Treated (left) and non-treated (right) soybean seeds. Photo credit: Paul Esker & Alyssa Collins, Penn State.
Hybrid (corn) or variety (soybean) selection is very important to successful crop production. Nonetheless, it can be challenging to review all of the seed guides produced annually, especially as they relate to making selections based on disease and stalk health ratings. The goal of this fact sheet is to provide some guidance related to three important concepts: (1) the key considerations related to selecting hybrids or varieties based on disease reaction, (2) understanding the different terminology that is used to describe disease resistance or susceptibility, and (3) understanding the different rating systems.
Above all, remember that the best approach starts with knowing your farm and fields and by asking excellent and direct questions of your local seed rep to identify the materials best suited for your farm based on your field history and disease issues. Furthermore, make sure that you select materials that are properly adapted for your production area.
Key considerations when selecting hybrids or varieties based on disease ratings and issues:
- Understand the field history, including things like crop rotation, plant population and tillage: many seed catalogs now make recommendations based on factors like corn-following-corn, no tillage, etc., which may (or may not) correlate with disease-related issues.
- Develop a database of the diseases that were noted in specific fields and check those against the hybrid or variety that was grown, as this way you will be able to understand how the rating system performed with your own observations.
- In corn, problems with standability or stalk rots may require that you place greater emphasis on these factors when you select hybrids. It is important to note that not all seed guides may list specific stalk rot diseases, so make sure to discuss this with your seed rep if you have specific stalk rot issues.
- If a fungicide was used in the field, and there was a check strip, look to determine if there were yield or quality differences between the treated and untreated areas.Not all hybrids or varieties will perform the same in every field. Check University and other variety or hybrid results to see how different selections perform across different environments. Knowing this can help you make better decisions for different fields on your farm.
Not all hybrids or varieties will perform the same in every field. Check University and other variety or hybrid results to see how different selections perform across different environments. Knowing this can help you make better decisions for different fields on your farm.
Terminology for disease reactions:
The different seed guides use different terms that may, or may not, mean the same thing. For example, several describe the disease reaction using words like “susceptible”, “moderately susceptible”, “moderately resistant”, “resistant”, which can tell you things about the relative performance across different environments (physical, geographical or climatological).
In other cases, the seed guide will make reference to “tolerance”, and this term may mean the same thing as “resistance”, but it can also mean something a little different. For example, in plant pathology, the term “tolerance” means that a plant can become infected, but without serious damage or impact on yield. It is a common way to describe a hybrid or variety when there is no resistant material. This is very common way to classify varieties in soybean for diseases like white mold.
In other situations, the terminology may be even simpler, ranging from “poor” to “excellent”, which we would interpret to mean “susceptible” to “resistant” or “tolerant”, depending on the disease.
Different rating systems:
Adding to the complexity is that different companies using different rating systems. Many companies use a 1 to 9 system, where 1 means susceptible or not recommended and 9 means resistant (or highly resistant, in some cases) or recommended, while others use the same numbering scheme, but in the opposite direction.
Furthermore, it is important to note that one companies “4” may not be the same as another’s since the “4” could mean something very different depending on how the company classifies the material, as many companies will combine several ratings into one of the broader classes described above to define resistance of susceptibility. Additionally, some companies use a simpler rating system that may range from 1 to 5 (or 1 to 4).
Finally, when a disease has no rating, there is often a note that will state something like the following, “not enough data or material has been evaluated for that trait or disease”.
In any of the cases, if you have any doubts about the numbering system, it is important to ask this question with your seed rep, since based on our experience, middle ratings can also imply that the reaction is dependent on location and environment (genotype x environment, or genotype x environment x management).