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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.