Over the past couple of weeks I have received several calls and observed numerous wheat fields with septoria leaf blotch. In addition, I’ve observed a number of wheat fields with glume blotch, much more this season than in previous years. At present, save for some rare situations where disease (other than rusts) has been observed on the flag leaves I don’t believe that Septoria, Stagonospora, or glume blotch are threatening the crop. As with many of the diseases present in MS at this time there are likely a few fields that might sustain minor yield loss as a result of the above diseases. However, much of this will likely be related to the environment over the next 3-6 weeks.
In addition to the abnormally warm spring temperatures being conducive for wheat rusts the environmental conditions have favored some other fairly common diseases.
Septoria leaf blotch
Typically, leaf blotch can be identified in many of our wheat fields. Oftentimes the disease is observed lower in the canopy, and more often than not on leaves that are shaded and appear nutrient stressed. Best way to distinguish Septoria leaf blotch from Stagnospora leaf blotch is the presence of brown to black pycnidia within the lesion (the small structures denoted in the attached photo that look like pepper grains). The reproductive structures can be observed without a hand lens, but best confirmation is with a 20× lens. One of the main differences between Septoria leaf blotch and the other diseases listed has to do with the shape of the lesion. Septoria leaf blotch lesions are typically abnormally shaped and unlike bacterial streak (listed below) will not tend to follow the veins.
Dry weather, or less frequent dews, would likely reduce the Septoria disease levels in all of our wheat fields.
Glume blotch (Stagonospora nodorum blotch)
Brown to black heads have been more prominent this year than in the past. I haven’t seen the discoloration go through the glume and onto the developing kernel. The fungus that causes the discoloration of heads can also cause a leaf spot, Stagonospora leaf blotch (see below description and photo). Even though I have observed the disease commonly during the past several weeks I have not seen what I would consider to be a severe case. In addition, the disease can be observed on the awns.
Stagonospora leaf blotch
The same fungus that causes glume blotch can also cause a leaf spot. As stated above, the big difference between Septoria leaf blotch and Stagonospora leaf blotch has to do with the presence of the dark reproductive structures of the fungus (pycnidia) present in the leaf spots caused by Septoria and NOT in those caused by Stagonospora (refer to photos). However, upon closer inspection, the leaf spots will have a darkish brown center surrounded by a yellow halo. After 5 seasons in Mississippi, I have only ever observed this particular disease a single time. But, I think it is important enough to mention because we can have the disease in our production system. Rotation and use of resistant varieties is likely one of the best methods of
Bacterial leaf streak
To make things a bit more confusing, bacterial leaf streak can produce a symptom similar to Septoria leaf blotch. However, the streaks will typically follow the veins of the wheat leaf and pycnidia will not be present within the lesions. Leaves may have lesions caused by more than one organism and this in some cases can compound the potential management strategy necessary to limit losses when the disease is first observed. The lesions typically cover a small area, can be confused with the lesions associated with stripe rust (due to the shape and size) and can coalesce to cover large areas of the leaf. With the temperature and heavy dews we have experienced over the past several weeks the environment has been conducive for the development of bacterial streak. Bacterial streak, like Stagonospora glume blotch can also be observed on the glumes. Management options are limited to resistant varieties.