In late-March, I visited a wheat field in southern Sacramento County at the request of a crop consultant. The consultant had previously brought some sample plants to me at my office that had irregularly shaped yellow and brown blotches on the leaves. We both thought the symptoms resembled those of Septoria Leaf Blotch disease (Septoria tritici, Mycosphaerella graminicola). By the time I visited the field, the symptoms were widespread, and the grower was chopping the field earlier than he wished to do. 

Septoria can be a serious problem of wheat in the Sacramento and northern San Joaquin valleys, particularly in wet years and in fields planted early. The disease can survive on crop residue and produce spores for several years if the residue does not fully decompose. Wet conditions cause spores to be released, which are blown with the wind or splash onto lower leaves. Infection can occur over a wide temperature range (41-94 degrees F). Fields planted early (i.e. October) have a longer period of exposure to wet conditions, and therefore, tend to be more susceptible if the disease is present. Leaf lesions are usually first visible on lower leaves. As lesions coalesce, particularly lesions on the flag leaf, the photosynthetic capacity of the plant is reduced. In grain fields, this can reduce grain filling and ultimately yields, and in forage fields it can reduce forage quality. Lesions may also appear on the glumes if conditions are favorable during heading.

Taking steps to try to diagnose the problem, with a hand lens, I observed what I thought to be black, spore-forming structures called pycnidia. I consulted with a couple colleagues to help gather additional information, including Farm Advisor Brenna Aegerter, whose expertise is plant pathology, and Small Grains Specialist Mark Lundy. Under the microscope, Brenna observed spores with an elongated shape, which is a characteristic of Septoria. Mark mentioned that he observed Septoria this year in areas of the Sacramento Valley. He also shared the 2025 UC variety testing results. While the variety I was observing is identified as moderately resistant to Septoria, environmental conditions likely impacted this field. Namely, rain came in early November and was followed by a few chilly weeks in late November and December when the fog never lifted (temperatures stayed in the 40s and the relative humidity exceeded 90 percent for days). While chilly, temperatures never reached freezing in many locations. Fast-forward to March, we had record-setting heat and only one storm since the New Year, which may have stressed the otherwise non-irrigated field. Taken together, these observations lead us to believe that the field was infected with Septoria, and the environmental conditions magnified the problem.

Crop rotation can help manage against Septoria in future plantings, but planting resistant varieties and properly timed fungicide applications are also good management practices. Septoria ratings are not a regular part of the UC statewide variety testing program, but this information may be available from seed companies. When warranted, fungicides should be applied between tillering and heading, with the purpose of protecting the flag leaf. Septoria only infects wheat, so other small grains like barley, oats, and rye are not affected by the disease. 

Of course, Septoria is not the only disease that can impact wheat. Leaf rusts may be more prevalent this year because of the April rain and cooler temperatures. Most new wheat varieties will have stripe rust resistance, which will mitigate its impacts. There is some breaking down of genetic resistance in the Pacific Northwest, but so far we have not observed any break-down here. Also keep in mind that while varieties may be developed for pest resistance, every year will have varying degrees of pest pressure, which is to say that new materials will be evaluated under different levels of pest pressure. UC variety trial results include a three-year summary. When making variety decisions, we recommend reviewing the three-year summary because the results from any particular year may have been skewed by unique circumstances. Please reach out to your local farm advisor for help with problem diagnostics in the field or assistance with variety information.