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Publisher Summary The genes whose products cause a number of the defense responses in infected plants are called “Avr-genes.” This chapter explains in detail Avr-genes and their products. The suppressor molecules only prevent the manifestation of resistance by allowing pathogens to penetrate a plant and grow in it. The suppressors can be less toxic for the cells, without causing a visual damaging effect, or highly toxic, causing damage and death of plant cells. Suppressors can be present both in the tissues of infected plants and in the pathogen cells. The suppressor glucans, isolated from the parasite races compatible with particular potato cultivars, do not cause irreversible suppression of the defense responses but only their temporary delay. Any parasite toxins, including nonspecific vivotoxins, are immunosuppressors. The secondary factors that intensify pathogenicity for particular plant species or cultivars are pathotoxins. This chapter discusses pathotoxin producers and pathotoxin-caused diseases. The characteristic features of all pathotoxins investigated so far include very low toxic concentrations toward the susceptible plants and high selectivity. In terms of the chemical composition, pathotoxins are classified into cyclic peptides, linear polyketols, glycoside, etc. Specificity and toxicity mechanism have also been explained in the chapter with examples. Plant resistance to pathotoxins is mostly determined by the absence of reception. This chapter also explains the biological role of pathotoxins and ecology of the producer along with the relationship between the two fundamental properties of parasites: induction and suppression. Also included are some pathways and degradation by phytopathogens such as degradation of phenol phytoalexins and degradation of terpenoid phytoalexins.

Publisher Summary The chapter discusses the molecular basis of plant immunization. There are several major areas of practical use of plant vaccinations such as the method of protection against viral diseases, prevention of tree infection by root rot causal agents, treatment of the chestnuts infected by the fungus Criphonrectria parasitica and treatment of seeds with rhizospheric bacteria. The principal ideas of studying immunization are based on the experimental data showing that the treatment of plants with attenuated strains of phytopathogenic microorganisms can induce resistance to more pathogenic strains. The section on immunization in plants explains how the chemical protection of plants has been traditionally based on the use of pesticides toxic for pathogenic organisms and how the use of fungicides has undergone three stages in its history. The first- and second-generation fungicides are nonspecific; they possess a broad range of effects on various fungi and do not damage plants. The third-generation fungicides are systemic organic products specific for particular groups of fungi and not toxic for plants. The chapter also describes the agents affecting parasite pathogenicity and the agents influencing plant resistance. Some of the ways of enhanced plant resistance against disease induced by chemical treatment are the building of a protecting film on the leaf surface; enhancement of cell wall resistance against pectolytic enzyme attack; and activation of phenol metabolism, oxidative enzymes, and phytoalexins.

Publisher Summary The chapter discusses the fungal plant pathogens, bacterial plant pathogens, plant viruses, and viral plant diseases. Fungi are not autotrophs and they have no chloroplasts; they can only use the energy stored in organic compounds. This chapter explains the properties and characteristics of fungi along with the system of fungi and fungal plant disease. Properties of phylogenetically independent taxons including fungi and fungaceous organisms are also explained. Bacteria are the smallest cellular organisms with a prokaryotic cell structure. Some bacteria (genus Bacillus) form endospores inside the cells, which are covered with a thick wall and can withstand harsh environments without losing viability. The chapter also explains a system of phytopathogenic bacteria and bacterial plant diseases. It outlines plant viruses and viral plant diseases, the structure of plant viruses, and symptoms of viral diseases. It also sheds light on the survival and propagation of phytopathogenic viruses. The parasitic nematodes as plant pathogens and the role of the nematode discharge in a plant–nematode relationship are also described in this chapter. Investigation of the feeding processes of the cyst nematode H. schachtii showed that the secretory proteins released by amphids in intercellular space between the nematode head and cell wall fix the nematode head to the syncytium wall.

Publisher Summary The nonspecific resistance factors in the horizontal pathosystem are divided into axeny and true resistance. Axeny is the resistance that occurs before a pathogen–plant cell contact, and true resistance is manifested after the pathogen contact with the cellular protoplast. The chapter explains how anatomical–morphological resistance factors are affected by plant habitus, downy leaves, structure and arrangement of stomata, and anatomic features of internal plant tissues. It also explains the regulation of ontogenesis and damage reparation in pathogens. Many plant parasites can penetrate into tissues only through wounds, and the rate of suberization of wound surfaces correlates with plant resistance. It also explains the nutrient value of infected organs and tissues. The chemical factors available to the plants for their own defense against diseases are diverse in chemical composition, conditions of production, and mechanisms of toxicity. This chapter discusses the chemical classification of phytoanticipins, which includes phenols, terpenoids, proteins, and amino acids. Plants possess not only chemically diverse defense compounds but also various strategies of their application. Protein inhibitors of viruses are a special group of vegetal antibiotic substances. The antiviral activity of a particular inhibitor is usually tested by mixing it with the viral suspension, the mix is used for inoculation of the leaves of a supersusceptible host plant, and then the number of infection spots that occurs is counted. Plant treatment with ribosome inhibitory proteins (RIPs) some time after the inoculation will not prevent development of the viral infection.

Publisher Summary The chapter discusses the parasite attack factors of the horizontal pathosystem. Different groups of parasites have both similar and different stages of pathogenesis and, therefore, similar and different weapons of attacking the host, such as germination on a plant surface, release of vivotoxins, development of infection structures, and so on. The only enzymes possessed by viruses are those necessary for their reproduction; therefore, they penetrate into plants in a passive way. This chapter discusses biosynthetic processes during fungal differentiation and penetration. It also explains mode of fungal attachment and mode of pathogen penetration into plant tissues as well as cell wall and cuticle degradation by pathogen-specific enzymatic machinery and penetration through the cell wall. The suppression of plant defenses has also been discussed. Regulation of the synthesis of the low-molecular secondary metabolites possessing vivotoxin properties are exemplified here by the trichothecene toxins of fusarial fungi. The significance of toxins in pathogenesis has been shown through mutants deficient in the ability of toxin production. This chapter also provides an explanation into the changes in metabolism of infected cells and tissues. Fungi and bacteria usually spread in the plant locally, in a short distance from the site of infection, through the intercellular spaces. The movement of viruses in the plant poses some particular problems, which have also been described in the chapter.