May 8, 2016
Seedling diseases could be a problem in Iowa this spring with the early planting of corn and soybeans. “While we all hope that seedling diseases will be a small problem and not a big one, it is important to be ready for them. It is also important to know how to properly sample corn and soybean plants in the field,” says Edward Zaworski, a plant diagnostician in the Iowa State University Plant and Insect Diagnostic Clinic at Ames.
SEEDLING STRESS: Rain, wet soils and cold temperatures this spring are promoting development of seedling blight diseases in corn and soybeans. The highest risk of these diseases showing up is in poorly drained soils.
When you are scouting fields early this season and decide you need to sample some sick-looking corn and soybean plants and send them to the ISU plant disease lab in Ames or another lab for diagnosis, or if you want to take the sample plants to your agronomist or crop consultant to have the problem diagnosed, remember these three tips: Dig the roots carefully; transport the sample quickly and keep it fresh; and avoid shipping samples to a lab on Thursday and Friday.
1) Seedling disease symptoms can look like other problems
Understanding the symptoms to look for and their effect on crop health will help you make management decisions to secure the greatest yield potential for your crop. Zaworski provides the following information and recommendations.
Seedling diseases can be difficult to diagnose, as symptoms of common seedling diseases (root rots and damping off) and disorders (chemical or environmental damage) may look alike. “It can be difficult to diagnose seedling diseases by symptoms alone, therefore, in the ISU Plant and Insect Diagnostic Clinic we rely on pathogen morphology and serological testing to identify different pathogens,” he says. “For example if you are trying to diagnose Pythium, Phytophthora and Rhizoctonia. Since the amount of plant tissue per plant is reduced due to the plant stage, it is important to collect a good sample.”
2) Dig the roots rather than pull them out of the ground
The most important consideration when sampling seedlings is to dig the roots rather than pull the plant out of the ground, he emphasizes. Seedling diseases, for the most part, are infecting the plant roots, which is why it is critical to get as much of the root system as possible. The more root tissue that is collected, the more likely a pathogen can be identified and identified correctly. When collecting seedlings, you should carry a small trowel or knife with you. That tool can help you dig around the seedlings.
Another thing to consider is how to package the sample. If soil is included with the sample, and not adequately separated, it can be difficult to get a good look at symptoms. This problem can be solved in one of two ways: wrap a plastic bag or tinfoil around the root ball to keep it separate from the aerial portion of the plant or shake as much of the soil off of the roots as possible without damaging the roots. Do not add moisture to a sample. Adding moisture can cause other non-pathogenic fungi and bacteria to grow, making it more difficult to isolate a pathogen causing the problem.
Seedling samples with no root systems can be nearly impossible to diagnose.
Gather as much of the root system as possible when sampling a corn or soybean seedling.
Keeping the soil separate from the leaves and stem portions of the corn or soybean plant gives the crop consultant or lab technician a better opportunity to observe symptoms and correctly diagnose the disease or other problem.
3) Another important consideration is sample transport
The time it takes for a sample corn or soybean seedling to arrive at the lab can be crucial for diagnosis. Collect the sample from the field and deliver it to its destination as soon as possible. If a sample cannot be delivered or mailed the same day it is collected, keep the sample cool, in a cooler or refrigerator, to slow the rate of deterioration. As with almost all samples, the more time the sample spends in transit, the less likely it will be to see the real symptoms and isolate the correct pathogen.
If you are located not too far from the Iowa State University campus at Ames and can drive there and deliver the samples to the ISU lab, keep in mind that fresh, hand delivered samples are ideal. Since this is not always possible, it is recommended that samples be shipped overnight and sent early in the week, avoiding Thursday and Friday shipments. Shipping late in the week can result in the crop sample sitting in a box over the weekend, sometimes in high heat, which will cause the sample to deteriorate before it is examined by the lab technician for diagnosis.
4) How long will it take to get results from the lab?
Sample processing normally takes up to two weeks, says Zaworski. This is because diagnoses typically require isolations and observation of fungal morphology and it takes time for the fungus to grow on the media in the lab.
Another option is expedited testing for Pythium, Phytophthora, and Rhizoctonia using serology tests. These tests allow for a two-day turnaround rather than a two-week turnaround, but a good representative sample of the corn or soybean plants is crucial for pathogen detection. In the ISU clinic, these serological tests will cost an additional $15 on top of the lab’s normal $20 fee for plant problem diagnoses.
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