Seedling diseases could be an obstacle for farmers this year with the early planting of corn and soybean. While we all hope that seedling diseases will be a small consideration, it is important to be ready for them. It is also important to know how to sample for them.
When you are out scouting early this season and decide you need to sample, remember these three tips:
- Dig the roots
- Transport the sample quickly and keep it fresh
- Avoid shipping samples on Thursday and Friday
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 Plant and Insect Diagnostic Clinic we rely on pathogen morphology and serological test to identify different pathogens to genus such as; Pythium, Phytophthora and Rhizoctonia. Since the amount to plant tissue per plant is reduced due to the plant stage, it is important to collect a good sample.
The most important consideration when sampling seedlings is to dig the roots rather than pull the plant out of the ground. 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. When collecting seedlings carry a small trowel or knife with you, so 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.
Keeping the soil separate from the aerial portions of the plant gives us a better opportunity to observe symptoms.
Another important consideration is sample transport. The time it takes a sample to arrive can be crucial for diagnosis. Collect the sample 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 nearby the Iowa State University campus, fresh, hand delivered samples are ideal. Since this is not always possible, it is recommended samples are shipped overnight and sent early in the week, avoiding Thursday and Friday shipments. Shipping late in the week can result in the sample sitting in a box over the weekend, sometimes in high heat, which will cause the sample to deteriorate before it is examined.
Sample processing normally takes up to two weeks. This is because diagnoses typically require isolations and observation of fungal morphology and it takes time for the fungus to grow on media. Another option is expedited testing for Pythium, Phytophthora, and Rhizoctonia using serology. These tests allow for a two-day turnaround rather than a two-week turnaround, but a good representative sample is crucial for pathogen detection. In the clinic, these serological tests will cost an additional $15 on top of our normal $20 fee for plant problem diagnoses.
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