Master Gardener Intern Training Basic plant pathology

Kevin Ong, PhD.

Associate Professor and Extension Plant Pathologist Director – Texas Plant Disease Diagnostic Laboratory College Station, TX

What is a plant disease? Textbook definition:

Injurious physiological processes induced by a continuous irritation of a primary causal agent resulting

in characteristic symptoms.

Three criteria: •  Injurious to the welfare of humans and/or the plant. •  Is the result of a continuous, not instantaneous, process •  Results in characteristic symptoms

•  Basic plant functions (left)

•  Disease interference with those functions (right)

CONCEPT #1: Disease triangle •  Three factors:

PATHOGEN (disease causing agent)

ENVIRONMENT

HOST P

LANT

DISEASE

Review of the Disease Triangle

PATHOGEN (disease causing agent)

ENVIRONMENT

HOST

PLA

NT

Susceptibility Aggressiveness Susceptibility Susceptibility Aggressiveness

Aggressiveness

Aggressiveness

Aggressiveness

Wet

Dry Disease Occurs

Disease

CONCEPT #2: Disease Cycle

Overseasoning/ Overwintering

Management

Management

Management

Infection

Host

Reinfection

Management

Management

Disease Cycle •  Example of a pathogen: cedar – apple Rust

Overwintering

Host Infection

Disease

Disease Cycle: Peach Brown Rot

Overseasoning

Host Infection

Reinfec

tion

Disease

Causes of plant diseases

•  BIOTIC (pathogenic) –  Fungi –  Bacteria –  Viruses –  Nematodes –  Parasitic plants

•  ABIOTIC (physiogenic) –  Temperature –  Moisture –  Light –  Nutrition –  Chemical

Plant Pathogens (Biotic)

* from Agrios, 1997

•  Majority microscopic

•  Fungi, bacteria, viruses, nematodes, parasitic plants, spiroplasmas

Kingdom Fungi

•  Morphology – Bodies constructed of filaments called hyphae

•  Septate hyphae have cells separated by cross-walls •  Aseptate (coenocytic) hyphae have no septa •  Haustoria are hyphae modified for absorbing nutrients from a host

(found in parasitic fungi)

– Hyphae interweave into mats called Mycelium

•  Sexual Reproductive Structures – Unique for each class of fungus

Fungi

•  The study of fungi is called mycology after the Greek word mykes which refers to fungi. Fungi are filamentous organisms that are for the most part microscopic, but some produce large structures such as toadstools or mushrooms.

•  Approximately 100,000 fungal species have been described and most of them are beneficial or benign.

•  There are only about 8,000 fungal species that cause plant diseases

Fungal Diseases

•  Most of the common diseases occurring on landscapes are caused by fungi.

•  85% of plant diseases caused by fungi. •  Majority of fungi are saprophytic. •  Characteristics of fungi

–  Absorb nutrients –  Multi-celled –  Radial growth as tubular filaments –  Reproduce and survive as spores –  May “overwinter” as sclerotia, rhizomorph or spores. May “overwinter” as sclerotia, rhizomorph or spores.

Images from the Plant Management Network Image Collections.

Diagnosis of fungal diseases

•  Presence of visible fungal structures – May be observed unaided or with

magnification.

•  Can usually be cultured on artificial media for identification – Exceptions: obligate parasite such as rust and

mildew fungi.

Water Molds

•  Aseptate Mycelium •  Swimming stage in life cycle •  Need water for reproduction •  Common diseases

–  Phytophthora – SOD, Potato Blight, Rhododendron blight and root rot

–  Pythium – root rots, seedling blights – Downy Mildew – foliar blights on rose, grapes

Ascomycetes

• Septate hyphae • Some reproductive structures are macroscopic

– Morels, truffles, cup fungi

• Many common diseases – Anthracnose – Leaf Spots – Cankers

Fungi- Botrytis

Marigold

Poinsettia Liniaris

Lisianthus

Basidiomycetes

• Septate hyphae • Some reproductive structures are macroscopic

– Mushrooms, Shelf fungi

• Many common diseases – Wood decay – Root rots – Rusts

Fungi

•  Brown patch-Rhizoctonia

Fungi •  Take-all- Gaeumannomyces

Bacterial Diseases

•  Bacteria are simple single celled organisms that are microscopic. Approximately 1,600 bacterial species have been described and about 80 species are plant pathogens.

•  Most bacteria can be grown in culture. •  Most of these bacteria are enveloped in a

protective layer of viscous gummy material and most also have flagella that are distributed in various patterns over the cell wall.

Bacterial Reproduction

•  Bacteria multiply and divide asexually by binary fission which means reproduction by splitting into two equal halves.

•  Bacteria can go through this process very rapidly and under favorable conditions may divide every 20 minutes.

•  At this doubling rate of reproduction one bacterium could produce one million bacteria in the time span of about ten hours.

Diagnosis of bacterial disease

•  Leaf lesions sometimes limited by veins (angular)

•  Ooze or streaming from cut tissue •  Soft rot (fruit) has foul odor •  Can be culture on media

–  Use of selective media for identification of pathogen.

Diagnosis of bacterial disease

Bacteria •  Blights

Syngonium

Blacking of the spathe in the ‘flower blight’ stage. Photo by T. Vowell.

Syngonium

Bacteria •  Wilts

Diagnostic

Crassula

Zucchini

Viral diseases

•  Made up of genetic material and protein coat

•  Replicate by “hijacking” plant DNA •  Require wound to enter plant cell •  Require living host •  Usually transmitted by a vector

Virus diseases: What to look for..

•  Eliminate other potential causes, such as bacterial, fungal diseases or insect damage

•  Indirect evidence: presence of vectors (eg. Insect known to transmit the virus)

•  Testing using laboratory methods (serological or genetic testing)

Viral symptoms

•  Mosaic

Viral symptoms

•  Rings

Viral symptoms

Plant Parasitic Nematodes

•  Characteristic of nematodes –  Very small animals (round worms) –  Typically in the soil –  Usually attack roots, sometimes

foliage –  Reproduces with eggs

Discriminating nematodes

Plant parasitic

Various shape and sizes of nematodes

Diagnosis of nematode diseases

•  Know the symptoms found on the host plant

•  Presence of the nematodes

Plant Parasitic Nematodes •  Root knot

Parasitic plants

•  Characteristic of parasitic plants – Obtain all or some of their needed nutrient

from other plants. – Many has little or no chlorophyll. – Cause relatively few problems when compared

to other disease problems.

Diagnosis of Parasitic Plant Problems

•  Know symptoms (typically stunting & unhealthiness)

•  Presence of the pathogen on the host.

Parasitic plants

Master Gardener Intern Training Plant Disease Control Principles

Kevin Ong, PhD.

Associate Professor and Extension Plant Pathologist Director – Texas Plant Disease Diagnostic Laboratory College Station, TX

Disease Control Principles

•  Resistance •  Exclusion •  Protection •  Eradication •  Avoidance •  Therapy

Resistance Employing genetics of the plant to naturally

resist pathogens – Resistant versus tolerance

•  Indirect Resistance – Adapted traits

•  Drought resistance, etc…

Where to find information? plant labels, internet, trade journals, research publications

Beware of the reliability of the information source!!!

Exclusion

Employing methods to keep pathogen out •  Using only healthy plants, buy certified plant

materials •  Maintain clean equipment •  Infrastructure to maintain clean growing area

Limitation: Cannot control everything

Protection Utilizing barrier to protect the plant from the

pathogen – Chemical barrier – Biological barrier – Physical barrier

Eradication Killing or getting rid of the pathogen

– Removing and/or destroying diseased plant debris (Sanitation or roguing)

– Fumigating area of planting (solarization)

Avoidance Finding ways to avoid the wrath of the

pathogen – Different planting sites – Different planting times – Utilitarian landscape design plan – Using adapted varieties

Therapy

Helping the plant to heal •  Systemic chemicals •  Pruning

The Texas Plant Disease Diagnostic Laboratory

What do we do?

•  Provide diagnostic support to: – AgriLife Extension personnel – State and Federal Department of Agriculture

personnel – Agriculture industry – Green industry – Homeowners

What is plant disease diagnostics?

•  An art of figuring out the cause of the disease problem disease problem

Fillable pdf form can be downloaded at website

Current and new services

•  General Diagnosis –  $30 routine –  $50 specialized test procedures

•  Nematode Detection Assay

•  Pierce’s Disease testing (TX grape growers only) •  Phytophthora [water catchment] Assay

(commercial)

More information at: hp://plantclinic.tamu.edu

Follow TPDDL on twier (txplantclinic)

Find us on facebook (Texas Plant Disease Diagnostic Lab)

•  NPDN mission •  Sample submission