PLP 120 Exam I

plant disease

injurious, causing harm by disrupting normal physiological processes

plant diseases are a result of ___

persistent or semi-persistent irritation, generally progressive

plant diseases are caused by ___

biotic (infectious) agents or abiotic factors that reduce the economic or aesthetic value

plant disease results in ___

abnormality in structure or function of a plant

seprophyte

acquires nutrients from dead organic material; decomposer

parasite

acquires nutrients from a living organism (lives in or on a living organism)

facultative xyz

they can be xyz but they don’t prefer to be

biotrophic

completes entire life cycle on a living host, lives and multiples only on another living organism, obligate parasite

necrotrophic

pathogen kills host cells using toxins or enzymes and obtains energy from them, facultative parasite/facultate saphrophyte

hemibiotrophic

first obtains nutrients from living cells in biotrophic phase then switches to necrotrophic phase and obtains nutrients from dead plant tissue, facultative saphrophyte

what are the three things in the plant disease triangle and what do they mean

host, pathogen, and environment; plant disease results when these three components interact

infection court

a site in or on a host plant where infection can occur ie a natural opening (stomata), wound, direct penetration

describe the steps of a basic disease cycle

1. dispersal of primary inoculum- movement from plant to plant
2. deposition
3. penetration
4. infection- pathogen establishes relationship where it obtains nutrients from host plant
5. colonization- pathogen spreads throughout host tissue
6. reproduction- pathogen produces new infectious units
7. survival- overwintering, persistence when preferred host/tissue is absent

which pathogen caused late blight/irish potato famine

Phytophthora infestans

Which pathogen caused coffee rust and where

Hemileia vastatrix, Sri Lanka

basic facts about bacteria

prokaryotes

single cellular

small 0.6 to 3.5 micrometers

lack well defined organelles

chromosomal DNA not organized into a nucleus

chromosomal DNA is closed circular

often contain plasmids

majority of known plant pathogenic bacteria are _____ __ (their preference for nutrient absorption)

facultative saprophytes

T/F majority of plant pathogenic bacteria are easy to culture on artificial media

true

possible advantageous traits carried on bacterial plasmid (extra chromosomal DNA)

antibiotic resistance, virulence factors, genes for production of a certain metabolic enzyme, ie copper resistance in Pseudomonas syringae pv. tomato

what is the shape of most bacterial plant pathogens?

rod (bacilliform)

monotrichous polar, lophotrichous, amphitrichous, peritrichous

most plant pathogenic bacteria are __ (what type of cell wall?)

gram-negative: thin layer of peptidoglycan surrounded by periplasmic space and plasma membrane, stains pink with counterstain Safranin

briefly describe the method through which bacteria reproduce and one reason why this method of reproduction might be beneficial to a plant pathogenic lifestyle?

binary fission:

asexual reproduction

can divide as rapidly as every 20 minutes

produces two identical daughter cells

strongly influenced by their environment

allows pathogen to colonize very quickly and then spread to other plants

methods through which genetic variation are introduced into bacterial populations

mutations and selection

bacteria are haploid so mutations always get expressed

3 types of lateral/horizontal gene transfer

conjugation- transfer from donor to recipient through cell-cell contact (pilus=bridge), often between 2 related species

transformation- bacteria takes up foreign DNA from the environment ie dead bacteria, random

transduction- bacteriophage (bacteria-infecting virus) picks up bacterial DNA while replicating in 1 bacterium and transfers it to a second

phylum containing important plant pathogenic bacteria

proteobacteria (gram -)

actinobacteria (gram +)

tenericutes (no cell wall)

which 2 bacterial species are gram +

Clavibacter spp.- potato ring rot, bacterial canker of tomatoes and wilting maize, wheat and alfalfa

Streptomyces spp.- scabs in potato and other root crops

Erwinia spp., Dickeya spp., Pectobacterium spp.

rods, peritrichous flagella, gram -, facultative anaerobes, wilts/blights/soft rots

Pseudomonas spp.

rods, polar flagella, gram -, some fluoresce under UV light (because they secrete ciderophores that chelate iron and then glow under UV), leaf spots/galls/cankers

Xanthomonas spp.

rods, single polar flagellum, gram -, slimy, yellow colonies on media, mostly cause leaf spots and some rots/wilts

Agrobacterium spp.

rods, few circumthecal flagella, gram -, soil and rhizosphere inhabitants, can survive for long periods of time without plant debris, cause growth abnormalities (galls, root proliferation)

Clavibacter spp.

rods, nomotile (no flagella), gram +, cankers/wilts/ring rot

Streptomyces spp.

filalmentous, gram +, small/dry/granular colonies on media, spore-forming, soil inhabitant (so crop rotation doesn’t work well as management technique), scab diseases

pathovar vs race

pathovar- differentiates bacteria that belong to the same species but infect different host plant species

race- differentiates bacteria that belong to the same species and pathovar but infect different plant cultivars (genotypes)

epiphyte

bacteria living non-parasitically on plant surface

phyllosphere

above-ground parts of plant that serve as habitat for microorganisms; including leaves, flowers, buds, fruit

rhizosphere

the microenvironment in the soil immediately surrounding the roots; soil directly influenced by root exudates

quorum sensing

bacteria detect and react to self-generated signal molecules to organize behavior in response to pop size (activate or repress expression of target genes)

bacteria secrete signal molecules which activates genes in close by bacteria (genes to secrete more EPS, turn on virulence factors, produce toxins, etc)

biofilm

a polysaccharide matrix in which bacteria are embedded that can assist in attachment, colonization, host invasion, survival

epiphytic phase of Erwinia amylovora

E. amylovora = Fire blight

bacteria build up on the stigma then invade through nectar thodes when bacteria reach high enough numbers, important phase in pathogenesis

dormant bacteria can overwinter in “holdover cankers”

epiphytic phase of Pseudoomonas syringae pv. tomato

bacteria build up on the surface of the leaf

management: avoid overhead irrigation, making the leaf surface a bad environment for them to spread to

what is one management strategy that does NOT work for managing rhizosphere inhabitants

crop rotation bc bacteria will survive in soil for long periods of time

Agrobacterium tumefaciens

crown gall (primarily a problem on perennial crops ie trees)

wide host range

soil inhabitant that survives for a looong time in the soil

can survive in soil even near plants that are NOT its host

Streptomyces scabies

common scab of tomatoes

gram + and filamentous

really good soil inhabitant

T/F: majority of pathogenic bacteria are NOT soil inhabitants

true

one strategy to attempt to clear out soil inhabitants and why, why is it also sometimes bad

turning the soil under, clearing out plant debris where the bacteria can overwinter, but it causes problems with carbon cycle/erosion

places where pathogenic plant bacteria can overwinter

in association with plant tissue (cankers, plant debris, seeds, propogative material)

guts of overwintering insects ie cucumber beetle (Erwinia tracheiphilia, cucumber wilt) and corn flea beetle (pantoea stewartii (Stewart’s wilt of corn)

ways that inoculum is spread from plant to plant and field to field

rain splashes, run-off, irritation/flooding, insects, contaminated seeds, infected transplants, animals, boots, tractors, pruning shears, knives

ways that pathogen can enter host

stoma, wound, hydathode, nectarthode

obligate insect transmission and examples

depend solely on insects for transmission to plants, they propagate inside the insect, tissue specialists, reduced genomes, depend on hemipterans (needles), infect plant and insect vector hosts

Xylella (gram -)

Candidatus Liberibacter (gram -)

Candidatus Phytoplasma

Spiroplasma

what does candidatus mean

never been cultured on artificial media = obligate parasite

genome degradation

in tissue specialists

the process by which a genome shrinks relative to its ancestor, lost biosynthesis pathways for substances which can be obtained from host

hemipteran vectors

insects with a needle-like piercing and sucking mouthpart (stylet)

Xylella fastidiosa

gram -

tissue specialist in the xylem

rippled cell wall

Pierce’s disease of grapevines and Bacterial leaf scorch

transmitted by sharpshooter

Candidatus Liberibacter asiaticus, africanus, americanus

Huanglongbing/citrus greening

transmitted by phloem-feeding psyllid

tissue specialist in the phloem

gram -

Candidatus phytoplasma

no cell wall

phloem tissue specialist

infect plant and insect hosts

transmitted by leafhoppers, psyllids, planthoppers, grafting

symptoms: witches’ broom, phyllody, virescence, yellowing/reddening of leaves/stems, stunting

diseases like Pear decline and Aster yellows

spiroplasma

mollicute (no cell wall?) with spiral morphology

transmitted by leafhoppers

culturable

phloem tissue specialists

diseases: citrus stubborn disease, corn stunt

general characteristics of fungi

eukaryotes with membrane-bound nucleus and organelles

haploid nuclei dominant phase

have cell walls with chitin and B-glucans

lack of chlorophyll and vascular system

store carbohydrates as glycogen

heterotrophic by absorption

6 vegetative structures of fungi

thallus- body, filamentous

hyphae- threads, mostly septated

mycelium- mass of hyphae