It has been an interesting year for field crop pathologists in the U.S. Although many seasons often pass without the discovery of any new diseases, this year they were treated with not just one but two new corn diseases in the country.
Tar spot and bacterial leaf stripe were both discovered late in the growing season — too late, in fact, to have any economic significance for producers in the affected areas. But next year?
What exactly is Tar Spot?
Early in September, Dr. Kiersten Wise and Gail Ruhl at Purdue University positively identified Phyllachora maydis in the U.S. for the first time.
This fungus is among the pathogens commonly known to cause tar spot, and had only been known to exist in the cooler, humid, higher elevations of Mexico, Central and South America and the West Indies.
Now that samples collected in Indiana by Monsanto breeders have been positively identified, plant pathologists are asking farmers and agronomists to be vigilant for symptoms in other regions too.
The overwintering form of corn rust and many saprophyte fungi that feed on dead corn tissues can be confused with tar spot, so now isn’t a great time to be looking for the disease.
In Mexico, tar spot is best found about two weeks before flowering but can show up as early as the eight-leaf stage, and it usually peaks about four weeks after flowering. Black, oval or circular lesions appear to sink into the leaf surface and really do look like tar. Small flecks may be the first to appear but lesions are able to merge and form blotched up to three-eighths of an inch across. Wise says one of the more distinctive signs of the disease is that an infected leaf will feel bumpy if you run a finger over the surface.
Though they’re easier to scout, don’t expect to see spots on husks or upper leaves first. The disease will start on the lowest leaves of the plant and work its way up, since it prefers 16-20 C and a mean relative humidity of 75 per cent or higher. Rain is not as conducive to infection, as it tends to wash spores off the leaf, but six to seven hours of dew or leaf wetness during the night is particularly good for breeding the disease.
If the temperature goes below 10 C or above 20 C, spore germination drops off, which has many disease watchers questioning whether tar spot can even overwinter in Illinois or Indiana.
“We suspect it’s not going to be able to overwinter, but we have to do some research to really understand that,” Wise says. “The good news is, even if it is maybe able to overwinter, we’ve only found a fungus that’s not reported to cause economic damage in the areas where it’s endemic.”
Where tar spot is prevalent, there are actually two pathogens which cause the disease and Monographella maydis, the worst of the two fungi, was nowhere to be found in the U.S. this year. Where both fungi are found, the USDA Research Service estimates yield losses average eight percent, although they can get up to 30 per cent in Mexico.
Albert Tenuta, field crop pathologist for the Ontario Ministry of Agriculture, Food, and Rural Affairs, says that the areas where tar spot would be most likely to appear first, if it ever came to Ontario, would be along the north shores of Lake Ontario, Lake Erie and the St. Lawrence. He says a good general rule of thumb would be to assume tar spot would thrive anywhere that white mould thrives, since the diseases enjoy the same environmental conditions.
“If we have a good white mould year, there’s also good potential for tar spot but we don’t have an inoculum source around here,” Tenuta says. “Spores would either have to be blown in or brought in on some contaminated product.”
And Leaf Stripe too?
Burkholderia andropogonis is a bacterium that can infect corn, Johnson grass, sorghum, rye, clover and other plant species. The bacterial leaf disease was confirmed for the first time in Champaign County, Illinois and was found even later in the year than tar spot was found in Indiana. The discovery was made so late in the growing season that there really is very little known about how much impact it has really had on the 2015 crop where it was found. But bacterial leaf stripe has been seen in the U.S. before, having been observed in South Dakota, Iowa, Kansas, Nebraska and Michigan between 1973 and 1975, when it appeared to cause no significant yield losses during.
“Bacterial leaf stripe has been detected wherever corn is, including Canada, but it’s not a big concern,” Tenuta says. “The key with bacterial stripe is that it is often confused with other diseases.”
The only way to really tell bacterial leaf stripe from Goss’s wilt and Stewart’s wilt is by lab testing.
“Neither tar spot or bacterial leaf stripe were detected in our annual surveys,” Tenuta points out.
Tenuta says Ontario is well positioned to detect and respond quickly because of those annual surveys. On average, the scouting team they form with Agriculture and Agrifood Canada collects samples in 175 to 200 commercial and seed corn fields every year throughout Ontario and into Quebec. Since most cases of corn diseases have historically started in this area, he says they’ve learned to keep a watchful eye out for new diseases. But with environmental changes come changes in pest pressure. Goss’s wilt is a disease that has bucked the trend, establishing in Mani-toba and southern Alberta while still undetected in Ontario.
Josh Cowan, Grain Farmers of Ontario, says their primary interest in helping to sponsor this annual survey is to follow changes in northern corn leaf blight.
“We entered this project because of the evidence that susceptibility to northern corn leaf blight was increasing in corn hybrids and the pathogens seem to be overcoming the resistance of the hybrids,” Cowan explains. “It also helps us understand the geographic extent where the resistance is breaking down so farmers get an understanding whether or not they need to pay attention to this if it’s in their area or not.”
This article was originally published in the January 2016 issue of the Corn Guide