Corn stunt disease was found last year in New York State. This disease used to be restricted to warmer regions like the southern U.S. and Latin America, but now it seems to be heading north. Though we have not officially detected it in Ontario yet, it’s close enough that we should start paying attention, especially if you grow field corn or sweet corn.
Read more: Watch out for Corn Stunt Disease!What causes corn stunt?
Corn stunt is caused by a pathogen complex. The corn spiroplasma (Spiroplasma kunkelii) is the main causal agent and three other pathogens are part of the complex:
- the Maize Bushy Stunt Phytoplasma
- the Maize Rayado Fino Virus
- and the Maize Striate Mosaic Virus
Those pathogens can occur alone or in combination, and they are all transmitted by the corn leafhopper (Fig 1).
The corn leafhopper plays a crucial role in spreading corn stunt disease. As they feed, they transmit the pathogens from plant to plant. It is uncertain if the leafhoppers can overwinter in Ontario, but they are highly mobile and can be carried long distances by wind.
Spiroplasmas and phytoplasmas are types of bacteria that lack cell walls and live inside the vascular system of plants, spreading through insect vectors. The main difference is that spiroplasmas have a unique spiral shape, while phytoplasmas have variable shapes. Viruses, on the other hand, are much smaller and consist of genetic material inside a protein coat. They can’t live or reproduce on their own and need to infect plant cells to multiply, spreading through insects, mechanical damage, or seeds.
Where does corn stunt come from?
Corn stunt was one of the biggest constraints for corn and sweet corn production in Latin American countries and southern United States from 1945 to 1980, but it wasn’t a major issue in North America until corn leafhoppers resurfaced in Texas, in 2016.
Last year, the disease was detected for the first time in Oklahoma and New York, across multiple sites. Researchers at Oklahoma State University and Cornell University are currently monitoring the spread of the pathogens and the corn leafhopper. They believe the leafhoppers were carried north by storm systems last year, but it’s unclear if they were able to establish new populations.
Heavy infestations of leafhoppers can cause severe feeding damage. However, economic losses come mainly from the disease spread, which can happen quickly even with small leafhopper populations. Major outbreaks in Florida in 1979-1980 led to over 90% yield loss and $60 million in economic losses.
Symptoms and impact
Corn stunt shows a variety of symptoms (Fig 2) that can be mistaken for other issues like nutrient deficiency, soil compaction, and drought. Symptoms include:
- Red streaks on leaf blades
- Leaf yellowing, noticeable at the beginning of the reproductive stage
- Stunted growth. Plants are shorter and spaces between leaves are smaller
- Chlorosis streaks on leaves
- Overproduction of ears with missing or underdeveloped kernels
These symptoms usually appear on the upper parts of the plants during the reproductive stages, later in the season. In contrast, corn leafhoppers feed on the lower parts of the plants earlier in the season. The honeydew they secrete can turn into black mold, which can help to identify the disease (Fig 3).
Monitoring and management
There is no available treatment that can directly target the corn stunt pathogens, so management strategies focus on controlling insect populations. In other countries, using insecticides to control corn leafhoppers led to higher production costs due to increased pesticide use, but their effectiveness is still uncertain.
In Ontario, efforts should focus on early detection of the pathogens and the vector. Accurate diagnosis of corn stunt must be confirmed through molecular testing. If you spot leafhoppers or suspect symptoms in field corn or sweet corn, reach out to OMAFA specialists for support with the diagnosis:
- Elaine Roddy – Vegetable Crops Specialist (elaine.roddy@ontario.ca)
- Caio Correa – Horticulture Pathologist (A) (caio.correa@ontario.ca)
- Hannah Fraser – Horticulture Entomologist (hannah.fraser@ontario.ca)
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