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Pathogen
Tar spot is caused by the fungus, Phyllachora maydis.
Hosts
Sweet corn and field corn.
Significance
Tar spot was introduced into the United States from Mexico and first reported in Illinois and Indiana in 2015 in field corn. In 2024, tar spot was reported in 18 states and 23 counties across western, central, and northern New York. The losses caused by tar spot in sweet corn have varied across regions and between years. Tar spot symptoms primarily appear on leaves causing reductions in photosynthetic area. The disease can also affect the size, shape, and uniformity of the corn ears. For processing sweet corn, recovery of kernels may also be reduced. Tar spot lesions on the husks may make them unsuitable for fresh market sales or incur extra labor costs due to the need to dehusk prior to sale.
Symptoms
Symptoms of tar spot are small, raised black spots with dark borders and a ‘tarry’ appearance that may be present on the leaves (Fig. 1A), leaf sheaths, and husks (Fig. 1B). As the spots become older, they may develop a ‘fisheye’ appearance (Fig. 2A). The tar spots are fungal structures called stromata which contain spores that are responsible for pathogen dispersal (see Disease Development). Tar spots usually first appear in the lower canopy and as disease severity increases symptoms spread to the upper canopy and are associated with premature leaf death (Fig. 2B). Multiple foliar diseases may also appear on the same leaf (Fig. 2C).
Figure 1: (A) Symptoms of tar spot appear on leaves as small, raised black spots with dark borders, often giving them a “tar-like” appearance.
Figure 1: (B) Symptoms can also appear on leaf sheaths and husks.
Figure 2: Tar spots usually first appear in the lower canopy and as (A) disease severity increases, symptoms spread to the upper canopy and are associated with (B) premature leaf death. (C) Tar spot lesions are raised from the leaf surface and go through the leaf so they are visible on both sides.
Tar spot can be challenging to find when scouting if the disease incidence is low. Tar spot lesions can also be easily confused with insect frass or other diseases such as some rust pustules. Tar spot lesions cannot be rubbed off the leaf with your fingers. The lesions are also raised from the leaf surface and go through the leaf, so they are visible on both sides (Fig. 2C). Use this key to determine if the symptoms are likely to be tar spot (Fig. 3). Please contact your local CCE office or a diagnostic clinic for assistance with symptom identification.
Figure 3: Three key factors for identifying tar spot of sweet corn in the field.
Disease Development
The fungus that causes tar spot, P. maydis, overwinters in infested corn residue as stromata. There are no alternative hosts, and the fungus can only infect field or sweet corn (see Alternative Hosts). Infections and disease spread are promoted by mild to moderate temperatures (59 to 70 °F), high humidity (>85%) and when leaves are wet to touch for at least 7 hours. The spores of the fungus that initiate new infections are contained within the tar spots, or stromata, of the fungus and are dispersed by wind-driven rain and storms.
Disease Cycle
Figure 4: Tar spot of sweet corn disease cycle.
Disease Management
An integrated management strategy for tar spot should break the infection cycles, reduce disease progress within the growing season, and ideally manage other foliar diseases (e.g., Northern Corn Leaf Blight and Common Rust) at the same time.
Inoculum Sources
- Susceptible Hosts. Corn (sweet and field) is the only host of the pathogen.
- Seed. The pathogen, P. maydis is not seed transmitted.
- Infested Crop Residues. The dominant source of inoculum for tar spot outbreaks is infested crop residues.
Management
Cultural Practices
- Crop rotation. Crop rotation should include three years between field or sweet corn.
- Residue management. Incorporate infested corn residue into the soil to encourage decomposition as soon as possible after harvest. This could be accomplished by discing and/or winter cover crops. The fungus cannot survive in the soil only.
- Irrigation. If irrigation is used, applying at midday to minimize the time that leaves are wet will reduce infection and disease spread. Ideally, leaves should not be wet to touch for longer than 7 hours.
Cultivar Selection
Little is known about the susceptibility of sweet corn cultivars to tar spot in New York. However, preliminary data suggests cultivars GH 9335 and GSS 2259P are tolerant to tar spot, while Early Cogent may be highly susceptible. Research is underway to characterize the susceptibility of sweet corn cultivars commonly grown in New York to tar spot.
Fungicides
In sweet corn, fungicides are typically applied at V7-V8 and tassel (Table 1). The aim of applying fungicides for the control of tar spot (and other foliar diseases) is to reduce defoliation during reproductive stages of growth and protect the ear leaves, and eliminate disease on the husks for fresh market sales.
Table 1. Selected conventional fungicides registered for tar spot control in sweet corn in New York.
OMRI-listed Fungicides. No efficacy data is currently available for tar spot control in sweet corn in New York.
Additional Information
Dr. Sarah J. Pethybridge (sjp277@cornell.edu)
Cornell AgriTech, 211 Barton Laboratory
Geneva, New York
(315) 744-5359 (cell)
Dr. Julie R. Kikkert (jrk2@cornell.edu)
Cornell Cooperative Extension
Cornell Vegetable Program
Canandaigua, New York
(585) 394-3977 x 404 (office)