Gray leaf spot is now appearing in southern Iowa seed cornfields. This disease and others can be very serious in seed corn production, and fungicidal control will be necessary in some fields. Seed corn presents different challenges (and opportunities) when it comes to disease management. Some unique features of seed production compared with grain production include the following:
- high value per acre,
- a broader range of leaf diseases cause economic damage,
- a need to grow specific genotypes regardless of susceptibility,
- leaf loss due to detasseling, and
- more fungicide options.
||Early symptoms of gray leaf spot.
||Common maize rust.
Some of these features lead to a greater need for fungicidal disease control in seed corn compared with field corn. The benefits of foliar fungicides on seed corn have been researched for a number of years at Iowa State University. Protecting susceptible inbreds with a fungicide has proven to be very profitable.
Leaf disease problems in seed corn include gray leaf spot (Cercospora zeae-maydis), eyespot (Aureobasidium zeae), common rust (Puccinia sorghi), northern leaf spot (Bipolaris zeicola, also known as Helminthosporium carbonum), and northern leaf blight (Exserohilum turcicum).
Guidelines for foliar disease control (see below) are based on scouting, relative susceptibility of the seed parent inbred, and weather considerations. In general, the most profitable results occur when sprays are initiated early in the season. Attempts to stop an epidemic will probably be unprofitable if the first fungicide application is made after detasseling.
- Do not plant seed corn in a field where corn was the previous crop, unless absolutely necessary.
- Know the susceptibility of the inbreds you are growing. This is a crucial point because the more resistant inbreds rarely need a fungicide. The more susceptible the inbred, the more likely fungicide use will be profitable.
- Scout fields early, when plants are about V6-V8. Observe at least 100 plants throughout the field. Record the average number of pustules or lesions per plant, disregarding the bottom three leaves.
- Scout every 1-2 weeks depending on weather and susceptibility. The interval should be shorter in wet, cool weather and on the most susceptible inbreds, and longer in hot, dry weather and on more resistant inbreds.
- When there is an average of 1-2 pustules or lesions per plant, and weather is favorable for disease (moderate temperatures and frequent rains or dews), begin spraying susceptible inbreds. Remember that fungicides are most effective when sprayed before infection takes place, so you must consider the weather forecast as well as previous weather.
- Leave an unsprayed area for comparison. There is always a temptation to protect everything, but an unsprayed check will provide valuable information on the effects of spraying.
- Follow label instructions for rates and spray intervals. Because symptoms of infection do not appear for 10-20 days, infections that occurred before you sprayed may continue to appear after you spray. So your decision to spray again should be based on the label instructions, weather, and disease development in unsprayed areas.
- Continue spraying until the proper preharvest interval or if weather turns hot and dry.
- If diseases have not appeared before tasseling, spraying is probably unnecessary.
There are four fungicides (chlorothalonil, copper, mancozeb, propiconazole) registered for use on corn for seed production. The fungicides differ in their efficacy against certain diseases and in their restrictions such as the preharvest interval and livestock feeding. Check the label to determine whether or not the fungicide may be applied, rates permitted, and for any restrictions of application. A recent change is that Tilt can now be applied up to 30 days preharvest (but forage and fodder may not be fed to livestock if Tilt is applied after silking). Tilt is the preferred product for gray leaf spot and eyespot control; it has protective and curative activity. Chlorothalonil (Bravo), Copper salts (Tenncop), and mancozeb products (Manzate, Dithane, and Penncozeb) have protective activity only. These products are effective against rust, northern and southern leaf blights and northern leaf spot. Penncozeb also specifies gray leaf spot control on the label.
This article originally appeared on pages 133-134 of the IC-482(18) -- July 12, 1999 issue.