Iowa State University has researched the benefits of foliar fungicides for eleven years. During the last four years, 27 experiments have been conducted in commercial seed production fields under natural conditions. The occurrence of diseases has varied substantially from year to year, according to weather and corn genotype.

Common rust has been the predominant disease the last two years. In 1992, a cool, rainy July resulted in an epidemic. Spraying started in late June or early July and continued until about 2 weeks after detasseling. This resulted in yield increases of 6 to 17 salable bags of seed per acre, with significant increases in bags of medium- and large-sized seeds. In 1993, the rust epidemic was already underway before spraying started, about 2 weeks before detasseling. Although heavy rust still developed in the sprayed plots, control was significant. We increased yield of salable seed by 14 to 112 percent. Because disease developed in the sprayed plots, we could not estimate total losses, but we increased yield by 20 to 65 bags of salable seed per acre.

The following guidelines for common rust control are based on scouting, relative susceptibility of the seed parent inbred, and weather considerations.

1. 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.

2. 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.

3. Scout every 1 to 2 weeks, depending on weather and susceptibility. Intervals should be shorter in wet, cool weather and on the most susceptible inbreds; longer in hot, dry weather and on more resistant inbreds.

4. Begin spraying susceptible inbreds when plants average 1 to 2 pustules or lesions and weather is favorable for disease (night temperatures below 75F and frequent rains or dews). This is when about 80 percent of the plants are infected. Remember, fungicides are most effective when they are sprayed beforeinfection takes place, so consider the weather forecast as well as previous weather.

5. Leave an unsprayed area for comparison. It is tempting to protect everything, but checking an unsprayed area provides valuable information.

6. 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 continue to appear after you spray. Base your decision to spray again on the label instructions and the weather.

7. Continue spraying until proper interval-to-harvest or the weather turns hot and dry.

8. 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, spraying nearly all seed parent inbreds for rust control would have been profitable. In 1992, profitable control of rust with fungicides occurred primarily with the highly susceptible inbreds. In all the years when common rust developed, the best control occurred when fungicide sprays were initiated early in the growing season and were continued periodically until 2 weeks after detasseling. Attempts to stop a rust epidemic are likely to be unprofitable if the first fungicide application is made after detasseling. Last year, seed 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 stop the disease effectively.

The guidelines in this article can also be used to control other diseases. Eyespot was commonly seen early in the season in the commercial seed corn fields where corn followed corn. It never developed into a serious problem in any of the fields, probably because of the resistance in the seed parents. If the seed parent inbred is very susceptible to eyespot, it is best not to plant it onto land carrying corn residue from the prior year. Wheneyespot-susceptible inbreds and hybrids were tested, yield losses of 15 to 20 percent were not uncommon. Fungicide sprays initiated by the 6 to 8 leaf stages gave good control.

We saw Northern leaf spot, also called Helminthosporium or carbonum leaf spot (Bipolaris zeicola or Helminthosporium carbonum) early in the season. It developed into an economic problem only on highly susceptible inbreds. Although the disease did not develop greatly until mid-August in 1992, sprays applied in mid-July were very beneficial for control of the disease. The weather in 1993 was ideal for Northern leaf spot, but the disease did not develop into an epidemic. It is our feeling that the rust in 1993 out-competed the other pathogens for the leaf tissue. You should scout for Northern leaf spot if the seed parent inbred is known to be highly susceptible. Take special care when corn is following corn in rotation. For effective control, the sprays must extend into late August on no more than 10-day intervals. Continue scouting beyond the flowering stage.

Gray leaf spot (Cercospora zeae-maydis) and Northern leaf blight (Exserohilum turcicum or Helminthosporium turcicum) has occurred erratically. Both diseases normally appear initially just before detasseling, but were seen several weeks before detasseling. These are potentially serious disease problems in seed production and should be part of your scouting program. Anthracnose leaf blight was not an economic problem in our studies. In 1994, it may be more prevalent due to the anthracnose stalk rot epidemic last year.

Four fungicides are labeled for use on corn for seed production. All of them 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, for rates permitted, and for any restrictions of application.

Copper thallate (Tenncop) can be applied up to harvest. In some cases, it appears to be phytotoxic to some inbred lines.

Chlorothalonil (Bravo) is a protective fungicide that can be applied up to 14 days before harvest, but the forage may not be grazed, and the husks and residue may not be fed to livestock.

Mancozeb products (Manzate, Dithane, Penncozeb) have protective activity and may be applied until 40 days before harvest. They differ slightly in their restrictions.

Propiconazole (Tilt), a systemic fungicide, may be sprayed only until silking.