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Plant Disease Musings

June 22, 2001
The disease situation has been pretty calm this season. So far we have had scattered occurrences of seedling blight on soybean, which was not too surprising, given the low germination scores of the seed this year. The wheat crop also escaped with very little damage from fungal disease, although this may have been because there weren't many leaves left after the armyworms visited. The corn crop is also starting off on a good foot from a disease-free perspective. Recent timely rains (or torrential, depending on your perspective), though, should keep you on your toes in scouting for foliar leaf blights in corn.

Common rust: The last two seasons saw quite a bit of common rust on field corn, which actually is quite "uncommon." This season I believe we will see a much more normal occurrence of this disease. We rarely worry about common rust causing many problems on field corn. Why? Because usually it doesn't show up with any density until we get into late July and that is past the point where it will make much difference in terms of final yield. Usually the only people who see it build up with any intensity are those who plant their favorite susceptible sweet corn right next to their field corn, and the sweet corn serves as an inoculum (spore) source to infect the field corn. Since we are seeing little development of this disease to the south, the airborne spores are not likely to reach here before their normal time.

Other fungal leaf blights: Our usual recommendation for all other fungal leaf diseases is to scout for them during the 2 weeks before tasseling to 2 weeks after tasseling. At that point you would be looking for about 15% whole-plant infection to justify a fungicide treatment. If fungal leaf blights should make a significant appearance before that point, what should you do? First, consider the weather. Fungi in general need free water (on the leaves) and continued wet weather about every 7 to 10 days to flourish. Next, consider the probability of multiple fungal leaf blights developing in the field. Knowing what type of resistance is in the hybrid is pretty helpful to answer this. Is the field continuous corn with high residue? Fungal leaf blights are more likely to develop under those circumstances.

Why do you need to know all this other stuff to decide whether or not to spray field corn for an early fungal leaf disease? You need to know the probability of other fungal diseases developing later because there isn't a big arsenal of fungicides for treatment. Spraying early in the season may leave you with few options or none when "other" leaf blights become active. The price of corn and cost per application also need to be considered in your decision. The bottom line is that the fungicide treatments are effective if applied in a timely manner. Fungicides are useful when conditions favor diseases, especially if you are producing hybrid seed. Multiple applications may be necessary when disease pressures are high. However, during periods of high temperatures or dry conditions, disease pressures will be low and additional applications may not be necessary. Application and fungicide cost money. The price of corn is low. The risk is greater in seed corn than in field corn.

Disease resistance note: As with all resistant hybrids, keep in mind that resistance does not mean the hybrid will not be infected by the disease. Resistance in a resistant hybrid can be expressed in a number of ways. Commonly, the effect of resistance will be expressed something along the lines of smaller lesions produced, fewer spores subsequently produced from those lesions, longer time for lesions to develop, or fewer overall lesions produced. The resistant hybrids will develop lesions, but the lesions will not "look" the same as a characteristic lesion on a susceptible hybrid.--Suzanne Bissonnette

Author: Suzanne Bissonnette

The Pest Management and Crop Development Bulletin
Executive Editor: Kevin Steffey, Extension Entomologist

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