1. Plant pathology review. Seed Pathology Overview Agron 338, Seed Science and Technology. Seed Quality Characteristics. What is plant disease?

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Seed Pathology Overview Agron 338, Seed Science and Technology Dr. Gary Munkvold Professor of Plant Pathology munkvold@iastate.

1 Seed Pathology Overview Agron 338, Seed Science and Technology Dr. Gary Munkvold Professor of Plant Pathology Seed pathology - terms Seed pathology (Agarwal & Sinclair, 1997) the study of seedborne diseases and pathogens (includes other propagative materials) Seed pathology (Munkvold, 2008) the study and management of diseases affecting seed production and utilization; disease management practices applied to seeds Seed pathology - terms Seed health refers primarily to the presence or absence of disease-causing organisms, such as fungi, bacteria and viruses, and animal pests, such as eelworms and insects, but physiological conditions such as trace element deficiency may be involved. Seed health testing deals with detection and identification of seed-borne microorganisms. Seed quality is a concept made up of different attributes, namely purity, moisture content, and germination. Source: Danish Seed Health Centre: Seed Quality Characteristics Germination ability Vigor Analytical (mechanical) purity Pure live seed = (%purity %germination)/100 Varietal (genetic) purity Seed health (absence of seedborne/seedtransmitted pathogens) Encyclopedia of Seeds, 2006 Seed Quality Characteristics cont. Moisture content Uniformity Additional seed quality parameters: Seed size Visual appearance Seed treatment quality (dose and uniformity) 1. Plant pathology review What is plant disease? Biotic vs. abiotic disease vs. injury Types of pathogens Disease triangle Effects of diseases on plants Encyclopedia of Seeds, 2006

2 What is plant disease? when the ability of the cells or part of a plant to carry out one or more essential functions is interfered with by a pathogenic organism or adverse environmental factor, the activities of the cell are disrupted, etc., the cells malfunction or die What is plant disease? Normal cell or plant functions are disrupted (efficiency is reduced) Caused by a prolonged interaction with another organism or abiotic factor A process, not a physical entity Disease triangle Plant pathogens Fungi & Oomycetes Bacteria Viruses and viroids Phytoplasmas Nematodes Parasitic plants Temporal, quantitative Fungi Kingdom Mycota Microscopic, filamentous or single-celled, eukaryotic Cell walls composed of chitin & other compounds No chlorophyll Spore-producing

Bacteria Kingdom Procaryotae Microscopic, single-celled,

fission Phytoplasmas Lack cell walls Viruses Nucleic acid

utilizing host biosynthesis processes Nematodes Nematodes

3 Bacteria Kingdom Procaryotae Microscopic, single-celled, prokaryotic Cell membrane and cell walls Reproduction by fission Phytoplasmas Lack cell walls Viruses Nucleic acid strands + coat protein Obligate parasites Reproduce by utilizing host biosynthesis processes Nematodes Nematodes Kingdom Animalia Microscopic, unsegmented worm-like organisms Stunt Nematode Lance Nematode

4 Stem rust disease cycle Pythium disease cycle 2. Seed Pathology Seeds implicated in plant disease for millennia Seed treatment suggested in 1733 Seed pathology initially consisted of detecting and identifying seedborne organisms Paul Neergaard first seed pathology publications and textbook

2. Seed Pathology overview Disease management in seed production Seed conditioning Seed infection processes Seed storage Seed health testing Seed

McGee, 1995 INFECTION Cultural practices Disease resistance Chemical control SEED PRODUCTION FIELD TRANSMISSION SPREAD PLANTED FIELD DISEASE DEVELOPMENT

Environment Association of pathogens with seeds Seedborne Pathogen accompanying, external, or internal Seed-transmitted Pathogen passes from seed to

transported or disseminated Seed as a pathway Seedborne or seed-transmitted Pathogen can be introduced and established through seed Classes of Seedborne

5 2. Seed Pathology overview Disease management in seed production Seed conditioning Seed infection processes Seed storage Seed health testing Seed transmission Seed treatment Seedling diseases Environments for Seed Pathogen Interactions Inoculum thresholds Cultural practices Disease resistance From McGee, 1995 INFECTION Cultural practices Disease resistance Chemical control SEED PRODUCTION FIELD TRANSMISSION SPREAD PLANTED FIELD DISEASE DEVELOPMENT HARVESTING PROCESSING STORAGE SURVIVAL Seed health tests Seed treatment Seed conditioning Environmental control Disease management in seed production why is it different? Crop value Seed quality Performance Appearance Quarantine issues No flexibility in genotype choice Limited location flexibility Limited crop rotation Environment Association of pathogens with seeds Seedborne Pathogen accompanying, external, or internal Seed-transmitted Pathogen passes from seed to plant Transmitted vs. transported or disseminated Seed as a pathway Seedborne or seed-transmitted Pathogen can be introduced and established through seed Classes of Seedborne Microorganisms 2 1. Pathogens for which seed is the major inoculum source, and when seed infection is controlled, the disease is controlled. 2. Important pathogens of a crop, but the infected seed is a minor inoculum source. 3. Seedborne microorganisms that have never been shown to cause disease as a result of their presence on seeds. 4. Pathogens that can infect seed either in the field or in storage, and reduce quality

Disease Control During Seed Corn Conditioning Hand sorting Discard insect-damaged and diseased ears Drying and shelling Arrest fungal development; discard glumes and damaged kernels Air screen

seeds and other potentially contaminated, lowdensity material Seed treatment Reduces contamination / impact of seedborne pathogens Germination tests Detect seed lots of questionable vigor Examples -

spp and S. sclerotiorum of soybean and Fusarium graminearum of wheat. Removal of purple soybeans infected to detect Cercospora kikuchii by optical sorting.

6 Disease Control During Seed Corn Conditioning Hand sorting Discard insect-damaged and diseased ears Drying and shelling Arrest fungal development; discard glumes and damaged kernels Air screen cleaning Discard glumes and damaged kernels Size grading Eliminate kernels physically altered because of disease Disease Control During Seed Corn Conditioning Gravity separation Removal of diseased seeds and other potentially contaminated, lowdensity material Seed treatment Reduces contamination / impact of seedborne pathogens Germination tests Detect seed lots of questionable vigor Examples - Disease Control During Seed Conditioning Removal of sclerotia of Sclerotinia sclerotiorum from soybeans Removal of ergot sclerotia from barley Removal of small/low density seeds infected by Phomopsis spp and S. sclerotiorum of soybean and Fusarium graminearum of wheat. Removal of purple soybeans infected to detect Cercospora kikuchii by optical sorting. Ergot sclerotia mixed with barley seeds Sclerotinia sclerotiorum contamination of soybean seeds Cleaner Output Example Corn Alan Gaul Seed health testing Phytosanitary certification Quality control Disease management through application of tolerances / thresholds Estimate stand establishment impacts Sampling!!!

Mechanisms of seed transmission (Neergaard) Intraembryal systemic infection Ustilago nuda Intraembryal local infection Ascochyta pisi Extraembryal systemic infection Drechslera graminea Extraembryal

Sclerotinia sclerotiorum Seed contamination saprophytism and systemic infection Fusarium oxysporum Seed contamination by structures from organ- specific seed infection followed by an extramatrical

vigor Seed exudates Insect interactions Climatic conditions Cultural factors Seedling disease management Seed quality Crop rotation, tillage, seedbed preparation, drainage Soilborne inoculum Soil

7 Mechanisms of seed transmission (Neergaard) Intraembryal systemic infection Ustilago nuda Intraembryal local infection Ascochyta pisi Extraembryal systemic infection Drechslera graminea Extraembryal local infection Alternaria brassicicola Seed contamination systemic infection Ustilago hordei Mechanisms of seed transmission (Neergaard) Seed contamination saprophytism and local infection Sclerotinia sclerotiorum Seed contamination saprophytism and systemic infection Fusarium oxysporum Seed contamination by structures from organ- specific seed infection followed by an extramatrical non-parasitic phase and later by direct organ- specific seed infection Claviceps purpurea Factors influencing seedling disease Seedborne pathogens Soilborne pathogens Seed structure Seed age Seed vigor Seed exudates Insect interactions Climatic conditions Cultural factors Seedling disease management Seed quality Crop rotation, tillage, seedbed preparation, drainage Soilborne inoculum Soil moisture & temperature Seed-soil contact Fertilizer and herbicide practices Avoid injury, promote plant vigor Planting date Escape inoculum due to unfavorable conditions for disease Resistance Available for some seedborne and soilborne pathogens Seed treatment, soil fungicides

8 Chemical seed treatments Fungicides To protect against seed & seedling pathogens; ensure stand establishment Primarily for soilborne pathogens Also for some seedborne pathogens (e.g., Phomopsis, Sclerotinia) Prevent seed transmission (smut fungi) Control of storage fungi Systemic activity to protect wheat seedlings from leaf infection Insecticides To protect against insects feeding on seeds & seedlings Systemic activity to reduce feeding on roots or above-ground plant parts Polymer coatings Planting & germination characteristics SAR inducers Seed Treatment Trends Insecticide seed treatments now standard on corn 6 active ingredients at once may become common Seed treatments will become standard for soybeans Target organisms number in the dozens for a single crop (microorganisms, insects, nematodes, birds, mammals) Low cost (relatively), improving efficacy, convenience, low environmental impact Plant Pathology 494 / 594, Seed Pathology Instructors: Gary Munkvold, ISU Seed Science Derrick Mayfield, ISU Seed Science Nancy Gonzalez, ISU Seed Science Lindsey DuToit, Washington St. Univ. Seed Technology & Business / Pl P 592, Seed Health Management (on-line) Gary Munkvold, ISU Seed Science Denis McGee, ISU Seed Science (ret.) Pl P 494/594 Outline Disease management in seed production Seed conditioning Seed infection processes Seed storage Seed health testing Seed transmission Seed treatment Seedling diseases

Seed Quality and Seed-Borne Diseases of Cereal Crops

Seed Quality and Seed-Borne Diseases of Cereal Crops Seed generally refers to grain intended for planting, while grain is the term for sale into the marketplace as food, feed or fuel. Quality has different