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 Disease Injurious change from normal biological function resulting from continual irritation by a causal factor
Disease
Environment
No Disease
Environment
2 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 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 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 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 • Biotic disease – Uniform distribution – Random distribution
6 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 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 2011 • things that you cannot do much about
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 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 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 http://pnwhandbooks.org/plantdisease/
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 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 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: http://www.res.bbsrc.ac.uk/cdm/plantpath/
12 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 Alfalfa with AMV • abnormal colors Photo by C. M. Ocamb, 1998 • abnormal patterns
Roots • reduced root system
Stems • discoloration of bark
Vegetative organs Cilantro with AMV • internal rotting or Photo by K. Buckland, 2017 discoloration
Whole plant • dwarfing
13 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 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 http://ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=17588
Abiotic Diseases
15 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 Disease outbreaks may wax and wane New pathogen introductions also occur
Black leg (Leptosphaeria = Phoma)
Phoma leaf spot (Black leg) on cabbage
17 Black leg (Phoma lingam; Leptosphaeria maculans) Pseudothecia (ascospores) Asci (sacs) with ascospores Leaf spot & pycnidia on infected stem residues
Canker & pycnidia form on stemS More leaf spots via rain Pycnidia oozing splashed conidia conidia
120 2015-16 Trap plant 2015 canola 2015 turnip-1 100 black leg results 2015 turnip-2 2014 canola 80
60
40
leaf spots/plant 20 Mean # Leptosphaeria 0 Oct Dec Feb Apr Jun
120 2016-17 Trap plant 2016 canola-1 2016 canola-2 100 black leg results 2014 canola 2016 turnip-1 2016 turnip-2 80
60
40
leaf spots/plantleaf 20 2015 2016 Mean # # Leptosphaeria Mean 0 Oct Dec Feb Apr Jun
Black leg of Brassica seedling due to infected seed Phoma black leg
18 • 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 Light leaf spot on radish
Light leaf spot (Turnip – Feb 2016)
Light leaf spot on plant stems
20 Stem lesion due to light leaf spot compared to black leg
Light Leaf Spot
Phoma Leaf Spot (Black leg)
21