This tough growing season will not leave us soon enough. The disease triangle is setting up with the cool and wet conditions we’ve had. The three elements of the disease triangle are a host plant (corn or soybeans in this case), the right environment and a pathogen. This triangle sets the stage for diseases to flourish.
The best way to stay ahead of diseases is to scout and look for symptoms and/or insect activity in growing crops. In addition, there are times when sending plant samples to labs for testing will be necessary. In many situations, especially early plant growth stages (through V6 in corn and 3-4 trifoliates in soybeans), it is best to dig and send whole plant samples. The lab can then examine the whole plant from roots to foliage.
Late planting adds another dimension to the disease triangle because plant maturities are pushed back farther into the summer. With much more green plant material available, diseases continue to attack plants. If cool, wet conditions continue into summer, this will slow growth habits and make younger plant tissue even more vulnerable.
So, what issues will delayed planting, slow growth and development cause for the 2019 season? This question pops up every year in areas that experienced a wet spring. However, this year, the wet area encompasses such a large swath of the central U.S., we should consider all of these potential hazards:
Delayed harvest with decreased yields
- With late-planted crops, we know yields decrease but harvest can also be delayed in scenarios where cooler, wetter conditions prevail past normal dates.
- Greater prevalence of insect pressure
- Yields can also decrease due to other factors
- Nitrogen loss
- Greater prevalence of disease pressure
- Greater prevalence of insect pressure
Less vigorous plants
- Due to saturated soils, oxygen can be depleted from soils
- Oxygen is necessary for plant respiration
- Plants deprived of oxygen may grow more slowly and be less healthy
We will discuss these issues in greater detail over the coming weeks to help you prepare for the worst possible scenarios and protect this year’s crops. We will break our discussion into issues to watch for in northern and southern latitudes for your convenience.
A word about nitrogen
Nitrogen loss is a concern in all areas every year, especially when we have excess moisture. Nitrogen is mobile within the soil and therefore has the ability to leach through the soil profile with moisture or be carried off fields with erosion. However, nitrogen loss may not be the only factor contributing to yellow, stunted plants. Cool growing conditions combined with a lack of sunlight contributes to slow growth, poor photosynthesis and stunting. Nitrogen may be available, but plants may not be accessing the depth the nitrogen has moved to.
Management Tip: Compare current soil samples from 6-, 12- and 24-inch depths to applied nitrogen to see if the nitrogen has been driven deeper in soils. As plants grow, roots will eventually reach these depths and interact with soil nitrogen. Supplemental applications may be made in the intervening periods, but at lower rates to sustain the plants until they reach deeper sources of nitrogen.
Corn Diseases (Northern Region)
Root rots from Pythium, Fusarium and Rhizoctonia
Tar spot
Common and southern rust
Eyespot
Northern corn leaf blight
Corn Diseases (Southern Region)
Anthracnose leaf spot
Gray leaf spot
Eyespot
Goss’s wilt (bacterial infection)
Crazy top is a distortion or stunting of the plant with a bushy tassel appearance and excessive tillering caused by prolonged flooding or intense rain periods. No treatment is necessary.
Stalk rots will be a major concern with saturated fields and flooding. These will be covered in a later blog.
Management Tip: All of these diseases are controllable through timely identification and treatment with fungicides in a timely manner. However, treatment is not always necessary based on timing and level of infection. Goss’s wilt will not respond to treatment as it is a bacterial infection. Therefore, proper identification is key at an early stage along with severity and attention to future potential for disease spread.
Soybean Diseases (Northern Regions)
White mold
Frogeye leaf spot
Septoria brown spot
Bacterial leaf blight
Soybean Diseases (Southern Regions)
Poor emergence or slow-growing soybeans
Bacterial leaf blight
Septoria brown spot
Frogeye leaf spot
Management Tip: Frogeye leaf spot is the only issue on this list that can be effectively treated this year. Septoria brown spot rarely makes it to the upper canopy to do yield-robbing damage; however, if it does infect the upper third of the canopy, treatment may be necessary. Fungicide selections should always include Strobilurin and Triazole (curative and preventative).