Tar Spot Advances: What is Known Now
RAPID ENTRY, QUICK SPREAD, YIELD LOSSES
Tar spot is a fungal leaf disease that infects corn leaves and causes lodging and yield losses. It has made an abrupt entrance to cornfields across the Midwest and Florida. First found in Illinois in 2015 and Indiana in and Florida in 2016, the disease has rapidly spread to Iowa, Wisconsin, and Michigan (Figure 1).
Figure 1. Midwest counties with confirmed incidence of tar spot as of October 2018 (University of Illinois, Purdue University, University of Wisconsin, Iowa State University, and Michigan State University Extension).
Preliminary data from across the Midwest collected from universities participating in the Crop Protection Network (cropprotectionnetwork.org) indicate that yield losses from tar spot can be severe. Where tar spot infection is high, yields are reduced by more than 30 bushel per acre. Several factors combine to contribute to yield losses: poor kernel fill and ear fill, loose kernels and vivipary - kernels sprouting while still on the cob. Yield losses also result from increased rates of stalk lodging.
CAUSES OF TAR SPOT COMPLEX
Tar spot in corn is caused by the fungus Phyllachora maydis, which was first observed in high valleys in Mexico. P. maydis has not typically been associated with yield loss by itself; however, it can form a complex with another pathogen, Monographella maydis, the combination of which is referred to as tar spot complex. In Mexico, the tar spot complex of P. maydis and M. maydis has been associated with yield losses of up to 30% (Hock et al., 1995).
In some cases, a third pathogen, Coniothyrium phyllachorae, has been associated with the complex. However, to date, only P. maydis is known to be present in the United States.
RANGE AND SPREAD
During 2016 and 2017, tar spot caused only relatively minor cosmetic damage and seemed a disease of little economic concern (Figure 3). However, 2018 brought significant outbreaks and severe yield losses.
Based on climate modeling by Mottaleb et al., (2018), areas at risk of tar spot infection beyond the current range of infestation first include central Iowa and northwest Ohio. Results indicate the potential for further expansion to the north and south but primarily to the east and west, including New York, Pennsylvania, Ohio, Missouri, Nebraska, South Dakota, eastern Kansas, and southern Minnesota.
TAR SPOT EPIDEMIOLOGY
- Much remains unknown about the epidemiology of tar spot, even in its native regions, and especially in the U.S.
- P. maydis is part of a large genus of fungal species that cause disease in numerous other species; however, P. maydis is the only Phyllachora species known to infect corn, and it appears to only infect corn (Chalkley, 2010).
- Tar spot has been reported every year since its initial confirmation, which suggests that P. maydis is overwintering in the Midwestern U.S.
- P. maydis is favored by cool temperatures (60-70 ºF, 16-20 ºC), high relative humidity (>75%), frequent cloudy days, and 7+ hours of dew at night.
- It appears to have windborne spores and tends to release them in periods of high humidity.
- So far, M. maydis has not been detected in the U.S.
- “Fish-eye” lesions, consistent in appearance with those caused by tar spot complex in Mexico, were observed in some Midwestern fields in 2018 (Smith, 2018; personal observation).
- M. maydis was not detected in association with fish-eye symptoms in these cases. The cause of the fish-eye symptoms and why they showed up in some fields but not others remains undetermined.
Figure 2. Corn leaves infected with tar spot in a field in Stephenson Co., IL; September 1, 2018. Tar spot was prevalent in this field, but symptoms appeared late in the season when senescence was already beginning. Stalk lodging was minimal in this field, and yield data suggested that tar spot likely had little to no impact.
IDENTIFICATION AND SYMPTOMS
Tar spot is the physical manifestation of fungal fruiting bodies, the ascomata, developing on the leaf. The ascomata look like spots of tar, developing black oval or circular lesions on the corn leaf. The texture of the leaf becomes bumpy and uneven when the fruiting bodies are present. These black structures can densely cover the leaf and may resemble the pustules of rust fungi (Figure 3).
Figure 3. Corn leaf with symptoms of P. maydis.
Tar spot spreads from the lowest leaves to the upper leaves, leaf sheathes, and eventually the husks of the developing ears (Bajet et al., 1994). P. maydis alone produces small, round, dark lesions; M. maydis causes a brown necrotic ring around the P. maydis ascomata. Together, they produce the characteristic “fish-eye” symptom of tar spot complex (Figure 4). Heavy infection rates may result in leaves becoming densely covered with tar spot ascomata (Figure 5).
Hybrids may differ in susceptibility however with only a couple of years of data available in small areas in the Midwest, much remains unknown. In hybrid trials in 2018, University of Illinois observations by showed that hybrids differed in severity of tar spot symptoms (Kleczewski and Smith, 2018). However, the extent to which differences in leaf symptoms may correspond to differences in yield is unknown at this time.
Due to its recent development in the United States, there are several unknowns about management of tar spot. Much of what is known about the disease comes from experiences in Mexico and Central America. In those areas, however, there are many differences in cropping environments, fungal populations and movement, hybrid genetics, and crop product management systems, each and all of which may influence disease development. However, a consortium of plant pathologists, university researchers, and public and private agronomists has issued some basic guidelines on managing tar spot.
Figure 4. A corn leaf demonstrating “fish-eye” symptoms of tar spot complex (P maydis + M. maydis).
- Manage Residue – tar spot causal pathogen appears to be overwintering in corn residue. The extent to which the amount of residue on the soil surface in a field affects disease severity the following year is unknown. Tilling fields to bury infected residue encourages it to decompose. This may help reduce the quantity of inoculum that can overwinter.
- Rotate to other crops – this may help residue to decompose and reduce primary inoculum. It is still unknown how long it will take to reduce inoculum.
- Fungicide considerations – it is unknown what fungicides may reduce tar spot. Little data exists about specifics. Always check labels for products that can be used to manage tar spot.
- Many questions persist on this disease new to the Midwest. Growers’ observations will be critical to ongoing trials planned for 2019.
Figure 5. Corn leaf under magnification showing dense coverage with tar spot ascomata.