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Septoria is the biggest yield-robbing foliar disease of winter wheat. In this second Arable Academy module, Rothamsted Research’s John Lucas explains why it is so prevalent

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What is septoria?
Septoria leaf blotch is the UK’s most important foliar disease of winter wheat. Caused by a fungus, Mycosphaerella graminicola, it is better known as Septoria tritici. Oblong patches of dead tissue develop on leaves containing numerous small black structures. In severe outbreaks these lesions may coalesce so the whole leaf becomes yellow-brown and desiccated.

Epidemics start during autumn and early winter with the arrival of airborne ascospores, released from sexual fruit bodies (ascocarps) on the remains of previous crops.

Ascospores infect emerging leaves of wheat seedlings to cause the first disease lesions. Within these lesions the fungus forms small, black, flask-shaped structures called pycnidia. These produce needle-shaped asexual spores (pycnidiospores) that ooze out on to the leaf surface during wet weather.

Rain-splash then spreads the spores to higher leaves as they emerge, where they infect and initiate further lesions containing pycnidia. Several cycles of infection can, therefore, take place, with the disease spreading up the plant. Later in the season the sexual ascocarps are formed in dead tissue, and these persist on crop debris to initiate disease on the following crop.

Septoria

The impact
Septoria depresses crop yield by reducing green leaf area and the capacity of the crop to produce sugars through photosynthesis. This is particularly damaging if the disease reaches the top three leaves, which contribute most to grain filling and yield. If not controlled, septoria can routinely reduce yields by 1-2t/ha and on susceptible varieties in severe disease seasons the impact may be as much as 3t/ha.

Factors affecting septoria
As rainfall plays a key role in dispersing septoria spores, the disease is associated with wet weather. Septoria is, therefore, more prevalent in western regions of the UK. The greatest risk of damaging levels of disease is linked to rain in spring and early summer that is sufficiently heavy to splash spores on to higher leaf layers. Cold winter weather can cut the level of disease present and delay the spring epidemic. Prolonged dry weather in summer also prevents the epidemic from progressing. As the ascospores initiating infection in the crop are wind-dispersed over significant distances, crop rotation has little effect on the disease. There are significant differences in the relative susceptibility of winter wheat varieties to septoria, but none of the available varieties are completely resistant to the disease.

Complicating factors
After septoria has infected a leaf there is a relatively long period (2-3 weeks) before visible symptoms develop. When assessing the disease there is a danger the extent of infection may be underestimated, and that fungicide treatments might be applied too late to be effective.Septoria has adapted to the fungicides used to control it. Resistance developed to the benzimidazoles fungicides in the 1980s and to the strobilurins since 2001. Neither of these groups now give effective control of the disease. There is evidence the performance of some triazole fungicides has shifted over the past 10 years, so higher doses are now needed to achieve full control. But the most effective triazoles still give good control if correctly timed using recommended rates.

The solutions
The two main ways of controlling septoria are the use of fungicides and resistant wheat varieties. After development of resistance to strobilurins, triazole fungicides are once again the mainstay of septoria control. The most effective triazoles should be used at recommended rates, as lower doses may no longer give complete control. Ideally, alternative mode of action fungicides, such as chlorothalonil and boscalid should also be included in the programme to reduce selection pressure on triazoles. Some studies have shown some strobilurins still give partial control of septoria. Fungicides should be timed to ensure protection of the top three leaves, so GS32 and flag leaf sprays are usually the most important.Due to problems with the adaptation of septoria to fungicides, more emphasis should now be placed on using wheat varieties that are less susceptible to the disease. The HGCA Recommended List includes three varieties (Robigus, Alchemy and Gatsby) with septoria ratings of 7, and further new varieties with improved resistance are in the pipeline. The more susceptible varieties should if possible be avoided in areas prone to high disease pressure. john.lucas@bbsrc.ac.uk

 

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