Horticultural Plant Disease

Chapter 4 Occurrence and Developmentment of Plant Disease Section 1 Pathogenicity of Plant Pathogens and Resistance of Host Plants

Section one- Parasitism of plant pathogen 1.Interrelation of plant and microbe 1.1 Mutualistic symbiosis （共生）  Two organisms can not live separately. One is beneficial to the other, they keep alive by living together.

1.2 Commensalism( 共栖，或偏利共生） Two organisms can live separately. When they live together, one can acquire more benefits, while another benefits less or none. 1.3 Parasitism （寄生） One organism lives on another organism, requiring another one to afford the nutrients. The organism which afford nutrients names as host; and another one that acquires nutrients is called parasite.

2. Parasitism and pathogenicity 2.1 Parasitism  The ability which pathogen gains the nutrients from alive host plant. 2.2 Pathogenicity  The competence that pathogen destroys the host plant and brings forth the disease.

Nourishing manner: Biotrophy( 活体营养 ）  The parasites obtain the nutrients from alive host, and do not kill the host cells and tissues immediately. These parasites are called obligate parasites. Necrotrophy （死体营养）  The parasites firstly kill the host cells and tissues, and then acquire the nutrients from the dead cells and tissues. These parasites are celled necrotrophic parasites.

3. Pathogenic mechanism of plant pathogen  Pathogenic microbes not only snatch the nutrients and water from host plant, but also inflict the mechanical pressure on host and produce the harmful metabolic products to affect host normal physiological activities. For example, enzyme, toxin, growth regulating substances produced by pathogens can induce a series of pathological changes to host plants.

3.1 Snatching the nutrients and water from host plant  Various pathogens have parasitism, and can gain the necessary living substances from host.  Parasites deplete （消耗） the nutrients of host plant, causing malnutrition, yellowing, stunting, even necrosis.  Hemiparasites mainly despoil( 掠夺 ) the water from host, causing mild harm to host.  Holoparasites bring forth tremendous damage to host, causing yellowing and necrosis promptly.

3.2 Mechanical pressure  Firstly hypha of fungi (germ tube) and radicel( 胚根 ) of higher parasitic plants touch the host plant and attach to the surface of plant; in succession, the top of the hypha swells to form appressorium, and the appressorium produces infection pegs to penetrate cuticle( 角质层 ) and cell wall, or excretes( 分泌 ) the enzymes to decompose cuticle and cell wall.

 Nematode penetrates the plant cuticular cell wall by stylet and the head or the whole body enters plant cell.  Some pathogenic fungi form the fruiting body( 子实体） under plant cuticle, and impose the strong mechanical pressure on plant cuticle, causing the cuticle to expand, plump up and rupture, eventually resulting in the fruiting body basseting.

3.3 Enzyme  The pathogen produces many enzymes related to pathogenicity, mainly including cutinolytic enzyme( 角质水解酶 ), cytolytic enzyme( 细胞降解 酶 ), protease, amylase( 淀粉酶 ), lipase( 脂肪酶 ), etc.  Many pathogenic fungi can produce a series of enzymes to catabolize the cuticle and cell wall.

3.4 Toxin  Toxin is a kind of poisonous compound which produces in metabolic process. It can disturb the normal physiological function of host plant in lower concentration.  Toxin is a kind of pathogenic substance with high efficiency. In lower concentration, it can induce the host to produce disease symptom.

3.5 Growth regulating substances  Many pathogens can the analogs of plant growth regulating substances to disturb the normal physiological activity of plant, inducing plants to produce many pathological changes in conformation, e.g. excessive growth( 徒长 ), stunting, malformation, defoliation( 落叶 ), inhibiting apical dominance( 顶端抑制 ), root top inactivity( 根尖钝化 ), etc.  Pathogen also can affect the normal function of plant growth regulating system to bring forth pathological changes.

Section 2 Plant disease resistance 1.Conception and types of plant disease resistance Plant disease resistance---The traits which the host plant avoid, cease, or counteract the invasion and expansion of pathogen to alleviate disease outbreak and loss degree.

Differentiation according to race specialization  Race specific resistance: host disease resistance only aims at several specific races; host varieties have specific interactions with pathogen races.  Race non-specific resistance: the plant disease resistance controls by minor genes, aiming at the whole group of pathogen; the host varieties have no specific interaction with pathogen races.

Differentiation according to resistance mechanism of host plant:  Passive resistance: Before touching with pathogen, the host plant has already had some characters to resist the invasion of pathogen.  Active resistance: Pathogen invasion induces the host to generate defense reaction.

 According to the character of resistant factors, the resistant factors can be divided into morphological and structural resistance, i.e. physical defense; physiological and biochemical resistant factors, i.e. chemical defense. 2. Plant resistance mechanism

2.1 Passive resistant factors  Inherent morphological and structural characters of host plant. The host resists the invasion and expansion in virtue of mechanical tenacity and stability to the enzyme produced by pathogen  Most plants have chemical passive resistance. Resistant plants may contain natural antimicrobial substances or inhibitors to enzyme produced by pathogen. Another possibility is that the resistant plants lack the required contents which pathogen parasitizes in host or causes disease.

2.2 Physical active resistant factors  Invasion of pathogen induces the metabolic changes of plant, causing morphological and structural changes of plant in subcellular, cellular, or textural level. Physical resistant factors can limit invasion of pathogen in cell wall, single cell, or part tissues.  Invasion and damage of pathogen can bring forth lignification( 木质化 ) and suberization( 木 栓化 ) of plant cell wall, and deposition of phenol and calcium ion.

2.3 Chemical active resistant factors  Chemical active resistant factors include hypersensitive necrotic reaction, phytoalexin( 植物保卫素 ) synthesis, toxin decomposition Hypersensitive necrotic reaction  The host plant is highly sensitive to invasion of non-compatible pathogen. The infected cells and adjacent cells become necrotic rapidly; the pathogens are restrained, killed or limited in the necrotic tissues.

2.3.2 Phytoalexin  A kind of low molecular weight, antimicrobial, secondary metabolites which produce or accumulate after infected by pathogen or stimulated by physiological and physical factors.  Phytoalexin is the induced product. Besides fungi, bacteria, virus, nematode, metallic ion, sodium azide( 叠氮 化钠 ), cycloheximide( 放线菌酮 ), mechanical stimulation also can activate the plant to produce phytoalexin. Lately the cell wall components of fungi, e.g. dextran( 葡聚糖 ), glycoprotein( 糖蛋白 ), or cell wall segments of hypha are discovered to have the activated effect.

3.Plant Induced resistance and resistant mechanism  Induced resistance--- acquired resistance, advanced inoculation or disposal by chemical or physical factors can bring the disease resistance to plant.  Cross-protection--- host plant firstly inoculates the weak pathogenic strains, and then inoculates the strong pathogenic strains. The host can resist the strong strains and alleviate the symptom.