Keep an eye out for Goss’s wilt
Despite June’s hot, dry weather, a couple of corn leaf diseases have already started showing up in Iowa. After last year’s outbreak, Goss’s wilt is on most every farmer’s radar.
In mid-June Goss’s had shown up in a field in Calhoun County in northwest Iowa and in a field in Story County in central Iowa.
Both of these would be considered high-risk fields, says Alison Robertson, Iowa State University Extension plant pathologist. During the 2011 growing season, a corn hybrid susceptible to Goss’s was grown on these fields and disease development was severe. Some crop residue is present on the soil surface this year and a Goss’s-susceptible hybrid is being grown there again in 2012.
To differentiate Goss’s wilt (a type of leaf blight) from other spots on corn leaves, look for freckles and ooze. If it’s Goss’s wilt, the freckles should be evident in the edges of the lesion. Ooze may or may not be present, says Robertson.
If you are scouting for Goss’s wilt, Robertson suggests to focus attention on fields that are planted to susceptible hybrids, have a history of the disease, have crop residue on the soil surface and have been recently injured by severe weather. Look for characteristic lesions of Goss’s wilt on the corn leaves. Strong winds that occur in an area are likely to cause damage to leaves and enable infection.
• Goss’s wilt was severe in some areas of Iowa last year.
• In 2012 farmers will want to scout fields for symptoms.
• Don’t confuse Goss’s wilt with other corn leaf diseases.
The Goss’s wilt bacterium (Clavibacter michiganensis nebraskensis, or CMN, for short) survives well in infested crop residue on the soil surface. “Infection is usually associated with severe weather events that injure corn leaves and enable entry of the bacterium into leaf tissue,” says Robertson. “In greenhouse studies we typically see symptoms on corn seedlings 10 to 21 days after inoculation with the bacterium.”
The most characteristic symptoms of Goss’s wilt are “freckles” within large reddish-brown lesions that usually occur along leaf edges. Bacterial ooze may occur on the lesion, giving it a wet or greasy appearance. When the ooze dries, it leaves a shiny residue on the lesion’s surface.
Planting a tolerant hybrid is the most effective way to manage Goss’s wilt, says Robertson. And spraying a fungicide won’t work because Goss’s wilt is caused by a bacterium, not a fungus.
Several foliar bactericide products are marketed for Goss’s wilt. Unfortunately, notes Robertson, no field data are available on their efficacy. Preliminary trials in the greenhouse on V3 corn seedlings indicated some products might slow disease development. Because greenhouse conditions are very different from field conditions, further evaluations in field situations are needed.
“This growing season we have field trials to evaluate foliar products at three locations in Iowa,” she says. On-farm trials in collaboration with the ISU-FARM [Farmer Assisted Research and Management] project also are being done in northwest Iowa. And participants at the ISU Crop Management Clinic and Corn Disease workshops, which will be held in July and August, respectively, at the ISU Extension Field and Education Laboratory near Boone, will evaluate foliar products in CMN-inoculated trials.
If you try a bactericide for Goss’s, Robertson recommends leaving two or three untreated strips in your field. These strips should not be in the end rows. Throughout the growing season, scout the field and make comparisons between the treated and untreated areas. If you see anything interesting, feel free to email her at email@example.com.
Don’t confuse Goss’s wilt with Holcus leaf spot. ISU Extension field agronomist Joel De Jong in northwest Iowa reported seeing Holcus leaf spot in corn in June. Symptoms of this disease are round spots, about 0.25 inch in diameter, that are pale yellow to white, with a water-soaked halo. Spots on some hybrids may have a purple or brown margin.
“Holcus is another disease of corn caused by a bacterium,” says Robertson. The pathogen is Pseudomonas syringae pv. Syringae — different from CMN. “Holcus infection of corn leaves occurs through wounds or stomates,” she adds. “But Holcus rarely gets severe enough to impact yield.”
Source: ISU Extension
This article published in the July, 2012 edition of WALLACES FARMER.
All rights reserved. Copyright Farm Progress Cos. 2012.