Outline. Goals: (1) Recognize and name disease symptoms (2) Where to get pathology information. Cindy Ocamb Extension Plant Pathologist

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1 Cindy Ocamb Extension Plant Pathologist Oregon State University Department of Botany and Plant Pathology Outline Definition of disease Plant pathogenic microorganisms - groups Abiotic causes of disease Diagnostic steps and associated disease patterns Overview of general types of symptoms Disease management strategies Sources of information Goals: (1) Recognize and name disease symptoms (2) Where to get pathology information 1

2 Disease Injurious change from normal biological function resulting from continual irritation by a causal factor Disease Environment No Disease Environment 2

3 Plant Pathogenic Organisms Fungi -- branching hyphae & spores many diseases Oomycetes -- water molds Downy mildew Bacteria -- water-loving microbes Bacterial canker Nematodes -- microscopic worms root diseases Viruses -- nucleic acids with protein coat Alfalfa mosaic Viroids -- ribonucleic acids Hop stunt viroid Phytoplasmas -- wall-less microbes Aster yellows Parasitic plants (no chlorophyll and/or roots) Mistletoes Fungi: mushrooms seen in lawns Conk of fungal pathogen infecting a tree Conk is a sign of the fungal pathogen Culturing (isolating) Fusarium species from corn seeds 3

4 Biotic pathogens have an infectious disease cycle Rose Rust (Phragmidium sp) Teliospores in telia (not shown) Uredospores produced in uredia throughout growing season (repeating stage) Rose Rust (Phragmidium sp.) Telia in February Uredia in May 4

5 Conidia in chains Can be windblown all season-long Powdery mildew on pumpkin temperature (cold injury) moisture wind (desiccation injury) light (sun scald) soil ph and soil structure chemicals (herbicides, etc.) Pathogen (infectious) vs. Abiotic (noninfectious) cause of disease Some abiotic factors cause specific symptoms but many abiotic diseases result in nonspecific symptoms. Diagnosis is difficult without probing about environment or weather, cultural practices, etc. 5

6 Is growth normal? What is normal growth? Compare problem plant to normal plant Healthy stem but juvenile tissue has overgrowths on bark Normal color variation, severe pruning can result in mostly green color Is growth normal Patterns? Abiotic disease Uniform distribution Biotic disease Random distribution 6

7 Is this uniform or random? Affected plant patterns aid in diagnosis Abiotic problem: more uniform distribution >1 plant species affected Biotic disease problem: generally a single host species affected less uniform distribution Is growth normal Patterns Part of plant affected? Symptoms? symptoms are expressed by the plant due to disease: can include leaf spot, leaf blight, shoot blight, fruit spot, fruit rot, gall, canker (lesion on stem), wilt, vascular discoloration, root rot, damping-off, etc. 7

8 Is growth normal Patterns Part of plant affected, symptoms Development of damage over time! Is growth normal? (know normal growth) Are patterns apparent? (plant, garden or field) Parts of plants affected? (look at entire plant) Symptoms or signs? (progression over time) Ask questions!! Cultural practices? When symptoms developed, extent? Are these representative samples? Synthesis of information diagnosis Disease Management things that you cannot do much about 2011 extra cool, wet springs or cool summers highly susceptible plants apricots or Red Haven peach in the Willamette Valley perennial plants growing outside of their natural range lemon trees in western Oregon swamp cypress on the east side of the Cascades 8

9 Disease Management things that you can do & should do to avoid problems cultural controls!! Examples of cultural controls site selection sunny, well-draining site ideal for vegetable gardens time of planting damping-off promoted by cold, wet soils plant resistant cultivars Frost peach has peach leaf curl resistance quality seeds and transplants don t sow garlic, potato from grocery plant spacing provide room for sufficient light and air movement fertility & water overwatering or too much N can lead to disease overhead irrigation enhances disease ~ drip remove diseased plant parts rake up scabby leaves/fruit in fall pull off tomato leaves with late blight etc. Control of Infectious Plant Diseases exclusion avoidance resistance eradication protection 9

10 Control of Infectious Plant Diseases exclusion conducted by governmental agencies quarantines, inspections, certification avoidance planting locale/time plant in warm soils pathogen-free transplants and seeds resistance genetic plant resistant varieties eradication sanitation remove infected plant material crop rotation tomato > lettuce > bean eliminate alternate host Cedar apple rust use pasteurized planting medium for starts protection chemical (fungicides) or biological Regional reference on plant disease Pacific Northwest Plant Disease Management Handbook website: On-line Guide to Plant Disease Control References/resources on plant diseases Factsheets, extension publications, etc. APS Press disease compendia, other books Additional on-line resources (public & private but be selective) County extension staff Accurate diagnosis is important! may need plant disease clinic submit good samples for diagnosis 10

11 Is growth normal (know normal growth) Are patterns apparent? (plant, garden or field) Parts of plants affected? (look at entire plant) Symptoms or signs? (progression over time) Ask questions!! Use all information resources Publications (paper and electronic) Land Grant University system (OSU) County Extension staff Plant Clinics to provide an answer to problem and prevention or management options. Powdery mildews Mostly host specific Infection by sexual spores promoted by high RH and/or free moisture; conidia inhibited by rainfall Repeating spore stage (conidia) produced on hyphal filaments Sexual spore stage produced in spherical ascocarp (cleistothecia) Photos by C. M. Ocamb Downy Mildews highly host specific promoted by wet conditions or high relative humidity Spores 11

12 Rusts obligate parasites host specific complex life cycles promoted by rainy weather spores produced in pustules Fusarium, Rhizoctonia, Pythium, etc. incite diseases No plant emergence -- seed rot or pre-emergence damping-off Post-emergence damping-off -- seedling emergences then falls over Root rot decay of roots, at any time usually present at low levels on healthy plants Seedborne Alfalfa mosaic virus found in cilantro in 2017 Alfalfa mosaic virus particles. Reproduced with permission from IACR Rothamsted: 12

13 Alfalfa mosaic virus vectored by aphids (at least 14 aphid species) Blue alfalfa aphid Acyrthosiphon kondoi Oregon, 1989 Pea aphid Acyrthosiphon pisum Oregon, 1982 Cowpea aphid Aphis craccivora Oregon, 1983 Cotton aphid Aphis gossypii Oregon, 1968 Spirea aphid Aphis spiraecola introduced to Oregon, 2001 Green peach aphid Myzus persicae Oregon, 1979 Alfalfa mosaic virus has a broad host range (attributed to multiple species of aphid vectors) 430 species of 51 dicot plant families are susceptible to AMV alfalfa (some varieties are asymptomatic) beans clover Susceptible crops grown pea in western Oregon potato tomato} Plant Symptoms Leaves abnormal colors abnormal patterns Alfalfa with AMV Photo by C. M. Ocamb, 1998 Roots reduced root system Stems discoloration of bark Vegetative organs internal rotting or discoloration Cilantro with AMV Photo by K. Buckland, 2017 Whole plant dwarfing 13

14 Phytoplasmas Are very small prokaryotes which are related to bacteria, but in contrast to bacteria they do not have a cell wall but rather are bound by a triple-layered membrane. They are pleiomorphic or filamentous in shape and less than 1 μm in diameter. Phytoplasmic DNA is distributed throughout the cytoplasm, instead of being concentrated in a nucleus. Phytoplasmas inhabit phloem sieve elements in host plants Members of Ca. Phytoplasma asteris (16SrI group phytoplasmas) are found in 80 monocot and dicot plant species Phytoplasmas cannot be cultured on artificial media Phytoplasmas will not survive in infected plant debris but will survive in crowns and roots of infected perennial plants Phytoplasmas are transmitted by 30 insect species and can overwinter in insects, especially leaf hoppers, and in perennial or biennial host plants, including native plants, ornamentals, and weeds such as Russian thistle (Salsola tragus), sowthistle (Sonchus spp.), prickly lettuce (Lactuca serriola), dandelion (Taraxacum officinale), etc. Phytoplasmas can be pathogenic to some insect hosts, but generally do not negatively affect the fitness of their major insect vectors Phytoplasmas can increase fecundity and survival of insect vectors, may influence flight behavior and plant host preference of insects 14

15 Phytoplasmas induce symptoms that suggest interference with plant development: witches broom (clustering of branches) of developing tissues phyllody (flower parts develop as leaf portions) virescence (green coloration of non-green flower parts) reddening of leaves and stems bolting (growth of elongated stalks) formation of bunchy fibrous secondary roots tap roots of carrots are thin, small, covered in many root hairs, and often taste bitter generalized yellowing, decline and stunting of plants phloem necrosis Photos by Steve Koike, UC-Davis at Abiotic Diseases 15

16 Blossom-end Rot -- Calcium deficiency Sunburn Rhododendron -- sun scald or desiccation injury Photo by C. M. Ocamb Copyright 2006 Melodie Putnam, Oregon State University 16

17 Disease outbreaks may wax and wane New pathogen introductions also occur Black leg (Leptosphaeria = Phoma) Phoma leaf spot (Black leg) on cabbage 17

18 Black leg (Phoma lingam; Leptosphaeria maculans) Pseudothecia (ascospores) on infected stem residues Asci (sacs) with ascospores Leaf spot & pycnidia Canker & pycnidia form on stems More leaf spots via rain splashed conidia Pycnidia oozing conidia Mean # Leptosphaeria leaf spots/plant Mean # Leptosphaeria leaf spots/plant Trap plant 100 black leg results canola 2015 turnip turnip canola 0 Oct Dec Feb Apr Jun Trap plant black leg results canola canola canola 2016 turnip turnip-2 0 Oct Dec Feb Apr Jun Black leg of Brassica seedling due to infected seed Phoma black leg 18

19 Plant only seed that has been certified to be free of black leg by official testing and has been treated with an approved fungicide or hot water (15 to 25 min at 122 F). Crucifer seed stock intended for planting in Oregon must be accompanied by an official test stating that the untreated seed is free from black leg. Homeowner seed packets, 0.5 oz size or smaller, are not exempt from black leg seed testing requirements. Black leg affects Brassicaceae weeds curvepod yellow cress: common host in western OR Light leaf spot of Brassica and Raphanus fungus: Cylindrosporium concentricum (Pyrenopeziza brassicae) Windblown ascospores produced on infected plant residues atop of soil Water-splashed conidia produced pycnidia on infected plant tissues (living tissues, above-ground) Seedborne 19

20 Light leaf spot on radish Light leaf spot (Turnip Feb 2016) Light leaf spot on plant stems 20

21 Stem lesion due to light leaf spot compared to black leg Light Leaf Spot Phoma Leaf Spot (Black leg) 21