Understanding Disease Resistance: IR and HR
What Is Disease Resistance in Seeds?
When selecting varieties, you'll often see HR and IR listed alongside a variety name. These codes describe how a plant responds to specific pathogens — and choosing the right resistance package can significantly impact yield consistency and input costs.
Important: Resistance is not immunity. No HR or IR rating guarantees a plant will never be affected by a pathogen. Both ratings describe the degree to which a variety can limit or slow disease development — not eliminate the risk entirely. Environmental conditions, disease pressure, and pathogen race all influence real-world outcomes.
HR — High Resistance
The variety strongly limits pathogen growth under normal conditions. Plants show little to no symptoms. Under extreme disease pressure or unusual environmental conditions, even HR varieties may show some symptoms — but yield impact is typically minimal. HR is not immunity.
IR — Intermediate Resistance
The variety reduces disease severity but will not prevent infection entirely. Symptoms may appear, but progression is slower and damage is less than in susceptible varieties. IR works best when paired with good cultural practices: proper spacing, humidity control, and sanitation. IR is not a substitute for crop management.
HR vs. IR at a Glance
| Feature | HR | IR |
|---|---|---|
| Pathogen growth limited? | Strongly | Moderately |
| Symptoms visible? | Rarely | Possible under pressure |
| Yield impact | Minimal | Reduced vs. susceptible |
| Immunity? | No | No |
| Best use case | High disease pressure | Moderate pressure or prevention |
Examples from the Paramount Seeds Catalog
Tomatoes
Grandice Res F1 Truss Tomato
HR: ToMV:0-2 / ToTV / Fol:1,2 / For / Pf:A-E / Sbl / Sl / Ss / Va:1 / Vd:1
IR: ToBRFV (validated) / On / Ma / Mi / Mj
Fol is HR with race specificity; ToBRFV is IR — multi-gene IR is currently the strongest commercial protection available against this virus, but it does not prevent infection under severe outbreak conditions.
Serenity F1 Truss Tomato
HR: ToMV:0-2 / ToTV / Fol:1,2 / For / Pf:A-E / Sbl / Sl / Ss
IR: ToBRFV / TSWV:0 / On
Adds IR to TSWV (race 0) — relevant where thrips pressure is a concern. IR means slower spread, not elimination.
Inspired F1 Beefsteak Tomato
IR: Powdery Mildew / Leaf Mold (races A–F)
IR to both major fungal threats across all commercially relevant races — can meaningfully reduce fungicide inputs in high-humidity greenhouses, but environmental controls remain essential.
Cucumbers
Verdon F1 Long Dutch Cucumber
IR: Powdery Mildew / CGMMV; demonstrated strength against Pythium and Fusarium
A lower-risk choice for off-season plantings or operations transitioning from a diseased crop. CGMMV IR slows spread but does not eliminate the virus — sanitation protocols remain critical.
Georgia F1 Long Dutch Cucumber
HR: CVYV / Ccu / Px | IR: CGMMV / CMV / Cca
Powdery Mildew (Px) is HR here — a step up from Verdon for operations with consistent mildew pressure, though not a guarantee under extreme conditions.
Peppers
Revolution F1 Bell Pepper
HR: Xcv:1-5 | IR: CMV / Pc
HR to Bacterial Spot across races 1–5 significantly reduces risk, but is race-specific — new races outside this range would not be covered. IR to Phytophthora Crown Rot supports tolerance under moderate moisture stress, not waterlogged conditions.
Common Resistance Code Reference
- ToMV / ToBRFV / TSWV — Tomato mosaic, brown rugose fruit, and spotted wilt viruses
- Fol / For — Fusarium Crown and Root Rot (race-specific)
- Va / Vd — Verticillium Wilt
- Px — Powdery Mildew (cucurbits)
- CMV / CGMMV — Cucumber Mosaic and Green Mottle Mosaic viruses
- Xcv — Bacterial Spot in peppers (race-specific)
- Pc — Phytophthora Crown Rot in peppers
Applying Resistance Ratings to Variety Selection
- Know your disease history — identify recurring pathogens from past crop records.
- Prioritize HR for known threats — especially where losses have been significant.
- Check race specificity — confirm the resistance covers the races present in your region.
- Layer IR with cultural controls — sanitation, airflow, and scouting remain essential regardless of resistance rating.
- Stay current — resistance packages evolve as new pathogen strains emerge.
Resistance ratings follow the International Seed Federation (ISF) standard notation. Always check the supplier's legend, as abbreviations are not always universal across catalogs.
Have questions about which varieties suit your operation? Contact our team or browse our full seed catalog.