Farmers and crop scouts are beginning to find some leaf diseases showing up on some cornfields in Iowa. "I've already had a few emails, tweets and phone calls from folks who are starting to discover disease symptoms on some of their 2013 corn crop," says Alison Robertson, Extension plant pathologist at Iowa State University.
It is very common to find anthracnose leaf blight on the lower leaves of young corn plants, particularly in corn-on-corn fields, she explains. "If you know the disease cycle of this pathogen, this should be no surprise given all the spring rain we have had," notes Robertson. "Colletotrichum graminicola survives in surface corn residue. In moist conditions in the spring, it produces spores on the crop residue and these spores are splash-dispersed onto the lower leaves. Infection and colonization of the leaf tissue occurs, and we see the dark irregular-shaped lesions on the bottom four to five leaves."
Once the corn canopy closure occurs, it is rare to see anthracnose leaf blight and, because it occurs so early in the growth of the plant, it does not affect yield, she explains. Furthermore, studies at both Iowa State and the University of Wisconsin have found no relationship between the incidence of anthracnose leaf blight and anthracnose stalk rot, says Robertson.
Common rust has also been reported at very low incidence in some fields in Iowa already this year. "Again, this is not too much of a surprise because we often see common rust toward the end of June," says Robertson. "What is different this season is that we are seeing it on very young corn plants because so much of the crop was planted late. Last year I compared common and southern rust. On July 1, gray leaf spot was reported in southwest Iowa."~~~PAGE_BREAK_HERE~~~
Robertson, as of July 1, also had one report of Goss's wilt from west-central Iowa. To distinguish this disease from some other look-alike problems (for example, burning on the corn leaves associated with ammonium nitrate side dressing), Robertson says you need to remember to look for freckles in the edge of the lesion. "I expect to hear of more reports of this disease if stormy weather continues," she says. "Look for Goss's wilt in the corn-on-corn fields that are planted to hybrids that are rated susceptible to Goss's wilt. That's the first place you should look for this particular disease."
If you see symptoms, check to make sure you have the disease properly identified, then consider making a fungicide application
* Disease management tips: Although some products are marketed for Goss's wilt management, there is still not much data available on their effectiveness. In 2012 Robertson and her ISU colleagues evaluated several products (Procidic, 42-Phi Cu, EcoAgra and Elixor). "But we were unable to detect a treatment effect because we had very low incidence of the disease at all our locations last summer," she notes.
While an application of a fungicide at the V5-V6 growth stage of corn plants would likely reduce common rust and anthracnose leaf blight, these diseases rarely cause yield loss on corn in Iowa, says Robertson. Most corn hybrids have good resistance to common rust, plus as the season progresses it usually gets too hot (greater than 80 degrees F) for the disease to continue to develop, she explains.
* Looking ahead for the rest of this growing season: Warm weather with frequent precipitation favors gray leaf spot and northern leaf blight, both of which reduce yield, says Robertson. "Moreover, the earlier in grain fill that these diseases occur, the greater the yield loss. Because much of the corn was planted late this season, tasselling will likely occur towards the end of July, so we could be at risk for yield loss due to disease."
"While you are out scouting your cornfields, check for disease development in the lower canopy, particularly gray leaf spot and northern leaf blight," she advises. "This is a good indication that conditions are favorable for disease development and a fungicide application between tasseling and 'brown silk' (blister stage) should be considered, particularly if the hybrid is susceptible to disease. Moreover, there is a greater chance of a return on investment with a fungicide application when disease risk is high."