Transcript 幻灯片 1

Sention 2
Infection Process
and
Disease Cycle
Section 1 Infection Process
 Pathogen contacts the infective part of plant,
and invades the host plant, reproduce, spread
in the plant to cause disease and appear the
disease symptom. Infection process also can be
considered that host plant suffer invasion of
pathogen and result in disease.
 In general, the infection process can be
divided into four stages: contact phase;
penetration phase; incubation phase;
symptom appearance phase.
1. Contact phase
1.1 The concept of contact phase
A period of time that pathogen contacts
host plant and arrive the rhizosphere(根围)
and phyllosphere(叶围) of host plant, then
move or develop to the infective part of
plant forming the invading structure.
1.2 The activities in contact phase
Secretions from plant roots or leaves attract the
pathogen to accumulate around the roots or leaves;
Secretions also stimulate or induce the resting body
germination of pathogenic fungi, bacteria, and nematode
in the soil, which benefits to the pathogen forming
invading structure and causing further invasion.
 In the contact phase, the pathogen keep up growing
and reproducing.
 Saprobiotic microbes in the rhizosphere and
phyllosphere can produce antimicrobial substances to
retrain and kill the pathogens; some saprobes and
avirulent strains of pathogen prevent the invasion by
occupying the infective spots of pathogen.
2. Penetration phase
2.1 The concept of penetration
A period of time from pathogen invading the host
to establishing the parasitic relationship.
2.2 Invading approaches of pathogen
Invasion by directly penetrating
The pathogen directly penetrate the cuticle and
cell wall of host plant. Besides nematodes and
parasitic seed plants, some fungi also can directly
penetrate, e.g. Erysiphe(白粉菌属), Colletotrichum
(刺盘孢属), venturia (黑星菌属), etc.
Natural openings invasion
Natural openings include stoma(气孔),
hydathode(水孔), lenticel (皮孔), stigma(柱
头) ,nectar(蜜腺), etc. many fungi and
bacteria can invade from natural openings.
Wound invasion
Pathogen can invade the host from various
wounds dispersed on the surface of plant.
Virus only can invade from tiny wound.
2.3 Invading time and amount of inoculum
 Pathogen invasion only needs short time.
Virus and bacteria invade as well as contact.
Fungi invade after germination and forming
germ tube, so need long time to accomplish
invasion, commonly less than 24 hours.
 A number of pathogen inocula are
required to accomplish invasion and cause
disease. The least inocula is called infection
dosage.
2.4 The relationship of invasion and
environment
The moisture decides the germination
and invasion of spores.
Temperature affects the rate of
germination and invasion.
Light determines the opening and closing
of stoma. So the light affects the pathogens
which invade from stoma.
3. Incubation phase
A period of time from establishing
parasitic relationship to appearing
obvious symptom.
The incubation period differs in
different plant disease. In general,
the incubation period is 10 days.
In the environmental factors, the
temperature gives more influence to
incubation period.
4. Symptom appearance phase
 Symptom appearance phase is the period
that pathogen enlarges damage and produces
more propagules (繁殖体).
Most fungi produce the spores in later period
of plant disease or in the dead tissues. Sexual
spores produce later. Some times sexual spores
produce or maturate after dormancy stage.
The sporulation of pathogen has intimate
relationship with temperature and moisture.
The new spores turn into the reinfection
source of plant disease.
Section 2 Disease Cycle
1. The concept of disease cycle
A process that plant disease occurs from
preceding growing season to next growing season,
including pathogen overwintering or
oversummering; inoculum release and
transmission; pathogenesis ; primary infection and
reinfection. Disease cycle also calls infection cycle.
2. Primary infection and reinfection
2.1 Primary infection
Pathogen passing through Overwintering or
oversummering causes the first infection in the new
generation plant.
 Primary infection effect on the initial infection
during the plant growing season.
 Most of the Systemic infection disease only have
primary infection.
 The diseases only having primary infection can
be controlled by preventing the primary infection, e.g.
head smuts can be controlled by seed treatment.
2.2 Reinfection
 The plant after suffering the first infection can
produce spores and propagules. The spores and
propagules can be transmitted to cause next infection.
 Many plant diseases may bring forth more
reinfections during a growing season.
Short incubation phase diseases can cause more
reinfections , and result in disease epidemic.
The diseases which have reinfection not only need to
control the primary infection but also the reinfection.
Effeciency of disease management have great difference.
3. Overwintering and oversummering of pathogen
3.1 The ways of overwintering and oversummering

Saprophyte: pathogen lives in the affected plant
debris or in soil.

Dormancy: pathogen survives in or out of the
plant by resting body.

Parasitism: some pathogen can overwinter or
oversummer in the affected plant by hypha, or in plant
debris or soil by resting oospore(卵孢子).
3.2 The places of overwintering and oversummering
3.2.1 The diseased plants in the field
 Various pathogens adopt different approaches to
oversummer or overwinter by living in the affected plants.
 The pathogen of Cucumber downy mildew diffuses and
damages by successive infection in the open field during
summer and in the protective field during winter.
 The pathogen of Chinese cabbage soft rot oversummers in
the turnip (芜菁) and overwinters in the stored Chinese cabbage.
 The field diseased plants include other crops, wild host,
alternate host(转主寄主).
3.2.2 Seeds, seedlings, and other propagating
materials
The resting body of pathogen can be mixed with
seeds, or attach on the surface of seed. The pathogen
can invade and lurk(潜伏) inside the seeds, seedlings
and other propagating materials by hypha.
Affected seedlings or propagating materials will
be the most effective primary infection source for
next year.
3.2.3 Soil
Soil is the main places of the pathogen to oversummer
or overwinter outside the plant.
The microbes in the soil can divide into soil
inhabitants(土壤习居菌) and soil invaders(土壤寄居菌).
Soil inhabitants have strong adaptability to soil, can
live longer in the soil alone, also can reproduce on the
soil organic substances. For example, Pythium(腐霉属)，
Rhizoctonia(丝核菌属)， and some fungi of Fusarium(镰
孢菌属)are representatives of soil inhabitants.
The soil invaders can live longer in the diseased debris
of plant , but can not live alone in the soil for a long
time. Most plant pathogenic fungi and bacteria pertain
to the soil invaders.
3.2.4 Diseased plant debris
 Most non-obligate parasitic fungi and bacteria can
live on the diseased plant deris, or live by
metatrophy(腐生营养).
 Pathogen can live longer on the diseased plant
debris because the pathogens fall under the protection
of plant tissues, having strong resistance to unsuitable
environmental factors, especially suffering less
antagonism(拮抗作用) of soil helpful microbes.
 Germination of resting spores and sporulation of
pathogen on affected plant debris have close
relationship with environmental conditions, especially
temperature and moisture.
3.2.5 Manure
The pathogen can interfuse the manure with
the affected plant debris. Resting body of
pathogen also can fall into the manure. If the
manure is not thoroughly decomposed, the
pathogen inoculum can live for a long time to
cause infection.
Some pathogens passing through the
enteron(消化道) of animals also have infectious
capability, e.g. smut pathogen(黑粉病菌).
4. Pathogen dispersal
4.1 Pathogen dispersal mainly depends
on exoteric factors, including natural
and factitious factors.
Natural factors are wind, rain, water,
insects, and other animals.
The important factitious factors are
transportation of affected seeds or
seedlings, agricultural activities,
agricultural equipments.
4.2 Various pathogens have different
approaches of transmission
Fungi are transmitted by air current.
Bacteria are transmitted by rain and
insects.
Viruses are transmitted by vectors.
Parasitic seed plants are transmitted
by birds and air current.
Nematode are spread by soil,
irrigation, water current.
4.3 The ways of pathogen dispersal
4.3.1 Air current transmission
 Air current transmission is the common way
of transmission.
 Fungi spores are tiny, light and abundant in
number. So fungi spores are suitable for air
current transmission. The bacteria and nematode
in soil also can be transmitted by wind .
 Air current transmission is long-distance
transmission. Many foreign pathogens are
transmitted by air current.
4.3.2 Rainwater transmission
 Plant pathogenic bacteria and
conidiophore(分生孢子) of melanconiales(黑盘
孢目)， Sphaeropsidales(球壳孢目) are
transmitted by rainwater. Zoospore of
mastigomycotina ( 鞭毛菌亚门)are also spread
by rainwater.
 Rainwater transmission is short-distance
transmission, only for tens of meters.
 Diseases transmitted by rainwater can be
controlled by destroying the local pathogens
or preventing the infection of pathogen.
 Irrigation water also can diffuse the
pathogens. So flooding must be avoided.
4.3.3 Biologic vectors
 Insects, acarids ( 螨 )， some nematodes are the
main transmitting vectors of plant pathogenic viruses,
especially insects and acarids have close relationship
with the transmission of viruses.
 Birds not only can carry the seeds of Ramulus
taxilli( 桑寄生 )and Herba visici( 槲寄生 )， but also
can disseminate Erwinia amylovora ( 梨火疫病菌 ).
4.3.4 Soil and manure transmission
 Soil and manure may be carried to other
places causing the transmission of pathogen. Soil
can transmit the pathogens which overwinter or
oversummer in the soil.Tuber and seedlings
adhering soil can transmit pathogen for a long
distance. Agricultural equipments , shoes can
disseminate pathogens for a short distance.
 Pathogens interfuse into the manure , if the
manure is not thoroughly decomposed, the
pathogen inoculum can live for a long time in the
manure, diffusing by manure.
4.3.5 Factitious factors dispersal
 In the factitious factors, allocating and
transporting the affected seeds, seedlings and other
propagating materials are most important. Products
and wrappers also can disperse the pathogen.
 Factitious transmission can disseminate the
pathogen for a long distance. Natural conditions and
geographical conditions can not limit the factitious
transmission.
 Plant quarantine can limit the factitious
transmission of pathogen, avoiding the pathogen from
affected area to disease-free area.