Disease Resistance Codes: How to Read Seed Descriptions

Letters such as HR, IR, PM, and ToMV can make a seed description look more complicated than it needs to be. These codes summarize information about a variety's response to particular diseases or pests.

They are useful when choosing plants for a known garden problem, but they are not a promise that a plant will never become sick. Read the code, crop, resistance level, and any listed races or strains together.

What Does Resistance Mean?

Resistance describes a variety's ability to restrict a specified pest or pathogen, or the symptoms and damage it causes, compared with a susceptible variety under comparable conditions. Weather, plant age, pest pressure, and the organism involved can affect the result.

The International Seed Federation distinguishes resistance from immunity. A resistant plant can still show symptoms or damage, especially under heavy pressure.

HR: High Resistance

High resistance means a variety restricts the specified pest or its effects to a high degree. It does not mean the plant cannot become infected, and it does not protect against every disease with a similar name.

IR: Intermediate Resistance

Intermediate resistance means a variety restricts the specified pest or its effects to a moderate degree. Under similar conditions, it may show more symptoms than a highly resistant variety but less severe symptoms or damage than a susceptible one.

IR is not simply a warning that infection is possible; that possibility also exists with HR. Both describe a degree of resistance to a particular target.

Read the Numbers and the Supplier's Key

Numbers or letters after a disease code can identify races, strains, pathotypes, or other defined groups. Resistance to one does not automatically cover another.

Do not assume that F2, F3, or another shortened notation has exactly the same meaning in every catalog. Race numbering and shorthand can differ. Consult the original breeder or supplier's explanation for the exact variety.

A botanical family name is also not a resistance claim for every crop in that family. A virus code listed for a cucumber does not establish the same response in every melon or squash.

Common Tomato and Pepper Codes

The following are reference meanings encountered in seed descriptions, including older shorthand. The supplier's crop-specific key takes priority.

• AS: Alternaria stem canker.
• EB: Early blight.
• F: Fusarium wilt; check the listed crop and races.
• FOR: Fusarium crown and root rot of tomato.
• CRR: Corky root rot of tomato.
• GLS: Gray leaf spot.
• LB: Late blight.
• LM: Leaf mold.
• N: Root-knot nematodes; verify the specified species or population.
• PST: Bacterial speck.
• V: Verticillium wilt; check the specific claim.
• BLS: Bacterial leaf spot, often used for pepper; race coverage matters.
• PC: Phytophthora root rot or blight, as defined for the crop.
• PeMV: Pepper mottle virus.
• PVY: Potato virus Y, including claims used for peppers.
• TEV: Tobacco etch virus.
• TM: Tobamovirus shorthand in some pepper descriptions; verify its definition and pathotypes.
• TMV: Tobacco mosaic virus.
• ToMV: Tomato mosaic virus.
• TSWV: Tomato spotted wilt virus.
• TYLCV: Tomato yellow leaf curl virus.
• ToANV: Tomato apex necrosis virus.

Common Cucumber, Melon, and Squash Codes

• ALS: Angular leaf spot.
• B: Bacterial wilt in some cucumber keys; confirm the supplier's usage.
• CMV: Cucumber mosaic virus.
• CVYV: Cucumber vein yellowing virus.
• DM: Downy mildew.
• PM: Powdery mildew.
• PRSV: Papaya ringspot virus; check the crop-specific claim.
• S: Scab in some cucumber descriptions.
• TSP: Target spot.
• WMV: Watermelon mosaic virus.
• ZYMV: Zucchini yellow mosaic virus.

Downy mildew and powdery mildew are different diseases. Resistance to one does not imply resistance to the other. Fusarium wilt claims also occur in melons and watermelons and must be read for the particular crop and races.

Common Bean and Pea Codes

• BBS: Bacterial brown spot.
• BMV: Bean mosaic virus shorthand in some older descriptions; verify the exact virus intended.
• BYMV: Bean yellow mosaic virus.
• HB: Halo blight.
• PEMV: Pea enation mosaic virus; E is used in some older keys.
• PLR: Pea leaf roll virus.
• F: Fusarium wilt, with crop-specific race information.
• PM: Powdery mildew.
• R: Rust in some bean keys.

Common Carrot, Beet, and Onion Codes

• AB: Alternaria blight of carrot in some keys.
• BB: Bacterial blight of carrot.
• C: Cercospora blight of carrot in some descriptions.
• CS: Cavity spot of carrot.
• CLS: Cercospora leaf spot of beet.
• CTM: Curly-top-related shorthand in some older beet descriptions; verify the exact designation with the supplier.
• FBR: Fusarium basal rot of onion.
• PR: Pink root of onion.

Common Lettuce, Spinach, Basil, and Brassica Codes

• BR: Black rot of brassicas.
• CR: Clubroot.
• FY: Fusarium yellows of brassicas.
• FW: Fusarium wilt of basil in some descriptions.
• Cv: Cladosporium leaf spot of spinach.
• DM: Downy mildew; the responsible organism and relevant races differ by crop.
• LMV: Lettuce mosaic virus.
• Rs: Corky root of lettuce in some keys.
• TBSV: Tomato bushy stunt virus, including references associated with lettuce dieback.
• WR: White rust of spinach.
• Nr: Lettuce leaf aphid resistance notation; check the stated biotype.
• Pb: Lettuce root aphid notation in some keys.

Not every resistance code concerns a disease. Aphids and nematodes are pests, not fungi or viruses. A single code can also represent different targets in different crops.

Common Corn Codes

• MDMV: Maize dwarf mosaic virus.
• NCLB: Northern corn leaf blight.
• SCLB: Southern corn leaf blight.
• SM: Smut.
• SW: Stewart's wilt.
• R: Common rust in some corn descriptions.

Seed Testing Is Not the Same as Plant Resistance

Notations such as MT0-10 or MT0-30 may describe seed-lot testing for lettuce mosaic virus rather than the growing plant's resistance. Consult the supplier for the sample size, method, and meaning.

A negative sample result is not proof that every seed in a lot is free of a pathogen. Likewise, a germination percentage describes a different test and does not establish disease resistance.

What If a Code Is Missing or Unclear?

Do not infer a resistance trait from a variety's name, appearance, or performance in one season. Ask for the exact claim and its source.

The older BS abbreviation, for example, appears in some crop keys but should not be assigned a universal meaning without context. This reference is not exhaustive, and terminology changes. The International Seed Federation's pest-code reference provides additional crop-specific information.

Resistance Is One Part of Garden Care

Choose resistance traits relevant to problems identified in your area. Continue using suitable spacing, watering, sanitation, and rotation practices. Good drainage and healthy roots still matter.

A fungicide does not treat every disease, and an insecticide does not cure a virus already inside a plant. Identify the problem before choosing any treatment. If a product is appropriate, follow its crop, pest, application, and harvest directions.

Frequently Asked Questions

Does HR mean immune?

No. Highly resistant plants can still show symptoms, especially under heavy pressure.

Is IR useful?

Yes. It indicates a moderate degree of resistance compared with a susceptible variety under comparable conditions, not an absence of protection.

Does disease resistance mean a seed is GMO?

No. Conventional breeding can produce disease-resistant varieties. Read our seed-type explanation for more detail.

Can I diagnose a sick plant from these codes?

No. Codes describe variety claims. Diagnosis requires looking at the symptoms, growing conditions, and possible causes, sometimes with laboratory help.

More Growing Guidance

Explore our cabbage growing guide. For the formal definitions behind HR and IR, see the International Seed Federation's resistance terminology.

Choose Varieties with Better Information

Explore vegetable seeds from David's Garden Seeds® and review each variety's listed characteristics before planting.

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