Glume blotch is a cereal disease that thrives in wet, humid growing conditions. It has generally been more prevalent in Ontario, but it does occur across the Prairies. Glume blotch symptoms on harvested grain can be confused with the fusarium damaged kernels (FDK) caused by fusarium head blight, and can negatively affect the quality of grain, especially wheat, and cause downgrading.
Glume blotch symptoms on wheat kernels are caused by septoria infections, and are very similar to those caused by fusarium. The septoria damaged kernels, however, do not contain mycotoxins that would be found in kernals damaged by the F. graminearum pathogen.
What is glume blotch?
Ron Howard, a plant pathologist with Alberta Agriculture and Rural Development, says leaf blotch is usually more common than glume blotch, since leaf material is available for infection for a longer period of time.
Most leaf spot diseases require temperatures between 15 to 30 C, with an optimum of 20 to 25 C and periods of high relative humidity or leaf wetness for 48 hours or more.
The pathogen that causes glume blotch occurs later in the season because of the timing of head emergence. The septoria leaf and glume blotch pathogens can overwinter on seed or crop residue, on volunteer wheat and the leaves of winter wheat.
Dark-coloured fruiting structures present on old infested residue release spores that are spread by wind and cause early spring infections. Later in the summer, infection is primarily caused by rain-dispersed spores. Rain splash helps spread the disease, which is favoured by high humidity and temperatures between 20 and 28 C.
The fungus cycles progressively on the crop as it develops, causing lesions which produce the fruiting bodies, releasing spores spread by rain splash to cause new lesions that begin the cycle all over again.
Stubble mulching and minimum till may increase the incidence of this disease, but short rotations, susceptible varieties and favourable weather conditions are the main risk factors for glume blotch development.
“Leaf diseases, in general, were quite prevalent in wheat fields across the Prairies from 2010 to 2012 because of higher-than-normal levels of precipitation,” says Howard.
Historically, glume blotch has been most severe on Red Spring and durum wheat in Manitoba and eastern Saskatchewan. Although it has shown up in plant surveys over the past two years, incidence and severity of glume blotch has been low in Manitoba, which has experienced generally dryer conditions. northwest Saskatchewan had very high levels of cereal leaf diseases such as septoria and tan spot in 2012 and at the heading stage glume blotch was present in many wheat fields sampled in the region.
Alberta surveys in mid-July to early August found severe leaf spotting on winter and spring wheat in central parts of the region and wheat leaf spotting was also quite severe in breeding nurseries, probably due to wet, warm conditions during May to July, but the symptoms could have been due to other leaf disease pathogens such as tan spot.
Some wheat varieties are moderately resistant to glume blotch. Seed treatments and foliar fungicides are available, but farmers should spray only in co-operative weather. Leaf and glume blotch development will be arrested during dry weather.
What to look for
Symptoms of leaf and glume blotch may first appear as small spots on the lower leaves of seedlings, although symptoms might not be detectable at the seedling stage depending on weather conditions. These spots grow into larger, yellow, lens-shaped lesions which later turn reddish brown. They become grey or greyish-brown and speckled as tiny black or brown fruiting bodies develop. Septoria produces lesions that can be confused with tan spot, which has oval, tan-coloured lesions, however the presence of pycnidia indicate Septoria leaf spots.
On the chaff or glumes, brown marks start at the tips, develop downwards and later produce pepper-like brownish dots, which are characteristic of septoria glume blotch. Infection can also occur on the stems at the joints or nodes.
U.S. data recommends a fungicide application if 25 per cent of the leaves have one or more lesions in three or five spots sampled in the field. If the disease is present, but this threshold is not reached, farmers are advised to watch weather forecasts, because the disease can spread rapidly during periods of heavy rainfall.
Recommendations from North Dakota State University indicate that it’s not economical to apply fungicide unless there is significant leaf disease present. Yield reduction will be anywhere from 10 to 40 per cent when the flag leaf is affected. †