Early in July Dave Nanda reported that it looked like it was going to be the year of gray leaf spot. Susceptible hybrids would be at risk. The consultant and plant breeder made that report after finding a significant amount of early pressure in fields on susceptible hybrids. At that time it was in the lower canopy. But nearly an entire growing season is a long time for a fungus to spread. Once it gets above the ear leaf and is severe, if it happens early enough in the growing season and shuts down maximum photosynthesis, it can affect yield.
His first field visit and report followed 5 consecutive days of 90 degree plus weather at the end of June in the eastern Corn Belt. Humidity and moisture levels were also still high. That's perfect weather for gray leaf spot, Nanda notes.
When he visited the same cornfields a month later, he was somewhat surprised, at least at first. There was very little additional progress by gray lead spot organisms, even on susceptible hybrids. By then pollination was nearing completion and since the lesions were still below the ear leaf, he advised against spraying fungicide. Only a month earlier he strongly suspected that susceptible hybrids would need to be sprayed to limit damage and preserve as much yield as possible.
That observation followed the coolest July on record in much of the Midwest, especially in the eastern and northern states. Gray leaf spot likes heat and humidity, not cool weather. Apparently, the cool weather arrested dev elopement enough that the corn got ahead of the organism.
Nanda was somewhat surprised again when he visited fields last week, and field a pre-harvest report. There were more Gray Leaf Spot lesions in many fields than he expected, based on his August observations. The last month ahs been mild, neither hot nor cold, across much of the Midwest. However, he still doesn't believe gray leaf spot will cause yield loss, because development didn't pick up again until later in the season. Obviously, it was more prevalent on hybrids that are more susceptible to gray leaf spot. Nearly every company ahs rating systems for major disease4s. However, all companies don't use the same system. Visit with your seedsman to review ratings on key diseases for the hybrids that you intend to plant next year.
What Nanda did find last week was an abundance of northern corn leaf blight. That's not surprising,. He says. Because it's a disease favored by the type of weather patterns that have emerged over the past three months, and although it was even on upper leaves, it may have little effect on yield in most fields, since it came in fairly late.
"What it may do is actually help corn dry down faster," Nanda says. "If it doesn't become severe until the plant reaches black layer, after which no more sugars come in or out of the kernel, and then it helps it dry up and dry down, it could actually be a plus in this particular season. Most fields are running far behind schedule, and while yields may be high, except where rainfall was limiting, corn will likely be wet at harvest and require lots of energy to dry it down."
The one negative to having any disease or insect damage in a plant is if predisposes the corn plant to secondary stalk rot infections. That's more of a concern with insects, such as corn borers, because the rots, also fungi, can enter the plant directly through open wounds.