A pathogen usually appears when the right set of circumstances is present, conducive to the infection of a given plant. Three main factors contribute in the emergence and development of diseases in plants, commonly referred as the Disease Triangle: presence of a pathogen, presence of a suitable host, presence of environmental conditions that favor disease development and maintain the interaction between the pathogen and its host. Some pathogens are more specific as to their requirements to develop, their ability to infect (more prevalent with vulnerable plants and whitin disturbed/unbalanced ecosystems), and are limited to certain habitats and regions.
To reduce the emergence and propagation of such seed borne pathogens and diseases is of the utmost importance. Specific procedures and precautions must be taken at each step of seed production processing. Garden tools and processing equipment must be thoroughly checked and cleaned between each use, especially if they are suspected of carrying seed borne pathogens. This will also limit potential/accidental mixing when cleaning seeds as well as disease propagation. Similar precautions are to be observed whenever seeds are stored, ensuring adequate labeling (inside and outside each container). In the event of seed falling on the floor prior to their storing or while taking them from storage, they should not be picked up and replaced in the container. Seeds from certain species such as the Brassicas are indeed known to share a very similar appearance and can be easily mistaken, and therefore significantly increasing the risk of seed mixing.
Specific treatments and techniques aiming to kill bacterial disease-causing organisms on or within seeds are discussed under the Storing section in Module V.
Seed Borne Microorganisms of Common Vegetable Crops
Disease Severity Classes
- 1 – Severe: Seed borne pathogens that represent the greatest risk, resulting in often highly destructive diseases for a number of crops. Infected seeds are usually the primary source of inoculum for these pathogens. If seed infection is controlled, then the disease is typically controlled.
- 2 – Intermediate: Seed borne pathogens that represent a significant risk to subsequent crops. While infected seeds may not always be the primary source of inoculum for disease outbreak, there are many cases in which seeds infected with these pathogens can lead to severe outbreaks when planted under environmental conditions that favor the growth and spread of a particular disease.
- 3 – Minor: Important crop pathogens that can be transmitted via seeds, but infected seeds are not usually the primary source of inoculum for subsequent outbreaks of disease.
Alliaceae
| Crops | Crop Species | Disease | Pathogen | Microorganism Type | Disease Severity Classes | Seed Treatment | Notes |
| Onion, leek | Allium sativa, A. ampeloprasum | Neck rot | Botrytis aclada | Fungus | 1 | / | / |
| Black mold | Aspergillus niger | Fungus | 2 | / | / | ||
| Smudge | Colletotrichum circinans | Fungus | 3 | / | / | ||
| Botrytis blight | Botrytis cinerea | Fungus | 3 | / | / | ||
| Basal rot | Fusarium oxysporum f. sp. cepae | Fungus | 3 | / | / | ||
| Stemphylium leaf blight | Stemphylium vesicarium | Fungus | 3 | / | / | ||
| Purple blotch | Alternaria porri | Fungus | 3 | / | / |
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Amaranthaceae
| Beet, Swiss Chard | Beta vulgaris | Phoma/Canker | Phoma betae | Fungus | 2 | 20 min. at 52°C | / |
| Downy mildew | Peronospora farinosa f. sp. betae | Fungus | 2 | 20 min. at 52°C | / | ||
| Bacterial leaf spot | Pseudomonas syringae pv. aptata | Bacteria | 2 | / | / | ||
| Cercospora leaf spot | Cercospora beticola | Fungus | 2 | 20 min. at 52°C | / | ||
| Fusarium wilt | Fusarium oxysporum f. sp. betae |
Fungus | 3 | / | / | ||
| Ramularia leaf spot | Ramularia betae |
Fungus | 3 | / | / | ||
| Spinach | Spinacia oleracea | Downy mildew | Peronospora farinosa f. sp. spinaciae | Fungus | 2 | 25 min. at 52°C | / |
| Fusarium wilt | Fusarium oxysporum f. sp. spinaciae | Fungus | 2 | 25 min. at 50°C | / | ||
| Verticillium wilt | Verticillium dahliae | Fungus | 2 | / | / | ||
| Cucumber mosaic virus (CMV) subgroup II | Virus | 2 | 25 min. at 50°C | / | |||
| Anthracnose | Colletotrichum dematium |
Fungus | 2 | / | / | ||
| Cladosporium leaf spot | Cladosporium variabile |
Fungus | 2 | / | / | ||
| Stemphylium leaf spot | Stemphylium botryosum |
Fungus | 2 | / | / | ||
| White rust | Albugo occidentalis | Fungus | 2 | 25 min. at 50°C | / | ||
| Pseudomonas leaf spot | Pseudomonas syringae pv. spinaciae |
Bacteria | 2 | / | / |
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Apiaceae
| Crops | Crop Species | Disease | Pathogen | Microorganism Type | Disease Severity Classes | Seed Treatment | Notes |
| Carrot | Daucus carota | Bacterial leaf spot | Xanthomonas campestris pv. carotae | Bacteria | 1 | 20 min. at 50°C | / |
| Black rot/Back crown | Alternaria radicina | Fungus | 1 | / | / | ||
| Alternaria leaf blight | Alternaria dauci | Fungus | 1 | 20 min. at 50°C | / | ||
| Cercospora leaf blight | Cercospora carotae | Fungus | 2 | 20 min. at 50°C | / | ||
| Itersonilia canker | Itersonilia perplexans | Fungus | 3 | / | / | ||
| Stemphylium leaf blight | Stemphylium vesicarium | Fungus | 3 | / | / | ||
| Celery, Celeriac | Apium graveolens | Northern bacterial blight | Pseudomonas syringae pv. apii | Bacteria | 1 | / | / |
| Cercospora (early) blight | Cercospora apii | Fungus | 2 | / | / | ||
| Septoria (late) blight | Septoria apiicola | Fungus | 1 | / | / | ||
| Black rot/Black crown | Alternaria radicina | Fungus | 1 | / | / | ||
| Cilantro | Coriandrum sativum | Bacterial leaf spot | Pseudomonas syringae pv. coriandricola | Bacteria | 2 | / | / |
| Parsley | Petroselinum crispum | Alternaria leaf blight | Alternaria petroselini |
Fungus | 2 | / | / |
| Black rot/Black crown | Alternaria radicina |
Fungus | 2 | / | / | ||
| Cercosporoid leaf blight | Passalora punctum | Fungus | 3 | 25 min. at 52°C | / | ||
| Septoria blight | Septoria petroselini | Fungus | 1 | 25 min. at 50°C | / | ||
| Itersonilia canker | Itersonilia perplexans | Fungus | 3 | / | / | ||
| Parsnip | Pastinaca sativa | Itersonilia canker | Itersonilia pastinaca | Fungus | 1 | 25 min. at 50°C | / |
| Black rot/Black crown | Alternaria radicina |
Fungus | 1 | / | / | ||
| Phoma canker | Phoma complanata |
Fungus | 2 | / | / | ||
| Cercosporoid leaf blight | Passalora pastinacae |
Fungus | 3 | / | / |
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Asteraceae
| Crops | Crop Species | Disease | Pathogen | Microorganism Type | Disease Severity Classes | Seed Treatment | Notes |
| Lettuce | Latuca sativa | Lettuce drop | Scelotinia sclerotiorum, S. minor | Fungus | 2 | 30 min. at 48°C | / |
| Lettuce mosaic virus (LMV) | Virus | 1 | 30 min. at 48°C | / | |||
| Septoria leaf spot | Septoria lactucae | Fungus | 1 | 30 min. at 48°C | / | ||
| Bacterial leaf spot | Xanthomonas campestris pv. vitians | Bacteria | 2 | / | / | ||
| Gray mold | Botrytis cinerea | Fungus | 3 | / | / | ||
| Endive, Escarole, Chicories | Chicorium endivia, C. intybus | Black leaf spot | Alternaria cichorii | Fungus | 2 | / | / |
| Gray mold | Botrytis cinerea | Fungus | 3 | / | / |
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Brassicaceae
| Crops | Crop Species | Disease | Pathogen | Microorganism Type | Disease Severity Classes | Seed Treatment | Notes |
| Various | Brassicacea spp., Raphanus sativus | Black rot | Xanthomonas campestris pv. campestris | Bacteria | 1 | 20 or 25* min. at 50°C | / |
| Xanthomonas leaf spot | Xanthomonas campestris pv. armoraciae | Bacteria | 1 | / | / | ||
| Xanthomonas leaf spot | Xanthomonas campestris pv. raphani | Bacteria | 1 | / | / | ||
| Black leg | Phoma lingam | Fungus | 1 | 20 or 25* min. at 50°C | / | ||
| Black spot | Alternaria brassicicola | Fungus | 1 | / | / | ||
| Alternaria diseases | Alternaria brassicae | Fungus | 1 | 20 or 25* min. at 50°C | / | ||
| Downy mildew | Peronospora parasitica | Fungus | 3 | / | / | ||
| Fusarium wilt | Fusarium oxysporum f. sp. conglutinans | Fungus | 3 | / | / | ||
| Peppery leaf spot | Pseudomonas syringae pv. maculicola | Bacteria | 3 | / | / | ||
| Verticillium wilt | Verticillium dahliae | Fungus | 3 | / | / | ||
| White rust/White blister | Albugo candida | Fungus | 2 | / | / |
*20 min. for broccoli, cauliflower, kale, collards, kohlrabi, rutabaga, turnip; 25 min. for Brussels sprouts and cabbage.
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Curcurbitaceae
| Crops | Crop Species | Disease | Pathogen | Microorganism Type | Disease Severity Classes | Seed Treatment | Notes |
| Cucumber, Squash, Melon |
Cucumis sativus, Cucurbita spp., Cucumis melo | Fusarium crown and foot rot | Fusarium solani f. sp. cucurbitae | Fungus | 2 | 20 or 25* min. at 50°C | / |
| Cucumber green mottle mosaic virus (CGMMV) | Virus | 2 | / | Cucumber only | |||
| Scab | Cladosporium cucumerinum | Fungus | 3 | / | / | ||
| Fusarium wilt | Fusarium oxysporum f. sp. cucurbitacearum | Fungus | 3 | / | / | ||
| Squash mosaic virus (SqMV) | Virus | 1 | / | / | |||
| Gummy stem blight | Didymella bryoniae | Fungus | 1 | / | / | ||
| Bacterial leaf spot | Xanthomonas campestris pv. cucurbitae | Bacteria | 2 | / | / | ||
| Charcoal rot | Macrophomina phaseolina | Fungus | 3 | / | Melon only | ||
| Melon necrotic spot virus (MNSV) | Virus | 2 | / | Melon only | |||
| Anthracnose | Colletotrichum orbiculare | Fungus | 2 | / | / | ||
| Watermelon | Citrullus lanatus | Bacterial leaf spot | Xanthomonas campestris pv. cucubitae | Bacteria | 2 | / | / |
| Bacterial fruit blotch | Acidovorax avenae subsp. citrulli | Bacteria | 1 | / | / | ||
| Fusarium wilt | Fusarium oxysporum f. sp. niveum | Fungus | 2 | / | / |
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Fabaceae
| Crops | Crop Species | Disease | Pathogen | Microorganism Type | Disease Severity Classes | Seed Treatment | Notes |
| Bean |
Phaseolus spp. | Bean common mosaic virus (BCMV) | Virus | 1 | / | / | |
| Gray mold | Botrytis cinerea | Fungus | 3 | / | / | ||
| Alfalfa mosaic virus (AMV) | / | Virus | 2 | / | / | ||
| Southern bean mosaic virus (SBMV) | / | Virus | 2 | / | / | ||
| Anthracnose | Colletotrichum spp. | Fungus | 1 | / | / | ||
| Cercospora leaf blotch | Cercospora canescens | Fungus | 2 | / | / | ||
| Bacterial wilt | Curbobacterium flaccumfaciens pv. flaccumfaciens | Bacteria | 1 | / | / | ||
| Charcoal rot/Ashy stem blight | Macrophomina phaseolina | Fungus | 3 | / | / | ||
| Angular leaf spot | Pseudomonas syrngae pv. syringae | Fungus | 2 | / | / | ||
| Bacterial brown spot | Pseudomonas syrngae pv. phaseoli | Bacteria | 1 | / | / | ||
| Common bacterial blight | Xanthomonas campestris pv. phaseoli | Bacteria | 1 | / | / | ||
| Leaf and pod blight | Phoma exigua var. exigua | Fungus | 2 | / | / | ||
| Halo blight | Pseudomonas syringae pv. phaseolicola | Bacteria | 1 | / | / | ||
| Edamame |
Glycine max | Anthracnose | Colletotrichum spp. | Fungus | 2 | / | / |
| Bacterial blight | Pseudomonas savastanoi pv. glycinea | Bacteria | 2 | / | / | ||
| Phomopsis seed decay | Phomopsis longicolla | Fungus | 3 | / | / | ||
| Septoria leaf spot | Septoria glycines | Fungus | 3 | / | / | ||
| Pod and stem blight | Diaporthe phaseolorum var. sojae | Fungus | 3 | / | / | ||
| Soybean mosaic virus (SMV) | / | Virus | 1 | / | / | ||
| Northern stem canker | Diaporthe phaseolorum var. caulivora | Fungus | 3 | / | / | ||
| Tobacco ringspot virus (TRSV) | / | Virus | 1 | / | / | ||
| Garden Pea | Pisum sativum | Ascochyta blights | Ascochyta spp., Mycosphaerella pinodes, Phoma medicaginis var. pinodella | Fungus | 1 | / | / |
| Gray mold | Botrytis cinerea | Fungus | 3 | / | / | ||
| Anthracnose | Colletotrichum spp. | Fungus | 2 | / | / | ||
| Fusarium wilt and root rot diseases | Fusarium spp. | Fungus | 2 | / | / | ||
| Pea early browning virus (PEBV) | / | Virus | 2 | / | / | ||
| Pea seed borne mosaic virus (PSbMV) | / | Virus | 1 | / | / | ||
| Downy mildew | Peronospora viciae | Fungus | 3 | / | / | ||
| Bacterial blight | Pseudomonas syringae pv. pisi | Bacteria | 1 | / | / |
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Poaceae
| Crops | Crop Species | Disease | Pathogen | Microorganism Type | Disease Severity Classes | Seed Treatment | Notes |
| Sweet corn |
Zea mays | Black bundle disease | Acremonium strictum | Fungus | 2 | / | / |
| Southern leaf blight | Helminthosporium maydis | Fungus | 3 | / | / | ||
| Black kernel rot | Botryodiplodia thobromae | Fungus | 3 | / | / | ||
| Gross’s bacterial blight and wilt | Clavibacter michiganensis subsp. nebraskensis | Bacteria | 3 | / | / | ||
| Anthracnose | Colletotrichum graminicola | Fungus | 3 | / | / | ||
| Fusarium diseases | Fusarium spp. | Fungus | 2 | / | / | ||
| Northern corn leaf spot | Helminthosporium carbonum | Fungus | 3 | / | / | ||
| Charcoal rot | Macrophomina phaseolina | Fungus | 3 | / | / | ||
| Maize dwarf mosaic virus (MDMV) | / | Virus | 3 | / | / | ||
| Seedling blight | Penicillium spp. | Fungus | 3 | / | / | ||
| Downy mildew | Peronosclerospora sorghi | Fungus | 3 | / | / | ||
| Head snut | Sporisorium holci-sorghi | Fungus | 3 | / | / | ||
| Ear rot; stalk rot; seedling blight | Stenocarpella macrospora, S. maydis | Fungus | 2 | / | / |
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Solanaceae
| Crops | Crop Species | Disease | Pathogen | Microorganism Type | Disease Severity Classes | Seed Treatment | Notes |
| Eggplant |
Solanum melongena | Charcoal rot/Ashy stem blight | Macrophomina phaseolina | Fungus | 3 | / | / |
| Anthracnose | Colletotrichum spp. | Fungus | 2 | / | / | ||
| Eggplant mosaic virus (EMV) | / | Virus | 2 | / | / | ||
| Phomopsis fruit rot; blight | Phomopsis vexans | Fungus | 2 | / | / | ||
| Phytopthora diseases | Phytophthora capsici | Fungus | 3 | / | / | ||
| Bacterial wilt | Pseudomonas solanacearum | Bacteria | 3 | / | / | ||
| Tomato mosaic virus (ToMV) | / | Virus | 2 | / | / | ||
| Pepper | Capsicum annuum | Anthracnose | Colletotrichum spp. | Fungus | 2 | 25 min. at 50°C | / |
| Alfalfa mosaic virus (AMV) | / | Virus | 2 | / | / | ||
| Charcoal rot/Ashy stem blight | Macrophomina phaseolina | Fungus | 3 | / | / | ||
| Phytopthora diseases | Phytophthora capsici | Fungus | 3 | / | / | ||
| Bacterial wilt | Pseudomonas solanacearum | Bacteria | 3 | / | / | ||
| Tomato mosaic virus (ToMV) | / | Virus | 2 | / | / | ||
| Pepper mild mottle virus (PMMoV) | / | Virus | 1 | / | / | ||
| Bacterial spot | Xanthomonas campestris pv. vesicatoria | Bacteria | 1 | 25 min. at 50°C | / | ||
| Tomato | Solanum lycopersicum | Anthracnose | Colletotrichum spp. | Fungus | 3 | 25 min. at 50°C | / |
| Alfalfa mosaic virus (AMV) | / | Virus | 2 | / | / | ||
| Bacterial spot | Xanthomonas campestris pv. vesicatoria | Bacteria | 1 | 25 min. at 50°C | / | ||
| Early blight | Alternaria solani | Fungus | 2 | 25 min. at 50°C | / | ||
| Bacterial canker | Clavibacter michiganensis subsp. michiganensis | Bacteria | 1 | 25 min. at 50°C | / | ||
| Stem rot; stem canker | Didymella lycopersici | Fungus | 3 | / | / | ||
| Leaf mold | Fulvia fulva | Fungus | 3 | 25 min. at 50°C | / | ||
| Fusarium wilt | Fusarium oxysporum f. sp. lycopersici | Fungus | 2 | 25 min. at 50°C | / | ||
| Phytopthora diseases | Phytophthora capsici | Fungus | 3 | / | / | ||
| Bacterial wilt | Pseudomonas solanacearum | Bacteria | 3 | / | / | ||
| Bacterial speck | Pseudomonas syringae pv. tomato | Bacteria | 1 | 25 min. at 50°C | / | ||
| Syringae leaf spot | Pseudomonas syringae pv. syringae | Bacteria | 2 | / | / | ||
| Tobacco mosaic virus (TMV) | / | Virus | 2 | / | / | ||
| Tomato mosaic virus (ToMV) | / | Virus | 2 | / | / |
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Credit (seed borne microorganisms list): Chelsea Green Publishing