PLANT PATHOLOGY - EPWS 310

Prokaryote Diseases of Plants

Contents

Readings

General Introduction

Kinds of Prokaryotes

Bacterial diseases - facultative saprophytes

General Characteristics

Major Genera

Specific examples of diseases caused by prokaryotes

Agrobacterium tumefaciens - Crown Gall

Clavibacter - Ring rot of potato

Fire Blight of Pome Fruits

Soft rot of numerous fleshy fruits- E. carotovora pv. carotovora

Southern bacterial wilt of solanaceous plants

Pseudomonas canker disease

Xanthomonas - Examples to know

X. campestris pv. Malvacearum - Angular leaf spot of cotton

Streptomyces - S. scabies - causes common potato scab

Xylella - Fastidious vascular bacteria

Mollicutes

 

Prokaryote Diseases of Plants

Readings:  Chapter 12

Lecture 1

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General Introduction

Prokaryotes - single celled microorganisms that have a cell membrane and a cell wall surrounding the cytoplasm. The latter contains only small (70s) ribosomes (eukaryotes have 80s and 70s ribosomes) and genetic material (DNA) not bound by a membrane, that is, not organized into a nucleus.

Kinds of Prokaryotes

I. Bacteria - cell membrane and cell wall

II. Mollicutes (previously known as mycoplasma-like organisms [MLO]). Lack a cell wall and have only a typical single unit membrane.

B. Bacterial diseases - facultative saprophytes

I. General characteristics-

1. Introduction

*1600 species known. Most are saprophytic decomposers.

* 80 species cause diseases in plants in 7 major taxa. Most are facultative saprophytes and can be grown on artificial media, although the fastidious vascular bacteria are difficult to grow in culture.

Agrobacterium - galls

Erwinia - soft rots and blights

Pseudomonas - leaf spots and blights

Ralstonia - wilts

Xanthomonas - leaf spots and blights

Xylella - wilts

Coryneform bacteria - Clavibacter, Arthrobacter and others

Streptomyces - Scab diseases

2. Morphology-

a. Almost all-phytopathogenic bacteria are rod-shaped except Streptomyces

b. Most are gram negative. Except coryneform bacteria and Streptomyces

b. Most have a polysaccharide slime layer or capsule surrounding them.

c. Most have delicate flagella.

*Polar flagella

*Peritrichous - over the entire surface.

3. Reproduction-

No sexual reproduction but conjugation does occur readily.

Asexual reproduction by binary fission.

Occurs by the inward growth of the cytoplasmic membrane toward the center forming a transverse membranous partition dividing the cytoplasm into two approximately equal parts.

Bacteria may divide every 20 minutes under favorable conditions. One million cells in 10 hours.

4. Ecology and Spread-

Plant pathogenic bacteria can develop in the host as a parasite, in plant debris or in the soil as saprophytes.

a. In host - e.g. Erwinia amylovora - fire blight of pome fruits. Plant-to-plant infection cycle.

b. Soil inhabitants - Agrobacterium tumefaciens - crown gall

Ralstonia solanacearum - bacterial wilt of solanaceous crops.

Streptomyces scabies - common scab of potato.

c. Soil invaders - survive as long as the debri in the soil. Most of the plant pathogenic genera. E.g. Xanthomonas and many Pseudomonas species.

Dissemination - water, insects, animals, humans.

B. II. Major Genera

1. Agrobacterium rod shaped, motile, gram negative, soil inhabitants - crown gall

2. Coryneform bacteria rod shaped to slightly curved, non-motile, gram positive, soil invader - wilts

3. Erwinia - rod shaped, motile (peritrichous), gram negative, facultative anaerobes, soil invader - two groups.

*Amylovora - blight or wilt diseases.

*Carotovora-soft rots, pectolytic activity.

4.Pseudomonas - rod shaped, motile, gram negative, soil invaders, blights, galls, leaf spots

*Fluorescent - e.g. P. syringae

5. Ralstonia rod shaped, motile, gram negative, soil inhabitants, wilts

6. Xanthomonas - Usually yellow on media, slow growing, and all plant pathogens - rod shaped, motile, gram negative, leaf spots and blights -

*All species are plant pathogens and are found only in association with plants or plant material.

7. Streptomyces - Slender branched hyphae without cross walls. At maturity the aerial mycelium forms chains of three to many spores. Gram positive, scab diseases.

8. Xylella - Single straight rods, non-motile, gram negative, aerobic, nutritionally fastidious, habitat is xylem of plant tissue, wilt diseases.

B. III. Specific examples of diseases caused by prokaryotes -

1. Agrobacterium tumefaciens - Crown gall. Note lab experiment. Pages 662 - 666

* Disease cycle Figure 12-36.

* Economic on pome, stone fruits and grapes

* First "genetic engineer"

* Model system for cancer studies in humans led to extensive knowledge although the mechanisms are quite different and these studies are no longer common.

* The Ti plasmid transforms the plant genomic DNA to produce special carbon compounds known as Opines, which are metabolized by the bacterium and cannot be used by another soil organisms.

* Survives in soil as soil inhabitant and colonizes rhizosphere soil competitively

* Used as a means to transform crop plants!

Control

* Biological control using Agrocin - 84 producing Agrobacterium radiobacter - 84

* Certification programs

* Avoid wounding

Lecture 2

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2. Clavibacter- Ring rot of potato Clavibacter michiganense subsp. sepedonicum. P. pages 649 - 651

* Dissemination - contaminated seed, Knives

* Entry- wounds, colonize xylem vessels

* Symptoms wilting late in the season - yellowing of leaves-interveinal area in tubers - light yellow vascular discoloration - bacterial ooze, rot develops -continuous ring of cavities.

* Overwinters- infected tubers

-Dried slime on machines, crates, sacks

-Do not overwinter in soil

*Control - healthy seed, sanitation, disinfects knives.

3. Fire Blight of Pome fruits- Erwinia amylovora, Figure 12-24; pages 641 - 647

Fire Blight - most destructive disease of pome fruits (mainly pear) in the world.

* Kills flowers and twigs during flowering and fruiting - colonizes nectary

* Dissemination by bees and other insects

* Entry into flower via nectary, leaves and stems via stomata and hydathodes and wounds

* Symptoms blight of flowers, stems and twigs

* Overwinters - on host in and around "cankers" on living wood under 1 cm diameter

* Control - removes all 1 cm twigs and cankers to remove overwintering inocula; reduce unnecessary prolific growth of the host i.e. reduce N applications etc

Chemical control to protect flowers e.g. Bordeaux Mixture and Streptomycin. Applied using computer model. Biological control using bees!

Lecture 3

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4.Soft rot of numerous fleshy fruits- E. carotovora pv. carotovora Figure 12-34; Pg. 656 - 662

5.Southern bacterial wilt of solanaceous plants (Moko disease of Banana) caused by Ralstonia solanacearum. Non-fluorescent. Pg. 647 - 651

*Location-present in tropics and warmer climates.

*Hosts-Banana, tobacco, tomato, potato, eggplant. Can also attack peanuts, soybeans, and plantains. Known as Granville wilt of tobacco or a brown rot of potato.

*Symptoms-Sudden wilt. Plants die rapidly

-Development of adventitious roots

-Cross sections of stem are black and ooze a whitish bacterial exudate

-Bacterial pockets are present around vascular bundles in the pith and cortex.

*Overwinters - diseased plants, plant debris

-Vegetative propagative organs

-Infected knives

*Entry - wounds, natural openings

*Control-resistant varieties

-Rotation

-Sanitation

-Diseased plants should be cut up and burned.

6. Pseudomonas canker disease - P. syringae pv. syringae - causes the bacterial canker of stone fruit and pome fruit trees. Fig 12-40; Pg. 667 - 671.

*Fungal cankers are often sunken and soft. Bacterial cankers often appear as splits in the stem, necrotic areas within the woody cylinder, or as scabby outgrowths on the surface of the tissue.

Common name- Bacterial canker and gummosis of stone fruit trees.

*Occurs all over the world. It also affects pear, citrus, lilac, rose, ornamentals, some vegetables and some small grains.

*Disease is known as bud blast, blossom blast, dieback, spur blight, and twig blight.

*Losses can be from 10-75% in a young orchard.

*Symptoms-forms cankers accompanied by gum exudation. Infection develops at the base of an infected spur and spreads upward and to a lesser extent down and to the sides. Cortical tissues are brown to bright orange. First noticed in late winter or early spring. As the tree brads dormancy gum is produced by the tree and breaks through the bark.

*The pathogen action- Produces a phytotoxin - syringomycin. The bacteria of many P. syringae strains are ice-nucleation-active, that is, they serve as nuclei for ice formation, and therefore cause frost injury to plants, at relatively high freezing temperatures. These same strains produce bacteriocins toxic against non-ice-nucleation active strains, thus assuring a competitive advantage for themselves.

*Development of disease- note overhead-

-Overwinters in active cankers, infected buds, and leaves, epiphytically (existing on the surface of a plant or plant organ without causing infection) on buds and limbs, and even on weeds and non-susceptible hosts.

-Infection takes place in fall or winter entering through pruning cuts. Bacteria move intercellularly and advance into the bark and into the medullar rays of the phloem and xylem. Cankers develop in the fall after dormancy sets in. At the end of cold weather the cankers develop quickly.

-Host response- Callus tissue. The ability to wall infection seems to be correlated with varietal resistance but is also affected by the age and succulence of the plant, the temperature and rainfall during a season, and the type of root sock on which the tree is growing.

-Control- use healthy budwood

-Graft to resistant root stock and graft high

-Chemical control copper and streptomycin.

7. Xanthomonas- Examples to know: X. axonopodis (formerly X. campestris pv. citri) - Citrus Canker Pg. 671 - 673

*It came to the U.S. from Japan. It hit Florida in 1910 and spread to the Gulf States and beyond. It was eradicated from Florida in 10 years. After the destruction of 1/4 million bearing trees, 3 million nursery trees to effectively eradicate it from Florida. It took 30 years to totally eliminate it from the U.S. by 1949. It showed up again in Florida in 1984. Destruction of 17 million nursery and young orchard trees by 1985 occurred before it was eradicated.

8. X. campestris pv. malvacearum- Angular leaf spot of cotton. Pages 630-633.

Local interest:

Symptoms - causes angular to irregular black spots on leaves and cotton bolls. - During hot humid weather the bacteria may invade and rot the bolls and cause them to rot, drop or to become distorted.

Lecture 4

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9. Streptomyces - S. scabies- causes common potato scab. Note disease cycle Figure 12 - 43; Pg. 674 - 675.

Soil inhabitant - very common

- Disease severity increased with pH and relatively dry conditions.

- Not favored by acid soil and wet conditions

Control

*Seed certification

* PCNB dusting of tubers

* Sulfur application to control pH and irrigation during tuber formation.

* Crop rotation

10. Xylella - Fastidious vascular bacteria previously known as rickettsialike organisms or RLO. Phloem limited and xylem limited. These are parasitic bacteria that cannot easily be grown on simple culture media. Nearly all are gram negative, although some important Phloem inhabiting prokaryotes are gram-positive. Page 678 - 682.

* Vectored by leaf hoppers and spittle-bugs

No effective control - resistance available

C. Mollicutes

About 200 diseases caused by these guys....still into the unknown

True mycoplasmas occur as vertebrate parasites but the plant pathogens cannot be distinguished on morphology. We await molecular studies.

Phytoplasmas: They resemble true mycoplasmas. They are pleomorphic (different forms or shapes).

Spiroplasmas are morphologically unique being helical mollicutes causing 6-7 diseases

ALL:

* Prokaryotes without a cell wall

* Usually vectored by leafhoppers

* Grow inside insect vector

* Vector behaves like persistent virus vector

Aster Yellows: Page 691 - 694

* General chlorosis, malformation, sterility, witches broom

* Worldwide caused by a classic phytoplasma, big deal on carrots!

* Leafhopper vector

* Phloem limited

* Overwinters in weed hosts

Control like viruses control weeds and resistance (limited)

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