In a recent issue of Integrated Crop Management, we discussed control of gray leaf spot in field corn. The focus of this article is control of leaf diseases in seed corn production. This production system presents different challenges (and opportunities) when it comes to disease management. Following are some unique features of seed production compared to grain production:
- 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
- greater fungicide options.
Some of these factors lead to a greater need for fungicidal disease control in seed corn compared to field corn. The benefits of foliar fungicides on seed corn have been researched for a number of years at Iowa State University. The occurrence of leaf diseases varies substantially from year to year, according to weather and corn genotype.
||Early symptoms of gray leaf spot.
||Common rust on corn.
Gray leaf spot (Cercospora zeae-maydis) and eyespot (Aureobasidium zeae) have been prevalent in recent years, while common rust (Puccinia sorghi) remains a significant problem. In some years, controlling these diseases with a fungicide increased yields by 14-112 percent and increased profits by $135-$230 per acre. Northern leaf spot (Bipolaris zeicola, also known as Helminthosporium carbonum) is a serious problem on some inbreds. Northern leaf blight (Exserohilum turcicum) is a sporadic but sometimes important disease.
Guidelines for leaf disease control 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 likely be unprofitable if the first fungicide application is made after detasseling.
Guidelines for leaf disease control in seed corn
- 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 knee-high (V8), or even earlier on very susceptible inbreds. Observe at least 100 plants throughout the field. Record the average number of pustules or lesions per plant, disregarding those on 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; 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, 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. Since symptoms of infection do not appear for 10-20 days, infections that occurred before you sprayed will continue to appear after you spray. Your decision to spray again should be based on the label instructions, weather, and disease development in unsprayed areas.
- Continue spraying until the proper pre-harvest 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, and propiconazole) registered for use on corn for seed pro-duction. All of the fungi-cides are effective, but some are less effective for certain diseases. They vary in some restrictions such as the pre-harvest interval and live-stock feeding. Check the label to determine whether or not the fungicide may be applied, rates permitted, and for any restrictions of application. Chlorothalonil (Bravo), copper salts (Tenn Cop), and mancozeb products (Manzate, Dithane, Penncozeb) have protective activity. Propiconazole (Tilt) has protective and curative activity. The labels of all the products specify rust, Northern and Southern leaf blights, and Northern leaf spot. Tilt is the preferred product for gray leaf spot and eyespot control because it is the only fungicide specifically registered to control these diseases.
This article originally appeared on pages 107-108 of the IC-478(14) -- June 23, 1997 issue.