Understanding Seed Borne Pathogens and Diseases

White fungal growth and sunken lesions on discolored seed – Photo credit: S. Markell, NDSU

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 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