What Do R, MR and MS Mean? How to Read Disease Ratings Before You Buy Seed
Every variety in the FP Genetics seed guide comes with a row of disease ratings: R, MR, I, MS or S beside stem rust, leaf rust, stripe rust, bunt and fusarium head blight. Those letters can tell you as much about a variety as its yield number does, but only if you know how to read them.
Here is what each rating means, which diseases matter most on the Prairies, and how to use the ratings when you are choosing seed.
The short answer
Disease ratings run on a five-step scale from R, resistant, to S, susceptible. They show how varieties compare when they are grown under the same disease pressure in provincial trials. An R-rated variety is not disease-free, but in a bad year it will hold up much better than an S-rated one.
The five-step scale
| Rating | Stands for | What it means in the field |
|---|---|---|
| R | Resistant | Holds up well under heavy disease pressure. Disease can still show up, but losses are usually small. |
| MR | Moderately resistant | Good protection. Worth scouting in high-pressure years, but a strong position to be in. |
| I | Intermediate | The middle of the scale. Some protection, but in a wet year or a high-risk field you may still need to manage the disease. |
| MS | Moderately susceptible | Limited protection. Plan on managing the disease when conditions favour it. |
| S | Susceptible | Little protection. Seed treatment or fungicide does most of the work. |
The five diseases that matter most
New wheat varieties in Western Canada are assessed against five “Priority 1” diseases before they are registered. These are the diseases with the most potential to cost Prairie growers yield and grade.
- Stem rust
- Leaf rust
- Stripe rust
- Common bunt, a seed-borne disease that gives grain a fishy smell and can lead to downgrading
- Fusarium head blight (FHB), which cuts yield and grade and can leave the toxin DON in the grain
When a variety is described as “MR or better for all Priority 1 diseases,” it means it rated MR or R on all five. That is a strong package.
How current FP Genetics varieties compare
| Variety | Class | Stem rust | Leaf rust | Stripe rust | Bunt | FHB |
|---|---|---|---|---|---|---|
| AAC Walsh | CWRS | MR | R | I | MR | MR |
| SY Manness | CWRS | R | R | I | S | I |
| AAC Hockley | CWRS | MR | R | R | R | MR |
| AAC Walker VB | CWRS, midge tolerant | R | R | R | MR | MR |
| AAC Hodge VB | CWRS, midge tolerant | R | R | R | R | MR |
| AAC Brigham VB | Durum, midge tolerant | R | R | R | R | MS |
| AAC Schrader | Durum | R | R | R | MR | I |
Source: FP Genetics 2027 Seed Guide, using 2026 Alberta, Saskatchewan and Manitoba seed guide data.
A few things stand out when you read the table this way:
- AAC Hodge VB is rated R for every Priority 1 disease except FHB, where it is MR.
- AAC Hockley is MR or better on all five.
- SY Manness is one of the highest yielding CWRS varieties in the lineup, but it is rated S for bunt. That is not a reason to pass on it. It means a seed treatment is worth planning on.
- In durum, FHB is the hard one. MS ratings are common in the class. AAC Schrader is rated I for FHB, which FP Genetics describes as the first I rating for FHB in durum globally. AAC Brigham VB is MS for FHB but R for the other four.
A rating is a comparison, not a guarantee
Ratings show how varieties stack up against each other under the same disease pressure. How much disease you actually see still depends on weather, field history and rotation.
Ratings can also shift a little between provincial guides and from year to year, so it is worth checking the guide for your province. And weigh each disease by your own risk. If fusarium has been a problem in your area, the FHB column deserves more attention than a small difference in yield.
What midge tolerance and VB mean
Orange wheat blossom midge larvae feed on developing kernels. Midge-tolerant varieties carry a gene called Sm1 that protects against that damage.
These varieties are sold as varietal blends, which is what the VB in the name stands for. About 90% of the seed carries the Sm1 gene. The other 10% is a similar variety without it, chosen to match the main variety in height and maturity so the crop develops evenly.
That 10% is called the refuge. It gives regular midges a place to survive and mate with any rare midges that manage to get past Sm1, which keeps the gene working longer.
The 90:10 ratio can drift when seed is saved and replanted over several generations. That is why certified seed is required for midge-tolerant varieties. It keeps the blend right and protects the gene for everyone. In the current lineup, AAC Walker VB and AAC Hodge VB are midge-tolerant CWRS, and AAC Brigham VB is midge-tolerant durum.
Putting it together
Yield tells you what a variety can do. Disease ratings tell you how likely it is to deliver that in a tough year. For help reading the yield side of the table, see How Genetic Gain Becomes Yield: Reading Performance Data.
If you are weighing two varieties, an FP Genetics Territory Manager or Product Specialist can walk you through the ratings for your area and the diseases you see most often.
Frequently asked questions
What does MR mean on a seed variety?
MR means moderately resistant. The variety offers good protection against that disease and holds up well under most conditions, though it is still worth scouting in high-pressure years.
What are the Priority 1 diseases in wheat?
Stem rust, leaf rust, stripe rust, common bunt and fusarium head blight. New wheat varieties in Western Canada are assessed against all five before registration.
Which FP Genetics wheat has the strongest disease package?
Among current varieties, AAC Hodge VB is rated R for every Priority 1 disease except FHB, where it is MR, and AAC Hockley is MR or better on all five. The best fit still depends on which diseases are the biggest risk in your area.
What does VB mean on a wheat variety?
VB stands for varietal blend. About 90% of the seed is midge tolerant and 10% is a similar variety without the tolerance gene, which acts as a refuge and helps the tolerance last.
Does an R rating mean I don’t need a fungicide?
Not always. An R rating means the variety holds up well under disease pressure, but weather and field history still matter. In high-risk years, scouting is still worthwhile.