Row Crop Disease Update – Corn & Soybeans

We are starting to tassel (VT) and move into reproductive growth stage on corn (R1), when the silks emerge from the little ears and the pollen starts falling from the tassels. For Seminole Agent Rome Ethredge writes, “The greatest production of pollen from a field will occur over 4 days, with some pollen shedding for a week or more. Over 2 million pollen grains fall from each tassel.  Peak pollen shed is mid morning and then towards late afternoon more falls, but if it’s raining or a heavy dew the plant won’t shed pollen until it dries. Also, some research shows that when the temperature goes above 86 degrees there isn’t much shed. If it’s cloudy and slightly cool, pollen may fall most of the day. Extreme heat (100 degrees) can kill some of the pollen.”

This is also a critical time in corn development where we don’t want stress on the plants. Stress during silking can reduce the number of kernel per ear. The number or rows will not be affected now as they were decided already. Disease is the main topic this week with corn, and here is an update from UGA Extension Pathologist Dr. Bob Kemerait on disease in CORN and SOYBEANS.

Here are a couple of notes on the current disease situation:

1. Our scouts have found Asian soybean rust on kudzu in Miller County today (May 17th). This is the earliest we have found it since 2005, and it likely means rust will be problematic for our soybean producers this year. Current conditions favor spread of the disease within kudzu.
2. Corn in the earliest planted fields is approaching the tassel growth stage and growers are beginning to ask questions about spraying fungicides.  Here are my thoughts:

• Tassel stage (or just prior to tassel) is an appropriate time to consider applying a protective fungicide.  This is especially true if a) southern corn rust has been detected in the area, b) northern corn leaf BLIGHT is problematic in a field (typically with a less-resistant hybrid), c) conditions have been favorable for disease (very wet), d) the corn was planted LATE or e) the grower is aggressive in a disease management program and wants to make sure the crop is protected.
• As of today (May 17th), we have not found SOUTHERN CORN RUST in Georgia and conditions have not been especially favorable for southern rust.
• As of today, we have had one report of common corn rust from Mitchell County. Common rust typically forms pustules on both sides of the leaf and does NOT need a fungicide application.

Common Rust, Photo by Andy Shirley, Mitchell County

• As of today, I have only had a report of northern corn leaf BLIGHT from Ty Torrance in Decatur County. Northern corn leaf blight can be an important problem that requires a fungicide treatment IF it is severe (e.g., a susceptible variety and favorable weather). The NCLB in Decatur County was confined to the bottom leaves and there were only a few lesions on about one plant out of 15. The grower is right to be aware of the problem but I do not think a fungicide is needed for NCLB in that particular field.

Northern Corn Leaf Blight, Photo by Ty Torrence, Decatur County

• Northern corn leaf SPOT has been found in Mitchell County by Andy Shirley.  Typically we do not spray for this disease, except in severe cases. The northern corn leaf spot in this field was confined to the lower leaves and did not appear to be spreading.

Northern Corn Leaf Spot – Photo by Andy Shirley, Mitchell County

CORN:  We have not detected southern corn rust in Georgia yet. Conditions over the next few days are more favorable for disease spread, but (overall) conditions have been unfavorable. I would not argue with a grower who wants to apply a fungicide at this time (to corn) as it reaches the tasseling growth stage; HOWEVER I think the grower is better advised to DELAY a fungicide application at this point and wait at least a week or so.