How, where and when does mildew infect wheat?
Widespread throughout the UK, wheat powdery mildew (Blumeria graminis f.sp. tritici) develops in the spring on green tissue of rapidly growing crops. Ideal growth conditions are temperatures of between 15-22°C. The disease is inhibited above 25°C. Mildew requires short periods of high humidity (NOT free water) alternating with drier periods for spore dispersal. Its development is favoured by high nitrogen and dense canopies.The usual life cycle duration for mildew is 5-7 days.
The disease overwinters as dormant mycelium on late tillerts, lower leaves and leaf shealths of volunteers and early sown autumn crops. Cleistothecia (black pigmented resting structures within established pustules formed in June) can overwinter on crop debris and produce sexual spores.
How does mildew impact wheat yields?
Mildew diverts nutrients from the plant to the fungus and increases respiration and transpiration. Severe early season infections, although very visible, don't impact on yields on a comparable scale to Septoria or rusts. Average yield reductions of 5% are possible, but severe early infections on suscpetible varieties have been known to reduce yields up to 20% due to reduced numbers of fertile tillers.
How to recognise mildew symptoms
One of the most easily recognisable diseases, mildew produces white fluffy pustules on the leaves, stem and sometimes on the ear. With time, these pustules turn brownish, and black dot cleistothecia become evident as shown above.
How to control mildew
Prothioconazole is the most mildew active azole, and will give very good protection against this disease where it is selected as the core component of a Septoria programme. This can be achieved by using Proline275 or Aviator235Xpro at T1 and Aviator235Xpro or AscraXpro at T2. In more curative, higher pressure early season scenarios, the addition of a specific mildewicide is a sensible precaution.