All the rain and prolonged wetness Ontario has experienced this year are likely the contributing factor to tar spot.
What is Tar Spot?
It is a fungal leaf disease originating in Central America. It was first discovered in Illinois and Indiana in 2015. In 2020, it was first introduced in Ontario, in the Ridgetown area. By 2021, it has now spread through SW Ontario.
How to identify Tar Spot?
- Look for small, raised, black spots scattered across upper and lower leaf surfaces. These are filled with spores.
- You cannot scratch it off. If still unsure what it is, conduct the “spit-test”. Wet the leaf and rub between fingers.
- Fisheye lesions can develop – tan lesions surrounding black fruiting bodies
- Generally spreads from lowest leaves to upper leaves and husks of developing ears
- Ears can have reduced weight and loose kernels
Conditions Favouring Disease:
- Cool temperatures with high humidity
- Increases greatly with 7 hours + of free moisture on leaves (rain, fog, humidity)
What is Tar Spot Implications?
- Yield losses of 20-50 bus/ac possible
- Losses depend greatly on level of infection as well as how early infection came in
- Stalk strength can be greatly weakened by tar spot infection
- Affects field corn, as well as sweet corn and seed corn
- Very weather dependent for how bad level of infection gets
How to Manage Tar Spot:
When thinking about how best to manage the disease, your first look should be a genetics. It is yet to be determined if resistant hybrids exist. Ongoing research has suggested that some hybrids are more tolerant than others.
Fortunately, most growers already have many fungicides in their management toolbox that are effective against tar spot. These types of fungicides are ones that are currently used to fight common rust, northern corn leaf blight, and grey leaf spot. Determining optimum application timing is still up in the air and more research is needed.
If you think that you may have tar sport in your field currently, it is important that you get in touch with your Agriculture Representative at Sylvite. We can then determine if immediate measures are required to stop the disease from gaining a foothold in the infected fields.