg/ha (Syngenta)  + Amistar 0.4-0.5 l/ha (Syngenta) is an alternative fungicide at boot stage particularly where severity of spots is low.  Chlorothalonil produced an effective delay in spot development but did not achieve the yield benefit seen from strobilurin fungicides.  Corbel (BASF)  applied at boot stage can reduce green leaf area so should be avoided unless Rhynchosporium or mildew are present at the time of treatment.