Septoria leaf blotch is the most important foliar disease of wheat in the UK. It is caused by Zymoseptoria tritici (previously known as Mycosphaerella graminicola and by the previous asexual stage name, Septoria tritici). Yield losses of 30-50% have been reported in susceptible varieties. The disease is initiated by wind dispersed ascospores, which are released continually from crop debris, mainly in late-autumn to mid-winter and again in late spring-early summer (see life cycle below, reproduced with permission from the AHDB Encyclopaedia of Cereal Diseases). In addition, the disease is intensified and dispersed onto newly emerging leaf layers by rain-splashed conidia (asexually-produced spores). This means that the crop is continually exposed to infection, except during dry periods when both types of the spores are unable to infect successfully. Risk is increased with early-sown susceptible varieties or crops drilled where straw debris is common.
The severity of septoria leaf blotch decreases with later sowing, and with more frost days in November. In contrast, high risk septoria periods (rain splash events) in May and June encourage the disease. A reduction in atmospheric SO2 concentrations in the 1970s and 1980s is thought to have caused switch from Septoria nodorum (Parastagnospora nodorum) as the main foliar pathogen of wheat in Europe to Zymoseptoria tritici (Mycosphaerella graminicola) in Europe. The impact of the disease depends on infection of the final leaves, which is heavily affected by spring rainfall.
Select wheat varieties with the best available septoria resistance. There are no AHDB recommended list varieties that are fully resistant but ‘KWS Siskin’ (group 2) and 'LG Sundance' (group 4) has a resistance rating of 7 indicating partial resistance. There are 13 other varieties with a resistance rating of 6 which include ‘Skyfall’, ‘Crusoe’ and 'RGT Illustrious' in nabim group 1.
Start a fungicide programme focusing on septoria in the spring to limit disease on lower leaves and use different modes of action to achieve the best eradicant and protectant control (e.g. azole + multi-site +/- SDHI). Earlier treatment is needed in more susceptible varieties and where disease pressure is high. Aim to protect the final three leaves (leaves 3, 2 and flag leaf). T2 Growth stage 39 (flag leaf fully emerged) timing is the most important to keep upper leaves disease free to optimise yield and quality. T3 treatment may be needed in wet conditions.
A mutation increasing resistance of Septoria to
SDHI fungicides has been found in the UK so it is important to follow fungicide
application guidelines. Currently the resistance type is only present at
low levels at one site but is likely to spread.
Autumn or winter applications of fungicide are not effective due to the continual re-exposure of the crop to infection by either ascospores or conidia. In the early 2000s, when wheat prices were very low (£75/tonne), experiments at Rothamsted found the most cost-effective regime was simply a single flag-leaf fungicide application. As the value of the crop increased, it became cost-effective to spray at least twice in spring, even more so if spring weather is unusually wet.
Please see: AHDB Fungicide Activity and Performance in Wheat for detailed information on fungicide choice.
Generally there has been a gradual erosion of efficacy of many triazole fungicides in the past ten years but some such as epoxiconazole and prothioconazole are still effective. It is recommended to mix products with two or more modes of action or to use products with different modes of action at different spray times in order to manage fungicide resistance. SDHI fungicides mixed with an azole are reported to have good efficacy. Resistance to SDHIs has been found in the UK and Ireland in February 2016 at isolated sites and low levels but this is likely to increase. Other modes of action that have reasonable efficacy include chlorothalonil, prochloraz, and mixtures of strobilurin and azole.
Disease severity is reduced by delayed drilling and when there are more frost days in November. In contrast, mild weather promoting inoculum build-up over winter and rain splash events in May and June encourage the disease. Yield loss depends on infection of the final leaves, which is heavily affected by spring rainfall.
- Septoria tritici in winter wheat
- AHDB disease monitoring
- AHDB Fungicide Activity and Performance in Wheat
- AHDB Recommended list