Bacterial Diseases of Wheat

• In wheat, bacterial infections can present similar symptoms to fungal infections but are not controlled by fungicides.
• Water-soaked lesions and bacterial exudates leaving a sheen on the leaf surface are indications of a bacterial infection.
• Bacterial leaf blight and bacterial leaf streak thrive under different conditions and are associated with different levels of yield loss.

Bacterial Leaf Blight

Bacterial leaf blight (BLB) is caused by the bacteria Pseudomonas syringae pv. syringae. This bacterium spreads through infected seed or water. Frequent rains and temperatures between 59 and 77 °F promote development of this disease.¹

Symptoms

Bacterial leaf blight begins as small water-soaked spots. After three to four days, these gray-green spots expand to form large, tan-white blotches.² Symptoms usually appear after boot stage, and whole leaves may senesce while grain heads and glumes appear healthy. The water-soaked appearance on or around the lesions identifies the bacterial infection, and slimy droplets can appear on leaves in humid conditions.

Effects on Wheat Yield

Typically, this bacterial disease is considered minor and not impactful on wheat yield. However, epidemics can occur in irrigated fields, especially with susceptible cultivars. South Dakota reported an epidemic with high infection rate causing at least 75% necrotic leaves.² The weather forecast should be considered when evaluating potential risks caused by this disease, as the bacteria is curbed by warmer temperatures.

Management

Management of bacterial diseases can be difficult once they appear, though bacterial leaf blight can by managed by planting tolerant wheat cultivars. Cultural practices such as reducing irrigation frequency and planting pathogen-free seed are recommended. Consider sending symptomatic tissue samples for testing at a local university for proper identification of the disease. Tan spot (a fungal disease) can look similar to bacterial leaf spot, except the former produces a yellow halo around lesions instead of a water-soaked appearance.

Bacterial Leaf Streak

Bacterial leaf streak (BLS) is a bacterial disease caused by Xanthomonas translucens pv. undulosa. Although the bacteria can survive a short time on the soil and in plant residue, it is considered seedborne. Plants become infected when bacteria either enter stomata and move into the coleoptile or enter wounds in the plumule created by strong wind and rain events. Temperatures above 78 °F and relative humidity greater than 30% favor the spread of this bacterium.³ After the initial infection, ideal temperatures, not humidity, are needed for X. translucens to multiply within plants.⁴ Irrigation can also increase the risk of infection due to splashing.

Symptoms

Symptoms of BLS typically appear after the flag leaf growth stage when storms have passed through the region. These include irregular translucent water-soaked streaks on leaves which turn yellow and then brown. Ooze exuded from the lesions is a key symptom of bacterial infection, which can cause leaves to appear shiny as the ooze dries. This disease can co-occur with other leaf diseases (tan spot, Septoria) making it difficult to distinguish.

Black chaff is caused when the bacterium that causes BLS, X. translucens pv. undulosa, infects glumes during grain fill. This stage of the disease is referred to as “black chaff” because dark purple to black streaks appear on the glumes. A purple to yellow lesion on the stem just below the head may also appear. Bacterial exudate helps distinguish this disease from Alternaria blotch or a genetic syndrome known as pseudo black chaff. Severe black chaff infection can cause discoloration in kernels.^3,5

Effects on Wheat Yield

The amount of infected leaf area on the flag leaf is an indication of the level of potential yield loss. If less than 10% of the surface area of the flag leaf is infected, yield loss may be five percent or less.^2,4 A flag leaf half covered by BLS infection by was observed to cause a yield loss of 20%. Yield losses between 30 and 40% have been reported in severe cases.^2,3,4

Management

Management is limited and includes using clean seed and planting resistant wheat cultivars. Single-gene resistance has not been identified; however, field and greenhouse screenings are being done to identify major quantitative trait loci with resistance to BLS.³ Commercial cultivars with high levels of BLS are not available yet.

Bactericides have been used with mixed results.⁴ While fungicides are not generally effective at treating bacterial diseases, Serenade^® ASO biological fungicide is labeled for foliar application on wheat for control of Xanthomonas spp. This product is part of a collection of biologicals from Bayer that gives growers flexible application and multiple sites of action for their integrated disease management programs.

Sources

¹2024. Wheat (Triticum aestivum)- bacterial leaf blight. In Pscheidt, J.W. and Ocamb, C.M., Eds. Pacific Northwest plant disease management handbook. Oregon State University. https://pnwhandbooks.org/plantdisease/host-disease/wheat-triticum-aestivum-bacterial-leaf-blight

²Tambong, J.T. 2022. Bacterial pathogens of wheat: Symptoms, distribution, identification, and taxonomy. In M.U.R. Ansari, Ed. Wheat – recent advances. IntechOpen. DOI: 10.5772/intechopen.102855

³Kaur, N., Ishimaru, C., Vinatzer, B.A., and Mehl, H.L. 2020. Bacterial leaf streak of wheat. The American Phytopathological Society. DOI: 10.1094/PHI-I​-2020-0512-01

⁴ Smith, M., Dill-Macky, R., Curland, R., Ishimaru, C., and Wiersma, J. 2018. Bacterial leaf streak and black chaff on small grains. University of Minnesota Extension. https://extension.umn.edu/small-grains-pest-management/bacterial-leaf-streak-and-black-chaff-small-grains#life-cycle--1373412

⁵Lux, L., Friskop, A., and Liu, Z. 2020. Bacterial leaf streak and black chaff of wheat. PP1566. North Dakota State University Extension. https://www.ndsu.edu/agriculture/extension/publications/bacterial-leaf-streak-and-black-chaff-wheat

Web sources verified 2/14/25. 1711_504273