B. Penetration through wounds

B. Penetration through wounds All bacteria, most fungi, some viruses, and all viroids can enter plants through various types of wounds. Some viruses and all mollicutes, fastidious vascular bacteria, and protozoa enter plants through wounds made by their vectors.

The wounds utilized by bacteria and fungi may be fresh or old and may consist of lacerated or killed tissue. Laceration or death of tissues may be the result of environmental factors such as wind breakage and hail; animal feeding, e.g., by insects and large animals; cultural practices of humans, such as pruning, transplanting, and harvesting; self-inflicted injuries, such as leaf scars; and, finally, wounds or lesions caused by other pathogens. Bacteria and fungi penetrating through wounds germinate or multiply in the wound sap or in a film of rain or dew water present on the wound. Subsequently, the pathogen invades adjacent plant cells or it secretes enzymes and toxins that kill and macerate the nearby cells. These pathogens may grow briefly on such tissue before they advance into healthy tissue.

Penetration through wounds

Penetration through wounds

a. Stomata

a. Stomata Stomata are most numerous on the lower side of leaves. They measure about 10–20 by 5–8mm and are open in the daytime but are more or less closed at night. Bacteria present in a film of water over a stoma and, if water soaking occurs, can swim through the stoma easily and into the substomatal cavity where they can multiply and start infection. C. Penetration through natural openings

Fungal spores generally germinate on the plant surface, and the germ tube may then grow through the stoma

Fungal spores generally germinate on the plant surface, and the germ tube may then grow through the stoma . Frequently, however, the germ tube forms an appressorium that fits tightly over the stoma, and usually one fine hypha grows from it into the stoma. In the substomatal cavity the hypha enlarges, and from it grow one or several small hyphae that actually invade the cells of the host plant directly or by means of haustoria. Although some fungi can apparently penetrate even closed stomata, others penetrate stomata only while they are open. Certain fungi, e.g., the powdery mildew fungi, may grow over open stomata without entering them.

b. Hydathodes

b. Hydathodes

Hydathodes are more or less permanently open pores at the margins and tips of leaves; they are connected to the veins and secrete droplets of liquid, called guttation drops, containing various nutrients. Some bacteria use these pores as a means of entry into leaves, but few fungi seem to enter plants through hydathodes. Some bacteria also enter blossoms through the nectarthodes or nectaries, which are similar to hydathodes.

c. Lenticels

c. Lenticels Lenticels are openings on fruits, stems, and tubers that are filled with loosely connected cells that allow the passage of air. During the growing season, lenticels are open, but even so, relatively few fungi and bacteria penetrate tissues through them, growing and advancing mostly between the cells.

Most pathogens that penetrate through lenticels can also enter through wounds, with lenticel penetration being apparently a less efficient, secondary pathway.

Methods of penetration and invasion by fungi.

Methods of penetration and invasion by fungi.

3. Establishment of infection

3. Establishment of infection

Infection is the process by which pathogens establish contact with susceptible cells or tissues of the host and procure nutrients from them. Successful infections result in the appearance of symptoms, i.e., discolored, malformed, or necrotic areas on the host plant. Some infections, however, remain latent, i.e., they do not produce symptoms right away but at a later time when the environmental conditions or the stage of maturity of the plant become more favorable.

The time interval between inoculation and the appearance of disease symptoms is called the incubation period. The length of the incubation period of various diseases varies with the particular pathogen–host combination, with the stage of development of the host, and with the temperature in the environment of the infected plant.

In most plant diseases, however, symptoms appear from a few days to a few weeks after inoculation.

Establishment of infection in a compatible reaction between a pathogen and its host plant.

Establishment of infection in a compatible reaction between a pathogen and its host plant.

4. Colonization (invasion), growth and reproduction

4. Colonization (invasion), growth and reproduction The spread of a pathogen into the host is called invasion. Various pathogens invade hosts in different ways and to different extents . Most fungi spread into all the tissues of the plant organs (leaves, stems, and roots) they infect, either by growing directly through the cells as an intracellular mycelium or by growing between the cells as an intercellular mycelium.

Intracellular colonization Intercellular colonization