Title: Lecture 23 – Crop Diseases and

Life Cycles of Pathogens

Speaker: Teresa Koenig

Created by: Teresa Koenig, Kim Kidwell

online.wsu.edu

Crop Diseases and Life Cycles of Pathogens

http://en.wikipedia.org/wiki/Plant_Virus http://en.wikipedia.org/wiki/Plant_disease

A. Plant Disease: Plant disorder resulting from a disease-causing factor.

1. Diseased Plant: an unhealthy condition of the plant from continuous irritation by a pathogen. 2. Pathogen: causal organism 3. Symptoms: the signs of plant disease.

Spring Wheat Infected with Stripe Rust

Photo courtesy of USDA ARS

B. Beneficial Effects of Pathogens:

1. Aid in decomposition of organic matter and nutrient recycling. 2. Symbiotic relationships with plants.

Ex. Rhizobium and legumes

C. Detrimental Effects of Pathogens on Crop Plants: Pathogens rob plants of water, nutrients and photosynthates.

1. Limits plant growth and development

a. Chlorosis: destroys chlorophyll which reduces rate of Ps b. Necrosis: death of plant tissue c. Block vascular system: disrupts translocation

d. Interfere with photosynthesis and respiration: decreases plant growth e. Affects water use efficiency: also decreases plant growth

2. Reduces plant quality:

a. Cause premature ripening of the grain or fruit. Grain does not fill completely, which reduces plumpness. b. Disease evidence on leaves, flowers, or fruit reduces overall attractiveness and quality.

c. Replace grain/fruit with fungal bodies Examples: Ergot and smut on wheat: fungus replaces grain. Has toxic effects on mammals.

Ergot on wheat spikes

d. Seed-borne diseases are transmitted to the next crop.

Examples:

Pea seed-born mosaic viruses; wheat smut

3. Indirect harvest losses:

Many diseases cause lodging, ear and fruit dropping or leaf loss.

D. Pathogens causing infectious diseases:

1. Fungi 2. Bacteria 3. Viruses 4. Nematodes

1. Fungi:

Responsible for the greatest number of crop losses.

Examples: Rusts, smuts, stalk rots, downy mildews

a. Reproduce via spores that are carried by the wind, crop residue and seed.

Powdery mildew on grapes

b. Commonly reproduce sexually so they readily adapt to or overcome genetic resistance in cultivars.

2. Bacteria:

Enter plant through natural openings like stomata or wounds.

Bacterial leaf spot on

strawberry

Photo courtesy of USDA ARS

a. Single celled organisms that reproduce by binary fission: one cell divides into 2 smaller replicas.

b. Colonize and parasitize host tissue. c. Symptoms are soft rots, leaf spots, bacterial galls or vascular disease.

Ex: Blight and wilt of beans, alfalfa and corn; potato scab

3. Viruses: Are not considered to be living organisms in themselves a. Obligate parasites:

They can only grow and multiply within a living host.

b. Invade cells and disrupt normal cellular functions. c. Most common symptom is an alteration of the leaf chlorophyll pattern.

Photo courtesy of USDA ARS

Mosaic virus on tomato leaf

Examples:

1.Mosaic or striped pattern, leaf roll, excessive branching. 2. Barley yellow dwarf: transmitted by aphids.

4. Nematodes:

Microscopic worms (roundworms) that attach to roots, stems, leaves and flowers.

a. Most feed on roots. b. Reproduce via eggs. Parasitize plant tissue.

Photo courtesy of USDA ARS

c. Cause galls, lesions, stunted root system or injured root tips. d. Cause the plants to wilt and growth is stunted. Ex: Soybean, corn, sugarbeet cyst nematodes