Septoria Tritici Blotch of Wheat

Symptoms occur on foliage as small gray dead areas that expand to irregularly shaped blotches. The blotches (lesions) begin as light green areas that appear to be limited at first by the leaf veins. The light brown lesions change to yellow areas in a few days, then to reddish brown, ending as grayish tan necrotic areas with small, black specks inside the dead areas. The black specks (pycnidia) within the lesions are the asexual fruiting structures of Septoria tritici and exude pinkish brown, column-shaped, jellylike masses of spores (conidia) upon wetting by rain or dew. These asexual conidia thus function as a source of inoculum to spread the disease within an infected field.

Septoria tritici blotch affects only wheat and is an important foliar disease of wheat in the Sacramento and northern San Joaquin valleys. It is particularly severe in years of higher than average rainfall and is especially damaging when late spring rains persist after emergence of the flag leaf.

The spores (ascospores) of the sexual stage of the pathogen, Mycosphaerella graminicola, initiate the first infections in each growing season when they are discharged into the air from sexual fruiting bodies in wheat debris remaining from previous crops. The maturation and discharge of ascospores occurs following the first fall rains. The ascospores, which are forcibly discharged and become airborne under drying conditions, serve to uniformly inoculate new plantings over wide distances. Current data indicate that ascospores can be discharged from October through April with the subsequent appearance of Septoria tritici pycnidia in lesions on wheat plants 3 to 4 weeks later. With two potential sources of inoculum for infection throughout the growing season, the major factors affecting severity of Septoria tritici blotch are temperature and moisture during the growing season. Spore germination and disease development are optimal at 60° to 77°F (16° to 25°C) when free moisture is present on the foliage. About 6 hours of leaf wetness are required for infection. Under favorable conditions of moisture and temperature, secondary cycles of infection occur every 21 to 28 days. Conversely, dry periods and warm weather prevent infection and disease spread. The disease reduces grain number, grain filling, or both, depending on whether the disease is severe only before or after anthesis versus the entire growth period of the grain.

The fungus survives between cropping seasons primarily as M. graminicola on wheat residue. The presence of the airborne ascospores, capable of long distance spread in the wheat growing regions, means that crop rotation will not afford escape from this source of inoculum. The impact of the disease is most severe in early planted wheat (October) because the plants are exposed to the pathogen over a longer period of time during a period when weather conditions are frequently favorable to disease development. Consequently, later plantings of wheat (Nov. to Dec.) are less likely to be severely affected.

Use resistant cultivars (see WHEAT CULTIVAR TABLE). Avoid early planting (October).

Although normally not economical, foliar fungicides can be used to control disease outbreaks and provide partial disease control. Applications should be made between tillering and heading with the objective being to protect the flag leaf. Depending on the weather conditions from tillering to early dough stage, one or more applications may be needed.