Not known to transmit any human or animal pathogens.

No known vector.

Infected heads lose their green color and appear bleached. This is the most characteristic sign, but all aboveground plant parts can become infected and show symptoms. 

Bleaching is in a characteristic pattern, wherein bleaching occurs above the point of infection. The fungus may infect the stem inside the wheat head, in which case the area above the point of infection appears bleached. To find the point of infection, remove the spikelets just below the point of bleaching to find discolored lesions, which turn from brown/black to dark gray due to heavy spore formation. When the pathogen infects the stem just below the wheat head, the entire wheat head appears bleached. Individual spikelets may also be infected. Grain development is limited in infected wheat heads (e.g., affected grains are shriveled/wrinkled with low test weight). 

Eye-spot or elliptical lesions typically form on older leaves, and the fungus can be isolated from lesions in the deteriorating basal leaves at the base of the plant. Initially, leaf lesions are water-soaked to gray-green. Leaf lesions are white with a reddish-brown margin from the top of the leaf, and gray from the bottom of the leaf where sporulation occurs.

See the CAPS Pest Datasheet for images of signs and symptoms.

Begin surveying wheat fields as soon as wheat heads begin to form and continue until harvest. Visual inspection of wheat is critical during warm, wet conditions that favor blast disease, specifically during periods of extended rain and heavy dewfall caused by warm daytime temperatures and cool nighttime temperatures. Dew provides the free water on the host surface that is needed for blast fungus infection.

The optimum temperature and wetting period for spore formation is 77-86°F after 25-40 hours of wetness. Infections are substantially less likely at temperatures less than 59°F and higher than 95ºF. Hot, dry weather does not favor blast disease development.

The most suitable method for detecting wheat blast is through visual inspection and collection of infected wheat plant parts (see Signs & Symptoms).

Prioritize examining areas of wheat fields exhibiting symptoms consistent with wheat blast. If such areas are not obvious at first, surveyors can consider using transects to better cover the whole survey area and visually inspect plants along those transects. Because it can be difficult to identify individual wheat plants, surveyors can use quadrats of any size (e.g., 1x1 ft., 3x3 ft.) and inspect plants found within those quadrats.

Surveyors should take precautions when surveying for wheat blast. Park vehicles away from infected or potentially infected plants, wear disposable clothing, and thoroughly disinfect all tools with bleach between survey sites to prevent contamination between properties during surveys.

Survey wheat fields, especially fields that have high fertility or use high rates of nitrogen fertilizer. Nitrogen-rich soils favor disease development. If possible, select fields of susceptible cultivars that have not been treated with fungicides.

Conduct initial visual inspection in areas of wheat fields where trees or windbreak barriers shade the field and allow dew to remain on plant surfaces. Search for symptomatic plants and examine them for characteristic grey spores. See the CAPS Pest Datasheet for images of signs and symptoms.

Collect at least 2-3 symptomatic wheat heads per field. When collecting a sample, avoid cross-contamination with previous/subsequent samples by sanitizing tools, changing disposable gloves, and/or using an everted collection bag to avoid direct contact. Double bag the samples in paper bags. Make sure there is no moisture inside the bag. Place silica gel into the outer bag so it's not in direct contact with the samples. Label the sealed bag with the host species/cultivar, the symptoms observed during collection, and the identifying code to be used in your records. Keep samples as cool and dry as possible in the field. Do not refrigerate. If possible, ship on the same day as collection, using a sturdy, crush-proof shipping container.

De Wolf, E., 2015. Identifying wheat diseases affecting heads and grain, Kansas State University, Kansas.

Harmon, P.F., Dunkle, L.D. and Latin, R., 2003. A rapid PCR-based method for the detection of Magnaporthe oryzae from infected perennial ryegrass. Plant Disease, 87(9): 1072-1076.

Pieck, M.L., Ruck, A., Farman, M.L., Peterson, G.L., Stack, J.P., Valent, B. and Pedley, K.F., 2017. Genomics-based marker discovery and diagnostic assay development for wheat blast. Plant Disease, 101(1): 103-109.

University of Minnesota, 2024. White heads in wheat. University of Minnesota Extension, https://blog-crop-news.extension.umn.edu/2019/07/white-heads-in-wheat.html.

Yasuhara-Bell, J., Pedley, K.F., Farman, M., Valent, B. and Stack, J.P., 2018. Specific detection of the wheat blast pathogen (Magnaporthe oryzae Triticum) by loop-mediated isothermal amplification. Plant Disease, 102(12): 2550-2559.

Wheat blast symptoms most closely resemble those of another type of wheat spike infection, Fusarium head blight (FHB), which is caused by Fusarium spp. fungi that are prevalent in the United States, especially the F. graminearum species complex. Due to the similar symptoms of these two diseases, wheat blast may be overlooked in the field. Close examination and training are essential to differentiate between wheat blast and FHB. When wheat blast infects the wheat head stems, it causes spike bleaching above the point of infection and creates spore-producing gray lesions at the point of infection. Conversely, FHB causes spike bleaching above and below the point of infection and may contain pink to orange masses of spores. Determine the point of infection by removing spikelets to find lesions which may produce spores.

Other diseases that may be confused for wheat blast include Fusarium crown rot (caused by Fusarium culmorum and F. pseudograminearum), take-all (caused by Gaeumannomyces graminis), and spot blotch (caused by Bipolaris sorokiniana). Although Fusarium crown rot can cause bleached head spikes, its primary signs and symptoms are pink mycelium under leaf sheaths and chocolate brown discoloration of the crown tissue and lower stem. These symptoms do not occur in wheat blast. Although take-all can cause white heads, it primarily causes chlorotic, stunted plants, with a dark stem base. These primary symptoms are distinct from wheat blast. Spot blotch causes dark brown or black discoloration on infected spikes and oval to elongated brown blotches on leaves and leaf sheaths. This dark brown discoloration on infected spikes does not occur in wheat blast.

Surveyors may also encounter PoL, which is not a severe pathogen on wheat, but individual wheat heads appear identical to PoT-infected heads. Any symptomatic tissues suspected of P. oryzae infection would need molecular diagnostics to confirm the pathotype.

Additionally, premature head bleaching may be due to wheat stem maggot (Meromyza americana), a minor pest widespread in the United States or abiotic factors, including frost injury, excess water, excessive heat, and micronutrient deficiencies. Plants affected by wheat stem maggot have bleaching that extends from the head to the stem below the head. If the head is pulled upward, it will separate from the rest of the plant just above the first node, revealing a ragged edge where the stem has been chewed.

See the CAPS Pest Datasheet for images comparing FHB and wheat blast signs and symptoms in wheat, as well as information about other mistaken identities