Figure 7. Common bunt of wheat
Common bunt or stinking smut is caused by the seed-borne fungus Tilletia laevis. The disease causes reduced wheat yields and grain quality. Common bunt was once the most prevalent disease of wheat in Ohio, however, today the disease is rarely found due to the widespread use of effective seed-treatment fungicides.
Heads of diseased plants may appear stunted and are noticeably thinner and darker in color than healthy, disease-free heads. "Bunt balls" (kernels containing millions of fungal spores) replace the kernels in the flowering heads and spread the glumes much farther apart than normal. Bunt balls approximate the size of normal kernels but tend to be more spherical and off-colored (typically gray-colored). When the bunt balls are ruptured at harvest, they release spores into the surrounding air and contaminate the surface of healthy seed.
Bunt fungi survive as resting spores on contaminated seed. When contaminated seeds are planted, bunt spores germinate in the presence of moisture and infect the wheat seedlings. Once established, the fungus grows systemically in the plant. At flowering, entire flowering heads may be infected, and the fungus converts the developing kernels into bunt balls. Healthy seeds are contaminated by the release of the spores at harvest.
Figure 7. Common bunt of wheat
Loose smut, caused by the fungus Ustilago tritici, has little effect on seed quality. However, it can result in substantial yield losses if left unmanaged. Symptoms are most noticeable between heading and maturity of the crop. Diseased flowering heads are conspicuously darker when compared to healthy, green heads. Also, diseased heads typically emerge earlier. Diseased spikelets may be entirely transformed into dark fungal spore masses. As the head emerges, floral tissue is torn and the spores are released. Eventually only a spike is left where a normally healthy head should be. Spores of the fungus are dispersed by wind to nearby healthy flowers where they initiate new infections during wet weather. Unlike bunt, the loose smut fungus infects the developing kernels without causing noticeable damage.
Figure 8. Loose smut of wheat
Ustilago tritici survives in infected wheat seed. The fungus becomes active when the seed germinates and grows into the growing point of the developing wheat plant. Fungal growth closely follows the plants' growth. When the flowers form, the fungus again sporulates and starts the cycle again. Use a systemic fungicide seed treatment to eliminate this pathogen from infected seed.
Head scab, caused by Fusarium spp., is frequently a major cause of poor quality wheat seed in Ohio. Infected seed appear whitish to reddish in color and are nearly always shrunken and light in weight. Scabby kernels may be dead or the young seedling may be killed by seedling blight after emerging from the soil. Roots of plants killed by seedling blight appear light to reddish brown in color and may be covered with mold. If they survive, they generally lack vigor and may produce only a few weak tillers. Planting infected seed early when soil temperatures are above 60 degrees F generally increases loss from seedling blight. The best control of the seed-borne phase of scab is to first clean seed lots to remove all lightweight seed, thus increasing test weights; and second, treat with a fungicide that is effective in controlling seedling blights.
Figure 9. Head scab of wheat
Stagonospora glume blotch, caused by the fungus Stagonospora nodorum has increased in the US in recent years probably due to expanded use of nitrogen fertilizers, semidwarf wheat varieties, and reduced tillage. Symptoms can develop throughout the growing season on all above-ground plant parts. Initial symptoms are small chlorotic flecks usually on lower leaves or those in contact with soil. The glumes of the wheat heads become infected in late spring before flowering. Lesions generally begin at the glume tips and are dark brown in color. During favorable weather, the fungus penetrates the glumes and infects the seed causing severe shriveling of the grain.
Stagonospora survives on seed, straw, and/or volunteer wheat. With the onset of moist weather, spores are produced and are spread to healthy wheat plants by splashing rain. Glume blotch is most common in relatively warm weather when the temperature is between 70 degrees F and 80 degrees F.