Wheat blast - Magnaporthe oryzae Triticum pathotype
Effective: April 22, 2020
Taxonomic Position: Magnaporthales : Magnaporthaceae
Pest Type: Fungi
Pest Code (NAPIS): FMAHMKT
This pest is a member of the following surveys: Small Grains
These Approved Methods are appropriate for: 2025, 2024, 2023, 2022, 2021
Pest Information
CAPS Pest Datasheet: https://download.ceris.purdue.edu/file/4026
Pest Tracker: https://pesttracker.org/pest/FMAHMKT
Hosts Identified in the CAPS Host Matrix:
Wheat
Pest is vectored by:
No known vector.
Survey

Approved Method(s)
Method Detail NAPIS Survey Method
Visual Collect symptomatic plant material. 3031 - General Visual Observation

Survey Recommendations
The following are recommendations for executing the survey using the approved methods for pest surveillance. The recommendations are developed through literature review and consultation with subject matter experts.
Signs and Symptoms:

The most telltale sign in wheat is head infection, but all aboveground plant parts can become infected and show symptoms. Infected heads appear bleached.

Infected rachises, or stems, are discolored, turning from brown/black to dark gray due to heavy spore formation. Bleaching and stunted grain development occur above the point of stem infection, but not below it.

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. Lesions are elliptical and have a white center and reddish-brown margin on the upper side of the leaf, and a gray appearance on the underside of the leaf where spore formation occurs.

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

Time of Year to Survey:

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.1,2 Dew provides the free water on the host surface that is needed for blast fungus infection.3

The optimum temperature and wetting period for spore formation is 25-30°C (77-86°F) after 25-40 hours of wetness.1,2 Infections are substantially less likely at temperatures less than 15°C (59°F) and higher than 35°C (95ºF). Hot, dry weather does not favor blast disease development.

Survey Site Selection:
Survey wheat fields, especially fields that have a lot of organic matter 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.
Sample Collection Instruction:
Conduct initial visual inspection in areas of the field where trees or windbreak barriers shade the plants and allow dew to remain on plant surfaces. Search for symptomatic plants and examine them for characteristic grey spores. Collect at least 2-3 symptomatic wheat heads per field. Store the samples at 4ºC (40ºF).
Key Diagnostic or Identification

Approved Method(s)
ID/Diagnostic:

The CAPS-approved diagnostic method is molecular identification, which is the only way to distinguish M. oryzae Triticum pathotype from other M. oryzae isolates.

A multiplex conventional polymerase chain reaction (PCR) assay is approved for sample screening. This test detects the species M. oryzae.

A Loop-mediated isothermal amplification (LAMP) assay is approved for rapid screening of M. oryzae Triticum pathotype.

A conventional PCR is approved for detecting M. oryzae Triticum pathotype and differentiating it from native M. oryzae strains.

Mistaken Identities:
Wheat blast symptoms 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. 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. Wheat blast causes spike bleaching only above the point of infection and produces spore-producing gray lesions at the point of infection. Conversely, spikes infected by FHB may become bleached above and below the point of infection and may contain pink to orange masses of spores. 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.

To request a copy of diagnostic protocols, email the S&T Beltsville laboratory at APHIS-PPQCPHSTBeltsvilleSampleDiagnostics@aphis.usda.gov and use the subject line 'Diagnostic protocol request'.

References
  1. Cardoso, C. A. d. A., E. M. Reis, and E. N. Moreira. 2008. Development of a warning system for wheat blast caused by Pyricularia grisea. Summa Phytopathologica 34(3):216-221.
  2. Kohli, M. M., Y. R. Mehta, E. Guzman, L. De Viedma, and L. E. Cubilla. 2011. Pyricularia blast–a threat to wheat cultivation. Czech Journal of Genetics and Plant Breeding 47(Special Issue):S130-S134.
  3. Wilson, R. A., and N. J. Talbot. 2009. Under pressure: investigating the biology of plant infection by Magnaporthe oryzae. Nature Reviews Microbiology 7:185-195.

If you are unable to find a reference, contact STCAPS@usda.gov. See the CAPS Pest Datasheet for all references.