Understanding Disease Resistance Codes for Greenhouse Crops
by Paramount Seeds on Jul 21 2026
When evaluating greenhouse seed varieties what do the disease resistance codes mean?
This guide explains what the codes mean, how to read them, and which resistances matter most for each major greenhouse crop.
How to Read Disease Resistance Codes
The international seed industry uses a standardized system developed by the International Seed Federation (ISF) to communicate disease resistance. Two levels of resistance are recognized:
- HR — High Resistance: The variety has the ability to highly restrict the growth and development of the specified pest or pathogen under normal conditions. HR varieties may still exhibit some symptoms under heavy disease pressure.
- IR — Intermediate Resistance: The variety restricts the growth and development of the specified pest or pathogen, but may exhibit a greater range of symptoms than HR varieties. IR varieties will show more disease expression under high inoculum pressure.
Resistances are listed using standardized abbreviations. For example, a tomato variety listed as HR: ToMV, Fol:0,1 / IR: TSWV has high resistance to Tomato mosaic virus and Fusarium crown and root rot races 0 and 1, and intermediate resistance to Tomato spotted wilt virus.
Always read resistance codes in the context of your local disease pressure and the specific races or strains present in your region.
Greenhouse Tomato Disease Resistance Codes
Tomatoes carry the most complex resistance packages of any greenhouse crop. Key codes to know:
| Code | Disease / Pathogen | Notes |
|---|---|---|
| ToMV | Tomato mosaic virus | Standard in most modern varieties; races 0, 1, 2 may be specified |
| ToBRFV | Tomato brown rugose fruit virus | Emerging resistance; critical for export markets and certified programs |
| Fol:0,1,2 | Fusarium oxysporum f.sp. lycopersici (crown/root rot) | Race specification is critical — confirm which races are present in your system |
| For | Fusarium oxysporum f.sp. radicis-lycopersici | Fusarium crown and root rot; distinct from Fol |
| Pst | Stemphylium (grey leaf spot) | Important in high-humidity greenhouse environments |
| Pl | Leveillula taurica (powdery mildew) | Distinct from Oidium neolycopersici; confirm species present |
| On | Oidium neolycopersici (powdery mildew) | Most common powdery mildew in northern greenhouse tomato production |
| TSWV | Tomato spotted wilt virus | Thrips-vectored; IR is common, HR rare |
| Va:0 | Verticillium albo-atrum race 0 | Soilborne; more relevant in soil-grown systems |
| Vd:0 | Verticillium dahliae race 0 | Soilborne wilt pathogen |
| Mi | Meloidogyne incognita (root-knot nematode) | Relevant in soil-grown and some substrate systems |
Browse our Greenhouse Tomato varieties with full resistance listings.
Greenhouse Cucumber Disease Resistance Codes
Cucumbers face significant pressure from powdery mildew and downy mildew. Key codes:
| Code | Disease / Pathogen | Notes |
|---|---|---|
| Px | Podosphaera xanthii (powdery mildew) | Most important resistance trait in greenhouse cucumber; race specification matters |
| Gc | Golovinomyces cichoracearum (powdery mildew) | Secondary powdery mildew species; less common in greenhouse |
| Pcu | Pseudoperonospora cubensis (downy mildew) | Highly destructive under humid conditions; new races emerging |
| Ccu | Corynespora cassiicola (target spot) | Increasingly important in heated greenhouse production |
| CVYV | Cucumber vein yellowing virus | Whitefly-vectored; relevant in warmer production regions |
| CMV | Cucumber mosaic virus | Aphid-vectored; broad host range |
| WMV | Watermelon mosaic virus | Aphid-vectored; less common in closed greenhouse systems |
Browse our Greenhouse Cucumber varieties with full resistance listings.
Greenhouse Lettuce Disease Resistance Codes
Lettuce disease resistance centers on downy mildew, with Bremia races evolving rapidly. Key codes:
| Code | Disease / Pathogen | Notes |
|---|---|---|
| Bl:1–36+ | Bremia lactucae (downy mildew) | Race numbers are critical — always verify current race coverage for your region with the BIS (Bremia Interaction Set) |
| Nr:0 | Nasonovia ribisnigri (lettuce aphid) | Resistance to the most damaging aphid species in greenhouse lettuce |
| Pb | Pythium (root rot complex) | Tolerance rather than true resistance; substrate and irrigation management remain critical |
| LMV | Lettuce mosaic virus | Seed-borne; most commercial seed is tested and certified LMV-free |
Important note on Bremia races: Downy mildew races in lettuce evolve faster than in almost any other crop. A variety with excellent resistance today may be susceptible to new races within one to two seasons. Always check current race coverage with your seed supplier before committing to a variety for a full season.
Browse our Greenhouse Lettuce varieties with full resistance listings.
Greenhouse Pepper Disease Resistance Codes
Peppers face pressure from Phytophthora, viruses, and fungal pathogens. Key codes:
| Code | Disease / Pathogen | Notes |
|---|---|---|
| Pc | Phytophthora capsici (Phytophthora root rot / blight) | Most destructive soilborne pathogen in pepper; HR varieties are highly valued |
| Tm:0,1,2 | Tobacco mosaic virus (pepper strains) | Race specification important; ToMV and TMV resistance often listed together |
| PVY | Potato virus Y | Aphid-vectored; common in greenhouse pepper production |
| CMV | Cucumber mosaic virus | Aphid-vectored; broad host range including pepper |
| TSWV | Tomato spotted wilt virus | Thrips-vectored; IR resistance available in some varieties |
| Lv | Leveillula taurica (powdery mildew) | Internal powdery mildew; difficult to detect early; resistance highly desirable |
Browse our Greenhouse Pepper varieties with full resistance listings.
Rootstock Resistance Codes
For grafted tomato and cucumber programs, rootstock resistance is a separate and equally important consideration. Rootstocks are selected primarily for soilborne disease resistance and vigor, not foliar disease resistance. Key rootstock resistance traits include:
- Tomato rootstocks: Fol (races 0,1,2), For, Vd, Va, Mi — providing protection against Fusarium, Verticillium, and nematodes that scion varieties may lack
- Cucumber rootstocks: Fusarium oxysporum f.sp. cucumerinum (Foc), Pythium tolerance, and general vigor under stress conditions
Browse our Greenhouse Seed catalog for rootstock varieties and their resistance specifications.
How to Use This Information
When selecting varieties for your greenhouse, match resistance codes to your actual disease history:
- Review your disease records from the past 2–3 seasons to identify recurring problems
- Identify the specific pathogens and races present in your system (lab testing can confirm)
- Prioritize HR resistance for your highest-pressure diseases
- Use IR resistance as a secondary layer, supported by cultural and biological controls
- Never rely on resistance alone — combine with sanitation, IPM etc
Contact the Paramount Seeds team — we're here to help you build a variety program that protects your crop from the ground up.
Browse our full Varieties for Greenhouse Production catalog.