What is a plant disease?

Transcription

1 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)

2 Three factors: CONCEPT #1: Disease triangle HOST PLANT DISEASE ENVIRONMENT PATHOGEN (disease causing agent) Review of the Disease Triangle Dry Disease Occurs HOST PLANT Aggressiveness ene ENVIRONMENT Susceptibility ggressivenes Wet PATHOGEN (disease causing agent) Management CONCEPT #2: Disease Cycle Management Overseasoning/ Overwintering Reinfection Host Management Management Disease Infection Management

3 Disease Cycle Example of a pathogen: cedar apple Rust Host Infection Disease Overwintering Disease Cycle: Peach Brown Rot Overseasoning Host Infection Reinfection D i s e a s e

4 Causes of plant diseases BIOTIC (pathogenic) Fungi Bacteria Viruses Nematodes Parasitic plants ABIOTIC (physiogenic) Temperature Moisture Light Nutrition Chemical Plant Pathogens (Biotic) Majority microscopic Fungi, bacteria, viruses, nematodes, parasitic plants, spiroplasmas * from Agrios, 1997 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

5 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. 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.

6 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 Lisianthus Poinsettia Liniaris

7 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

8 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.

9 Blights Bacteria 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

10 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 ms Rings

11 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

12 Various shape and sizes of nematodes Diagnosis of nematode diseases Know the symptoms found on the host plant Presence of the nematodes Root knot Plant Parasitic Nematodes

13 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

14 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!!!

15 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)

16 Avoidance Finding ways to avoid the wrath of the pathogen Different planting sites Different planting times Utilitarian landscape design plan Using adapted varieties Helping the plant to heal Systemic chemicals Pruning Therapy The Texas Plant Disease Diagnostic Laboratory

17 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 Fillable pdf form can be downloaded at website

18 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: h p://plantclinic.tamu.edu Follow TPDDL on twi er (txplantclinic) Find us on facebook (Texas Plant Disease Diagnostic Lab) NPDN mission Sample submission