Recovery Plan for Tar Spot of Corn, Caused by Phyllachora maydis

Authors: ROCCO DA SILVA, CAMILLA - Purdue University; CHECK, JILL - Michigan State University; MACCREADY, JOSHUA - Michigan State University; ALAKONYA, AMOS - International Maize & Wheat Improvement Center (CIMMYT); BEIRIGER, ROBERT - University Of Florida; BISSONNETTE, KAITLYN - University Of Missouri; COLLINS, A. - Pennsylvania State University; CRUZ, CHRISTIAN - Purdue University; ESKER, PAUL - Pennsylvania State University; Goodwin, Stephen - Steve; MALVICK, DEAN - University Of Minnesota; MUELLER, DAREN - Iowa State University; PAUL, PIERCE - The Ohio State University; RAID, RICHARD - University Of Florida

Submitted to: Plant Health Progress
Publication Type: Literature Review
Publication Acceptance Date: 7/2/2021
Publication Date: 11/24/2021
Citation: Rocco Da Silva, C., Check, J., Maccready, J.S., Alakonya, A.E., Beiriger, R., Bissonnette, K.M., Collins, A., Cruz, C.D., Esker, P.D., Goodwin, S.B., Malvick, D., Mueller, D.S., Paul, P., Raid, R. 2021. Recovery Plan for Tar Spot of Corn, Caused by Phyllachora maydis. Plant Health Progress. 22:596–616. https://doi.org/10.1094/PHP-04-21-0074-RP.
DOI: https://doi.org/10.1094/PHP-04-21-0074-RP

Interpretive Summary: Tar spot is a destructive leaf disease of corn caused by the pathogenic fungus Phyllachora maydis. It was first detected in the U.S. in 2015 and since then has spread across the corn-growing region, including Florida, Illinois, Indiana, Iowa, Michigan, Minnesota, Missouri, Ohio, Pennsylvania, and Wisconsin, plus southwestern Ontario, Canada. Losses in the U.S. were estimated at 241 million bushels from 2018 to 2020 but potential losses can be much higher. To develop a U.S. recovery plan for tar spot, all available literature was analyzed including the origin and biology of the pathogen and numerous measures for disease control. The results showed that one source of resistance is known and fungicides are effective, but the overall level of knowledge is woefully inadequate and much research needs to be performed on the biology of this pathogen in the U.S. Recommend actions include continued collaboration, screening for sources of resistance, incorporating weather, other environmental factors and fungicide sprays in disease-prediction models, plus a better understanding of pathogen biology and enhanced extension efforts to disseminate knowledge. This information will be useful to plant pathologists and agrochemical companies trying to manage this disease, and to scientists to plan out research that will help to mitigate loses caused by tar spot in the future.

Technical Abstract: Tar spot is a recently introduced foliar disease that threatens U.S. corn production. It was first identified in Mexico in 1904 but was not detected in the U.S. until 2015. Since then tar spot has spread across the corn-growing region, and by late 2020 had been found in Florida, Illinois, Indiana, Iowa, Michigan, Minnesota, Missouri, Ohio, Pennsylvania, and Wisconsin, plus southwestern Ontario, Canada. Losses in the U.S. were estimated at 241 million bushels from 2018 to 2020 but potential losses can be much higher. In the U.S. tar spot appears to be caused by the obligate fungal pathogen, Phyllachora maydis, which cannot be cultured on media, making laboratory manipulations difficult. This pathogen can overwinter on corn residue to produce spores that are disseminated over short distances by rain splash or air currents and likely up to thousands of meters via wind, in addition to movement in infested plant material. The pathogen causes lesions that cover the leaf surface and reduce the green tissue area needed for yield. Tar spot can be diagnosed by a black, glossy, raised stroma on the leaf surface; this may or may not be surrounded by a necrotic halo termed a fisheye lesion. Stroma can be dissected to reveal the fungal perithecia, which produce asexual spores. Its widespread establishment and spread across multiple states and years makes tar spot eradication in the U.S. impossible. Instead, efforts have been focused on disease management, based largely on research conducted in Central and South America. Currently, all corn hybrids can be infected by P. maydis but some have partial resistance. A single source of genetic resistance to tar spot has been identified (qRtsc8-1), but fully resistant hybrids are not available. Chemical control is effective, with multiple popular mixed-modes-of-action fungicides providing protection, but understanding optimal fungicide application timing will be crucial. Cultural management also has potential for disease control. Recommend actions include continued collaboration, screening for sources of resistance, incorporating weather, other environmental factors and fungicide sprays in disease-prediction models, plus a better understanding of pathogen biology and enhanced extension efforts to disseminate knowledge.