Tar spot disease of maize found for the first time in the USA
Tar spot of maize (Zea mays) is a disease caused by a complex of several fungal pathogens which has been observed in America causing serious yield losses in some countries. Yield losses exceeding 50% annually have recently been noted in Colombia, El Salvador, Guatemala, Mexico and Nicaragua. Initial symptoms of tar spot are brownish lesions on maize leaves. Black spore-producing structures (ascomata), protruding from the leaf surface, appear later and can densely cover the leaf surface. Lesions may coalesce to cause large areas of blighted leaf tissue. Symptoms and signs of tar spot can also be present on leaf sheaths and husks. The disease reduces photosynthetic potential and therefore plant vigour. Affected ears have fewer kernels which may germinate prematurely on the cob.
At present, three fungal species have been found in association with this disease: Phyllachora maydis, Monographella maydis, and in some cases, Coniothyrium phyllachorae. In the areas where this disease is commonly found (e.g. Mexico, Central and South America), Phyllachora maydis is usually the first species found in association with leaf lesions. While Monographella maydis is usually considered to be a common saprophyte on leaf surfaces, it becomes pathogenic in association with Phyllachora maydis and forms necrotic rings around Phyllachora maydis lesions. Coniothyrium phyllachorae might be a hyperparasite of the other two species, but its role is not fully understood yet.
In September 2015, the presence of Phyllachora maydis (only) was confirmed for the first time in the USA, in samples collected from a maize field in Indiana (US). A few days later, P. maydis was also detected in leaf samples collected from commercial maize fields in 3 counties of Illinois (LaSalle, DeKalb and Bureau). It is pointed out that the causal agents of tar spot are not known to be seed borne. Studies will be carried out to determine how this fungus arrived in the USA and whether measures need to be taken to prevent future disease outbreaks.
Sources
INTERNET
- ProMed posting (no. 20150920.3658925) of 2015-09-20. Tar spot, maize – USA: 1st rep (IN). http://promedmail.org/direct.php?id=3658925
- Purdue University. Tar spot on corn confirmed in the United States. http://extension.entm.purdue.edu/pestcrop/2015/Issue24/Issue24.pdf
- University of Illinois Urbana-Champaign. The Bulletin. Pest management and crop development information for Illinois. Corn disease alert: New fungal leaf disease ‘Tar spot’ Phyllachora maydis identified in 3 northern Illinois counties. http://bulletin.ipm.illinois.edu/?p=3423
- USDA-ARS. Invasive and emerging fungal pathogens. Diagnostic Fact Sheets. Tar spot of corn-Phyllachora maydis. http://nt.ars-grin.gov/taxadescriptions/factsheets/index.cfm?thisapp=Phyllachoramaydis