Smut diseases, caused by fungi in the order Ustilaginales, pose a serious threat to hay and grass seed production. In recent years, loose smut has led to yield losses, reductions in forage quality, and concerns over contamination of feed and field equipment. This fact sheet addresses loose smut disease in hay and other grains. Producers will learn how to identify loose smut, how to distinguish it from other diseases and how best to manage it.
Identification
Typically, smut is not noticeable until seed heads form. Black, powdery masses of fungal spores called teliospores replace seed heads. These masses are fragile and easily disintegrate, releasing spores into the environment. Affected fields will have stunted plants, missing seed heads and clouds of black spores during mowing (Figures 1 and 2).
A video shot by an affected grower in the Willamette Valley shows dark, sooty smut spores rising from the mower.
Disease cycle
The disease cycle of loose smut (Ustilago spp.) begins when wind-dispersed fungal spores infect healthy grass flowers and colonize developing embryos. These infected seeds carry dormant fungal mycelium that becomes active upon seed germination, growing systemically within the plant and eventually reaching the developing seed head.
As the plant matures, the fungus replaces the ovaries with masses of black spores, which are then released to infect new flowers. Since the pathogen is seedborne, contaminated seed is the primary source of new infections. The pathogen thrives under cool, moist conditions that favor prolonged flowering and plant infection.
Unlike foliar diseases, loose smut does not spread from plant to plant during the growing season but relies on floral infection and seed infestation for transmission. This makes seed treatment and the use of pathogen-free seed critical to breaking the disease cycle in hay and grass seed production systems.
Management strategies
Seed treatment
Some growers use seed treated with fungicides to manage smut diseases in major host plants in Oregon.
|
Host |
Smut type |
Fungicide |
Product example |
|---|---|---|---|
|
Grass hay and seed |
Loose smut | Thiram | 42-S Thiram® |
|
Grass hay |
Kernel smut | Thiram | 42-S Thiram® |
|
Grass hay |
Stripe smut | Thiram | 42-S Thiram® |
|
Oats |
Loose and covered smut | Difenoconazole | Difenoconazole 3L-ST ® |
|
Barley and oats |
Loose smut | Tebuconazole + Mefenoxam | Raxil Pro MD ®. Broad-spectrum control; often combined with other fungicides. |
|
Barley, oat and wheat |
Loose smut | Triticonazole | Stamina® F³ Cereals Fungicide Seed Treatment |
|
Barley |
Loose smut | Tebuconazole | Raxil Pro MD® |
-
Loose smut is seedborne, so fungicides must be applied as seed treatments to be effective. (Foliar sprays do not work.)
-
Resistance management: Rotate and mix fungicide modes of action. For example, avoid consecutive use of triazoles in Group 3.
For up-to-date pesticide registrations, check the Oregon Department of Agriculture Pesticide Database and see current pesticide recommendations in the PNW Plant Disease Management Handbook .
Resistant varieties
Use resistant grass species where available. Do not farm-save seed from heavily infested areas and always plant certified seed.
Crop rotation
Avoid continuous grass hay cropping to reduce inoculum in soil and volunteers.
Sanitation
- Do not haul infected hay: Spores can spread to clean fields during transport.
- Burning is effective but prohibited in many parts of the Willamette Valley.
- In-field management:
- Flail-mow infected stands finely.
- Immediately incorporate residue via deep disking.
- Follow with a dense nonhost cover crop.
Monitoring and early detection
Scout during heading. Look for abnormal heads and spore clouds during mowing and harvesting.
Equipment hygiene
Thoroughly clean mowers, balers and transport equipment between fields. You can use a solution of one part bleach to nine parts water (EPA reg. no. 5813-500), or 70% ethanol (Klercide 70/30 Pharma Ethanol, EPA Reg. No. 1677-252) followed by a thorough water rinse. Follow the directions on the disinfectant label, especially the prescribed contact time needed for effectiveness. For example, the contact time for 70% ethanol is five to 10 minutes, meaning that the equipment needs to be visibly wet throughout that period.
Prescribed hot water treatment
This method can kill internal smut mycelium in infected seeds. It typically involves soaking seeds in water at 52°C–54°C (125°F–130°F) for 10–15 minutes. You must carefully manage this treatment to avoid damaging seed viability.
Differential diagnosis
Loose smut in hay can be confused with several other diseases. This next section briefly discusses the common ones.
Covered smut
Causal agent: Tilletia species
Distinguishing features
- Smutted grains remain enclosed in intact glumes (seed covers).
- When crushed, they emit a foul, fishy odor (especially in wheat).
- Black spores only become visible if glumes break open.
How it differs: Loose smut spores are exposed and powdery; covered smut remains hidden.
Flag smut
Causal agent: Urocystis agropyri
Distinguishing features:
- Appears earlier in the season, often from seedling stage.
- Causes dark gray-to-black streaks on leaves and leaf sheaths.
- Plants become twisted, stunted or fail to produce seed heads.
How it differs: No powdery spore masses on heads; symptoms focus on foliage.
Leaf smut
Causal agent: Entyloma spp. and related smut fungi.
Distinguishing features:
- Produces yellow stripes or lesions on leaves.
- Heads may be malformed, but spores are not massed or released in clouds.
How it differs: No black powder or head replacement; symptoms are foliar.
Head smut
Causal agent: Sporisorium reilianum
Distinguishing features:
- Common in corn (Figure 6) and sorghum.
- Replaces entire seed heads with large, solid, dark smut galls. The tassel and occasionally the leaves are infected as well.
- Galls often have a silvery membrane before bursting.
How it differs: Galls are much larger and firmer, unlike loose smut, which is powdery.
Ergot
Causal agent: Claviceps purpurea
Distinguishing features:
- Dark purple to black sclerotia (hardened fungal bodies) replace individual seeds.
- Sclerotia are firm and oblong and are not powdery.
- Produces toxic alkaloids that are harmful to livestock.
How it differs: Ergot produces firm, black bodies, while dusty, black spores are symptoms of loose smut. Ergot is toxic; loose smut is not.
If you suspect you have ergot, please contact the OSU Endophyte Service Lab.
Sooty head mold
Causal agents: A variety of saprophytic fungi, including Cladosporium, Alternaria, Epicoccum and Stemphylium.
Distinguishing features:
- Appears as dark, olive-green to black fungal growth on mature wheat heads.
- Develops under humid or wet conditions, especially during grain maturation or delayed harvest.
- Growth is superficial and does not penetrate plant tissues.
- Often associated with senescing tissue or environmental stress.
How they differ: Loose smut systemically infects the plant and replaces the developing seed with masses of black spores. Sooty head mold is a surface-level fungal growth that does not invade or replace seed tissue, though it may contribute to grain discoloration (black point, for example) if conditions persist.
Black chaff
Causal agent: Xanthomonas translucens (bacterial)
Distinguishing features:
- Irregular black streaks and blotches on glumes and awns.
- Often associated with stress or drought.
- No powdery spore production or head replacement.
How it differs: Caused by bacteria, not fungus. No smut spores or reproductive cycle.
What to do with plants infected by loose smut
- Do not feed or sell affected hay if quality is overly compromised. Loose smut is associated with reduced palatability and other digestive issues. (Note that this is not the same as the toxic ergot alkaloids.)
- Destroy on site: Bush hog and bury. Avoid spreading spores.
- Rest the soil from host crops for about two to three years.
- Composting is unreliable unless piles reach above 130°F and are actively turned.
Summary
Loose smut is a growing issue in field crops, including hay and grass seed systems. Effective management hinges on pathogen-free, certified seed; seed sanitation; prescribed hot water (steam) treatments; equipment hygiene; crop rotation and in-field residue suppression. Consult local Extension professionals for updated fungicide registrations and variety recommendations.
References
- Agriculture and Horticulture Development Board. Loose smut in cereals (the importance of clean or fungicide-treated seed). Retrieved June 13, 2025.
- Bockus, W.W., et al. 2010. Compendium of Wheat Diseases and Pests, APS Press.
- Falloon, R.E. 1979. Further studies on the effects of infection by Ustilago bullata on vegetative growth of Bromus catharticus. New Zealand Journal of Agricultural Research 22:621-626.
- Menzies, J.G., P.L. Thomas and S. Woods. 2014. Incidence and severity of loose smut and surface-borne smuts of barley on the Canadian prairies from 1972 to 2009. Canadian Journal of Plant Pathology, 36(3), 300–310.
- Menzies, J.G., T.K. Turkington, R.E. Knox. 2009. Testing for resistance to smut diseases of barley, oats and wheat in western Canada. Canadian Journal of Plant Pathology 31:265–279.
- Mohan, S.K., P.B. Hamm, G.H. Clough and L.J. du Toit. 2013. Corn smuts, PNW 647. Oregon State University Extension Service.
- Moorman, G.W. 2023. Disinfecting tools, equipment, pots, flats and benches. Penn State Extension.
- Morton, D.J. 1961. Percentage yield loss as related to percentage loose smut in barley.
- Pacific Northwest Pest Management Handbook. 2025. Grass for Seed-Head Smut (Loose Smut). Oregon State University, University of Idaho, Washington State University.
- Sosnowski, M.R., J.D. Fletcher, A.M. Daly, B.C. Rodoni and S.L.H. Viljanen-Rollinson. 2009. Techniques for the treatment, removal and disposal of host material during programmes for plant pathogen eradication. Plant Pathology, 58(4), 621–635.
- Loose Smut of Wheat and Barley. University of California Agriculture and Natural Resources, Integrated Pest Management Program. Retrieved June 13, 2025.
Acknowledgments
Special thanks to the Willamette Valley growers who reached out and shared their observations and experiences. Your curiosity and collaboration keep Extension work grounded in reality. In particular, Richard Walker has been instrumental in shaping this resource through his keen observations, field videos and photos. He reflects the true spirit of Extension. Appreciation also goes to the many experts who generously allowed their photos to be used in this publication.
