GRAY LEAF SPOT
Overview: Gray leaf spot, caused by the fungus Cercospora zeae-maydis, leads to leaf tissue loss, a decrease in plant sugars and decreased grain production. It is considered the world’s most yield-limiting disease in corn, causing yield losses from 5-40 percent. All corn hybrids have some susceptibility to the disease.
What You Should Know About Gray Leaf Spot:
- Gray leaf spot overwinters in residue on the soil surface. Spores develop on residue in late spring when temperatures and humidity increase. Rain and wind transport spores onto the corn plant, where they mature and move up the plant.
- Gray leaf spot infection occurs when relative humidity is more than 90 percent and the leaf surface remains wet for more than 11 hours.
- Soon after tasseling, orange-red lesions with narrow yellow halos appear. As lesions mature, they appear tan or brown in color and rectangular in shape. The release of gray fungal spores in mature lesions give the disease its name. Lesions are ½ to 4 inches long and vein-limited, giving them a straight-edge appearance.
- Increased use of reduced tillage and no-till production practices has increased the incidence of gray leaf spot. Weedy fields trap humidity and prevent leaf drying by blocking air movement through the canopy.
Action Steps for Managing Gray Leaf Spot:
- Manage residue: Crop rotation and clean plowing are effective methods to control fungus levels. A two-year crop rotation away from corn is effective under reduced tillage. A one-year rotation is sufficient with clean plowing.
- Apply fungicides early: Fungicides are recommended when susceptible hybrids are planted in fields with a history of gray leaf spot. Apply fungicides early in the season before significant leaf damage occurs.
- Select tolerant hybrids: Using tolerant hybrids provides an earlier, more extensive source of inoculums for gray leaf spot development.
Overview: Goss’s wilt (Clavibacter michiganensis subsp. nebraskensis) is a bacterial disease that affects corn plants. Goss’s wilt has been prevalent in areas of the Western Corn Belt for decades and has expanded in recent years to additional areas of the Central and Eastern Corn Belt.
What You Should Know About Goss’s Wilt:
- Goss’s wilt infection can occur at any time during the corn growing season. If Goss’s wilt infections occur during key crop development early in the growing season, yield potential could be reduced by up to 50 percent.
- Seedling plants that are infected will have a slimy residue on the leaves. Larger plants will have oblong lesions with wavy margins and dark green to black “freckles.”
- Goss’s wilt bacteria require an “entry point” to infect a corn plant. Crop injury from hail, wind, insects or even heavy rain can allow bacteria to enter. Humid, wet weather is another risk factor, because wet or moist leaves can allow infection to spread.
- Goss’s wilt bacteria can overwinter in residue and can survive on the soil surface for one year. Because it is a bacterial disease, it cannot be treated with fungicides.
Action Steps for Managing Goss’s Wilt:
- Reduce residue: Rotate crops and utilize tillage to bury residue and reduce the probability of infection. Infected fields should be tilled or harvested last to prevent infected material from spreading to uninfected fields.
- Select appropriate hybrids: The best way to control the disease is to plant hybrids with strong Goss’s wilt tolerance.
- Goss’s wilt is a bacterial disease that affects corn plants.
- Goss’s wilt infection can occur at any time during the corn growing season.
- Goss’s wilt infection can reduce yields by up to 50 percent.
- Planting Goss’s wilt-tolerant hybrids and reducing corn residue are the best ways to control Goss’s wilt.
IRON DEFICIENCY CHLOROSIS OR IDC IN SOYBEANS
Iron chlorosis symptoms are interveinal chlorosis of the leaves with leaf veins remaining green as shown in the photo below. These yellow leaves tend to show up when the soybean plant is in the first to third trifoliate stage. Iron chlorosis in plants is more prevalent in poorly drained soils containing high pH or calcium carbonate, high salt or saline soils.
Soils in most instances have enough iron (Fe) for production, the issue that arises is that as pH increases the soluble form of Fe decreases due to high pH especially those with high calcium carbonates and in some instance’s salts. As air space reduces in soils (wet soils) the respiration of carbon dioxide from roots and soils increases, when that happens you increase bicarbonate levels at the same time. Higher bicarbonate levels (I always think higher soil pH in this instance) correlates with higher IDC.
Other factors that may increase bicarbonate levels are crop residues and manures. As soil microbes are breaking down organic matter from the crop residue or manure, they release carbon dioxide which again raises bicarbonate levels. Higher soil nitrate levels and increased IDC, in my opinion, is very complex, as a soybean root takes in nitrate it exchanges it with bicarbonate ion, again increasing bicarbonate soil levels. Another aspect is as the plant brings in nitrate it converts it to ammonium in the leaves which increase leaf sap pH levels and then reduces the level of usable Fe available to the leaf and plant.
Best Management Practices for Iron Deficiency Chlorosis:
- Select a variety(s) that has a good to very good IDC scores
- Utilize an ortho-ortho-EDDHA fertilizer at planting in IDC prone areas of fields
- Increase plant populations in IDC prone areas of fields this spreads salt and bicarbonate uptake over more plants reducing their levels in individual plants
- Utilize a companion crop such as oats at planting to reduce nitrate and water levels, terminate this companion crop prior to it reaching 8-12 inches in height
Other factors that may reduce Iron Deficiency Chlorosis severity:
- Increasing soil water drainage in IDC prone areas
- Decreasing soil nitrate levels via increased Nitrogen Management
- Increasing crop rotation intervals away from soybean production
As always, if you have questions about this or other agronomic issues, please contact your local Dairyland Seed Agronomist.
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