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Principales of Plant Protection

Practicalpart (Sec3)

Plant Diseases Caused by Viruses and

Prokaryotes

Mohamed


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Plant viruses

Like all other viruses, plant viruses areobligate intracellular parasites that do

not have the molecular machinery toreplicate without a host.

Viruses are extremely small and canonly be observed with an electronmicroscope. The structure of a virus is

given by its coat of proteins, whichsurround the viral enome.


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Viruses also cause manyimportant plant diseases andare responsible for hugelosses in crop production and

quality in all parts of theworld. Infected plants mayshow a range of symptomsdepending on the diseasebut often there is leaf

yellowing (either of theYellow mosaic symptoms onsquash caused by Squash mosaicYellow vein-banding symptomson grapevine caused by


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Bacteria

The plasma membrane (in Gram-positivebacteria) and both membranes in Gram-negative bacteria are phospholipid bilayers

prokaryotic (nomembrane-enclosednucleus)

no mitochondria orchloroplastsa single chromosome,closed circle of double-

stranded DNAIf flagella are present, theyare made of a singlefilament of the proteinflagellin.


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Caused by:Agrobacteriumtumefasciens

SYMPTOMS: Rough abnormal

galls develop on roots,crowns, and occasionally on

aerial parts of stone fruittrees.

Young trees become stuntedand older trees oftendevelop secondary woodrots.


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DISEASE CYCLE

The bacteria survive in gall tissue, in soil, or in apparentlyhealthy root of host plants.

The bacterium may persist in field soil for at least one year,or considerably longer if large amounts of infected root

residues remain in soil after removal of trees. The pathogen penetrates only through wounds. The most

susceptible wounds on stone fruits are the crown end rootsand rarely on trunks and limbs.

The bacteria attach to the wounded host cells and thentransfer T-DNA into the cell.

Infections are favored by moist , alkaline, poorly drainedsoils and can be stimulated by the feeding of plant parasiticnematodes.

At temperatures above 20C galls become obvious in 2-4


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DISEASE MANAGEMENT

Proper sanitation and cultural practices:the use of certified disease-freetransplants, careful handling to avoidinjury as much as possible during planting

and life of trees in the orchard, andplanting in well drained soils.

Biological control, using a non pathogenic

strain, A. radiobacter K84, that producesthe antibiotic agrocin 84, is effectivepreventive treatment.


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Phytoplasma is a prokaryote.

wall-less intracellular bacteria.

paleomorphic shape.

cannot be grown in vitro (absolute parasite).

multiplication by binary fission or budding.

localized in the phloem sieve tubes of infectedplants from where it is acquired by the vector forsubsequent transmission.

it invades systemically all plant organs.

Phytoplasma


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Examples

Witches Broom Disease of citrus byPhytoplasma


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Spiroplasma

Citrus Stubborndisease


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Citrus Stubborn disease

Causal agent

First mollicute of plant origin to becultured (1970).

Characterized as Spiroplasma citriin 1973.

(Charbonneau & Ghiorse,

Characteristics: Wall-less prokaryote Transmitted to plant phloem by

leafhoppers Motility and helicity:

cytoskeleton

(Yokomi et al.,

GEOGRAPHICAL DISTRIBUTION f it t bb di


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Europe France (Corsicaonly) GreeceItaly (few records;

Sardinia, Sicily)SpainTurkey

AfricaAlgeria EgyptLibyaMorocco

Tunisia

CyprusIranIraqIsraelJordanLebanon

PakistanSaudiArabia

SyriaTurkeyYemen

Asia

NorthAmerica

Mexico

USA (Arizona,California, Illinois,Maryland).

Argentina

(Tucumn)Brazil (So Paulo)PeruSurinameVenezuela.

SouthAmerica:(Unconfirmed)

Oceania

New Zealand(isolated

GEOGRAPHICAL DISTRIBUTION of citrus stubborn diseaseBased on EPPO Data Sheets


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Develop best andshow symptoms

under hotconditions

S. citri multiplies in its vector, thatbecome infective 10-20 days after

acquisition

Acquiring S.citri from

other hosts

Infectphloem

sieve tubes


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Tree symptoms:

Fruit symptoms:

Suppressed fruitingLopsided, Acorn shaped.Styler end green.aborted seeds

Stunted growthUp-right foliationShorter, chlorotic leaves.

short internodes

Symptoms

C lt i


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S

Culturing


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Biologicalindexing


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S l i l


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Commercialkit (Sediag

Italy)Coating (IgG)

Sample plant extract

Conjugate (AP-IgG)

Substrate(P-nitrophenylphosphate)

PhotometricalMeasurment at 405

nm

Washing &Incubation

Washing &Incubation

washing

(Clark and Adams, 1977)

Serological assayELISA


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Management

Certification

Production of

healthybudwood.

Surveynurseries

andyoungorchards

Rougingand

replacingsymptomatic trees

Vectorcontrol

Trapplants

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