Last year, sweet corn growers suffered severe losses to common rust (Puccinia sorghi). This and other foliar diseases, particularly Northern leaf blight (Exserohilum turcicum), can be economic problems on susceptible varieties to some extent each year. Since rust does not overwinter in Iowa, we do not necessarily expect another severe rust year, but that will depend on the weather. Although resistance is available for these diseases, it is not always completely effective and some of the most desirable varieties are susceptible. Therefore, it can be profitable in many cases to use foliar fungicides to control these diseases.
Guidelines for common rust control have been based primarily on research with susceptible varieties. They rely on scouting and consideration of relative susceptibility of the variety and weather. Similar guidelines can be used for Northern leaf blight control, although it will typically show up later in the season.
- Know the susceptibility of the varieties you are growing. This is a crucial point because the more resistant varieties rarely need a fungicide. The more susceptible the variety, the more likely fungicide use will be profitable.
- Scout fields early, when plants are about knee-high (V8). Observe at least 100 plants throughout the field. Record the average number of pustules or lesions per plant.
- Scout every 1-2 weeks depending on weather and susceptibility. Interval should be shorter in wet, cool weather and on the most susceptible varieties, 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 (night temperatures below 75 F and frequent rains or dews), begin spraying susceptible varieties. This will usually be when about 80% of plants are infected. 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 about 10 days, infections that occurred before you sprayed will continue to appear after you spray. So your decision to spray again should be based on the label instructions and weather.
- Continue spraying until proper interval-to-harvest or if weather turns hot and dry.
- If the threshold (see number 4 above) is not reached before tasseling, spraying is probably unnecessary. Leaves are much less susceptible to rust after tasseling but they remain susceptible to other diseases.
In 1993 it would have been profitable to have sprayed nearly all sweet corn varieties for rust control. Best control occurs when sprays are initiated early. Attempts to stop a rust epidemic will likely be unprofitable if the first fungicide application is made after tasseling. Last year, seed corn growers reported mixed results with fungicide applications. Some were very successful, others were disappointed. Some disappointing results were due to two factors: the disease pressure was extremely high, and many fungicide applications were made too late to effectively stop the disease.
There are four fungicides labelled for sweet corn production. All of the fungicides are effective, but some are less effective for certain diseases. They vary in the interval-to-harvest required. Check the label to determine whether or not the fungicide may be applied, rates permitted, and for any restrictions of application. Chlorothalonil (Bravo), mancozeb products (Manzate, Dithane, Penncozeb), and maneb have protective activity. Propiconazole (Tilt), a systemic fungicide, was just recently registered for sweet corn.
This article originally appeared in the June 8, 1994 issue, pp. 1994 issue, pp. 88-89.