Wheat Leaf Diseases

Bob Schroeder
CPS Tangent

Pycnidia of Septoria tritici
Lesions of Stagonospora nodorum

Many of you may have read the article last year on the blog that described the identity of Stagonospora nodorum.  On the head of the wheat plant it is commonly referred to as glume blotch.  With the conditions we have experienced so far this season, we again are seeing and will continue to see large amounts of nodurum as well as Septoria tritici.  Septoria tritici is primarily a leaf pathogen, but it too can affect wheat heads.  The Stagonospora nodurum commonly affects both foliage and heads.  Both fungi can occur within the same field and on the same plants.  Yield reductions can be greater than 10-30% where fungicides are not used.  Pycnidia are the spore producing structures of the fungus which in the case of S triciti are visible to the naked eye.  They are most noticeable following periods of rain and dew and are spread in these wet and windy conditions.  S. nodurum infections start as small dark brown flecks that expand into lens shaped lesions.  Lesions are usually surrounded by a distinct yellow halo.  The lesions have pycnidia also but are difficult to see without the use of a hands lens.  Lesions of both Septoria tritici and Stagonospora nodurum can rapidly expand and kill portions of leaves or entire leaves.
When one or both fungi invade the heads of wheat, the disease is referred to as glume blotch.  Infection by tritici is highly dependent on temperature and cool wet conditions of which we have had a lot this year so far. Some of the heavier seeded fields, with thick canopies of vegetation and high fertility have provided the disease with ideal conditions to be a problem.  Infections start in the lower leaves of the plant and move up until high temperatures become limiting.  The Nodorum occurs over a wide temperature range but is highly dependent on the plant state of development.  Infections increase about the time that plants are in the "boot stage" of development.

Foliar infections serve as a launching point to head infection which makes protecting the flag leaf especially important as well as the F1 and F2 leaves that precede the flag leaf.  The wet and windy weather contributes to the spreading of the diseases whereas the dry weather tends to slow it or completely halt it.
Timing of fungicide application plays an important role in determining the level of disease control.  For the best results, scout your fields early and often to monitor the amount of Septoria tritici and Stagonospora present while at the same time paying attention to the weather trends and stage of plant growth for all varieties of wheat in the Willamette Valley.