Introduction to Plant Diseases

Damon L. Smith Assistant Professor and State Extension Specialist Department of Entomology and Plant Pathology,

Oklahoma State University

Plant Disease

Plant Disease: any malfunctioning of the growth or reproduction of a plant caused by continuous irritation by a primary causal agent that results in the development of symptoms

Plant Disease

Continuous irritation : Very important point; this separates disease from injury

• Injury = acute

• Disease = chronic

Broad classification of Primary Causal Agents

• Biotic (infectious): living organisms (pathogens) that have the ability to infect plants and cause disease

• Abiotic (non-infectious): disorders induced by non-living causes such as environmental conditions (85% of our lab samples are identified as abiotic disorders)

Infectious Disease Agents (Pathogens)

• FUNGI: cause the great majority of plant diseases; thousands have been reported

• BACTERIA: six genera are known to cause plant diseases

• VIRUSES: Submicroscopic obligate parasites

• NEMATODES: small non-segmented worms

Fungi

• Most common of plant pathogens

• Attack all plant parts

• About 16,000 fungal plant pathogens

• Unit of infection = spore or mycelium

• Multi-celled

• Visible – microscopic

• E.g., cedar-apple rust, black spot of rose

• Spread by wind, insects, water, tools

Fungi

FUNGI grow apically by linear extension of their thread-like bodies from the tips. A single strand of this fungal body is termed a hypha (plural hyphae), and the hyphae taken all together are called a mycelium. Nuclei are found in the hyphal cells.

Most fungal hyphae have crosswalls; some do not.

Multinucleate hypha

Mycelium on leaf surface

chlamydospore

Examples of types of fungal conidia (asexual spores).

These conidia are microscopic

and easily carried by air currents.

Fusarium

Aspergillus

Pencillium Fusarium

Botrytis Verticillium

macroconidia

microconidia

Spore germination and infection

• Spores require high humidity for development (>80%) • Spores require water film on the host tissue to

germinate

• Many germinated fungal spores penetrate plant tissue directly, or enter through the leaf pores (stomates) or wounds.

• Most conidia can survive from several hours to several days if conditions necessary for germination are not met.

• Resting spores with thick walls may survive for years.

• Quite small and single celled

• Spread by water, tools and seed (sometimes)

• Enter wounds or natural openings

• E.g., soft rot of vegetables, fire blight, crown gall

Bacteria

Bacteria

Crown gall of grape

Bacterial scorch of oak

Bacterial cells grow by dividing into two cells from one (called binary fission)

Bacterial

cell

Dividing

cells

Plant pathogenic bacteria do not form spores

Bacteria are spread by: • Water splash and runoff • Insects (honey bees, insect larvae, chewing insects,

etc.) • Tools (such as pruning tools, trowels) • Contaminated flats, pots, or soils (in greenhouses

etc.) • In contaminated or infected seed • By cuttings taken from infected plants, or in infected

planting stock (tubers, bulbs, etc). • Not spread by wind (exception of wind-driven rain

drops)

How Bacteria Spread

How bacteria gain entry into plants

Bacteria cannot penetrate plant tissue directly

Bacteria usually enter plant tissue by means of:

• Wounds

• Natural openings on plant leaves or stems: lenticels, hydathodes, etc; Water saturation aids entry through natural openings.

• Insects or insect larvae are common vectors of bacterial pathogens.

• Bacteria may “piggy back” and gain entry through

cankers (open wounds) caused by fungi.

Viruses

• Very small, need electron microscope to see

• Spread by insect, seed, contact (handling, propagation), nematodes, root grafts

Viruses = nucleic acid (RNA or DNA) surrounded by a protein coat.

Viewed with electron microscope.

Shapes include: short rods, long flexuous rods, or polyhedrons.

Exist and multiply only in living tissue.

Transmitted from plant to plant 1. insects (mostly aphids and leafhoppers) 2. mechanically through small wounds 3. parasitic plants like dodder

Virus Diseases of Plants

SYMPTOMS CAUSED BY VIRUSES

1. Rarely kill plants 2. Some degree of stunting of entire plant 3. Symptoms usually systemic (throughout the plant) rather than localized 4. Symptoms usually appear on leaves as a. mosaic (yellow areas intermingled with normal green areas) b. ring spots (chlorotic or necrotic rings) c. line patterns (chlorotic or necrotic lines) d. lighter colored areas mingled with normal color on flowers or fruit. e. yellows (overall chlorosis) f. tumors g. pitting or flattening of stems or fruits

Viruses

http://www.oznet.ksu.edu/dp_hfrr/extensn/problems/tomswv.htm

TSWV on Tomato

Leaf roll virus of grape

TSWV on Peanut

Nematodes

• Non-segmented, round worms

• Most feed on roots; some on foliage & stems

• Usually microscopic

• E.g., root-knot nematode

Nematodes • 15,000 described species. Size ranges from 3/1000th of

an inch to 27 feet in length.

-Most are nonpathogenic, but several hundred species (10% of described species) are parasitic on living plants.

-Plant parasitic nematodes typically measure from 1/25th to 1/50th of inch in length

• Females lay eggs that hatch into small nematodes.

• Characteristically cause stunting of plants, may also cause cysts, galls, or lesions on roots.

• Spread by anything that moves soil. i.e. wind, water, tools, animals, man

Root Lesion Nematodes Endo-migratory

Cyst Nematodes Ecto-sedentary

Root Knot Nematodes Endo-sedentary

Sting Nematode Ecto-migratory

Spiral Nematode

Ring nematode

Plant Parasitic Nematodes

Stylet (spear)

Plant pathogenic nematodes have hollow stylets or spears as mouthparts with which they withdraw

their nutrients from plant cells.

Root Knot Nematode: Meloidegyne incognita

Symptoms Resulting from Nematode

Feeding

Root knot nematode;

two small, individual galls (G)

G

G

Root knot nematode

Watermelon: advanced stage, heavy infection

Plant Disease Management

Principles and General Concepts

Pine Trees (many species)

Host

Pathogen Environment

Plant Disease Triangle

Fungus: Diplodia pinea Wet Conditions; cool to moderate temperatures

Diplodia Tip Blight

Manipulate the Host Component

Host

Pathogen Environment

Host

Pathogen Environment

• Genetic resistance most powerful disease management tool

• Use native plants (fewer disease problems)

• Select ornamentals and trees adapted to Oklahoma conditions (Oklahoma Proven Varieties)

• Plant stress management – Stress predisposes plants to disease

Manipulate the Environment

• Do not over fertilize – many diseases are more severe when there is an excess of nitrogen available

• Reduce competition for water, light, nutrients

• Provide adequate air movement and light penetration

• Irrigate in morning

• Apply irrigation water to the soil

(e.g. drip irrigation)

Host

Pathogen Environment

Host

Pathogen Environment

Reduce Amount of Pathogen

Host

Pathogen Environment

Host

Pathogen Environment

• Exclusion – Preventing the introduction of a pathogen (beware of exotics!)

• Eradication – Constant observation and early diagnosis is critical

• Sanitation – Removing pathogen infested plant material from the area

• Pesticide Use

Fungicides

• Protectant/contact – remain outside the plant and only protect the tissues to which they are

applied – chemical does not move after it dries – removed by pruning, often degraded by the environment – Mancozeb (Fore, Formec)

• Systemic – chemical moves through the plant – slightly misleading – fungicides move upward – only true systemic is fosetyl-aluminum (Aliette, Flanker, etc.)

• Local systemic (penetrant)/mesotemic – Triazole and strobilurin chemistries – absorbed into tissues but have limited movement – Propiconazole (Propiconazole, Fertilome Systemic Fungicide, Banner),

trifloxystrobin (Compass)

Fungicide Resistance Management

•Fungicide resistance can be a problem if MOA rotation is not used, proper spray volume is not used, off-label rates are used, etc. Recommended Fungicide Use

Patterns

= Resistant Individuals = Sensitive Individuals

Non-recommended Fungicide Use Patterns

Accurate Diagnosis

• Submit sample to OSU Plant Disease Diagnostic Lab

• Proper identification will lead to the right management methods

• Send a good sample!

Jennifer Dominiak-Olson, Plant Pathology Diagnostician Contact:

Room: 119 NRC Phone: (405) 744-9961 FAX: (405) 744-6039 http://www.ento.okstate.edu/pddl/