|Method||Product / Instructions||NAPIS Survey Method|
|Visual||Collect symptomatic plant material.||3031 - General Visual Observation|
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.
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.
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.
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'.
If you are unable to find a reference, contact STCAPS@usda.gov. See CAPS Pest Datasheet for all references.