Xylem vessels

Fungi that cause vascular wilts invade the xylem vessels of plants . Some fungi, such as those causing scab and black spot produce mycelium that grows only in the area between the cuticle and the epidermis (subcuticular colonization); others, such as those causing powdery mildews, produce mycelium only on the surface of the plant but send haustoria into the epidermal cells. Xylem vessels Subcuticular colonization

Surface of the plant

Nematodes

Bacteria invade tissues intercellulary, although when parts of the cell walls dissolve, bacteria also grow intracellularly. Bacteria causing vascular wilts, like the vascular wilt fungi, invade the xylem vessels. Nematodes

Most nematodes invade tissues intercellularly, but some can invade intracellularly as well. Many nematodes do not invade cells or tissues at all but feed by piercing epidermal cells with their stylets. Intracellular invasion of nematodes

Viruses, viroids, mollicutes, fastidious bacteria, and protozoa invade tissues by moving from cell to cel

Viruses, viroids, mollicutes, fastidious bacteria, and protozoa invade tissues by moving from cell to cell intracellularly. Viruses and viroids invade all types of living plant cells, mollicutes and protozoa invade phloem sieve tubes and perhaps a few adjacent phloem parenchyma cells, and most fastidious bacteria invade xylem vessels and a few invade only phloem sieve tubes.

Individual fungi and parasitic higher plants generally invade and infect tissues by growing on or into th

Individual fungi and parasitic higher plants generally invade and infect tissues by growing on or into them from one initial point of inoculation. Most of these pathogens, whether inducing a small lesion, a large infected area, or a general necrosis of the plant, continue to grow and branch out within the infected host indefinitely so that the same pathogen individual spreads into more and more plant tissues until the spread of the infection is stopped or the plant is dead.

In some fungal infections, however, while younger hyphae continue to grow into new healthy tissues, older ones in the already infected areas die out and disappear so that a diseased plant may have several points where separate units of the mycelium are active. Also, fungi causing vascular wilts often invade plants by producing and releasing spores within the vessels, and as the spores are carried in the sap stream they invade vessels far away from the mycelium, germinate there, and produce a mycelium, which invades more vessels.

Bacteria, mollicutes, viruses, viroids, nematodes, and protozoa

Bacteria, mollicutes, viruses, viroids, nematodes, and protozoa

All other pathogens, namely bacteria, mollicutes, viruses, viroids, nematodes, and protozoa, do not increase much, if at all, in size with time, as their size and shape remain relatively unchanged throughout their existence. These pathogens invade and infect new tissues within the plant by reproducing at a rapid rate and increasing their numbers tremendously in the infected tissues.

The progeny may then be carried passively into new cells and tissues through plasmodesmata (viruses and viroids only), phloem (viruses, viroids, mollicutes, some fastidious bacteria, protozoa), or xylem (some bacteria); alternatively, as happens with protozoa and nematodes and somewhat with bacteria, they may move through cells on their own power.

Reproduction

Reproduction Plant pathogens reproduce in a variety of ways. Fungi reproduce by means of spores, which may be either asexual (mitospores, i.e., products of mitosis, roughly equivalent to the buds on a twig or the tubers of a potato plant), or sexual (meiospores, i.e. products of meiosis, roughly equivalent to the seeds of plants). Parasitic higher plants reproduce just like all plants, i.e., by seeds.

Bacteria and mollicutes reproduce by fission in which one mature individual splits into two equal, smaller individuals. Viruses and viroids are replicated by the cell, just as a page placed on a photocopying machine is replicated by the machine as long as the machine is operating and paper supplies last. Nematodes reproduce by means of eggs.

The great majority of plant pathogenic fungi and oomycetes produce a mycelium only within the plants they

The great majority of plant pathogenic fungi and oomycetes produce a mycelium only within the plants they infect. Relatively few fungi and oomycetes produce a mycelium on the surface of their host plants, but most powdery mildew fungi produce a mycelium only on the surface of, and none within, their hosts.

The great majority of fungi and oomycetes produce spores on, or just below, the surface of the infected area of the host, and the spores are released outward into the environment.

Plant pathogenic plasmodiophoromycetes, however, such as the clubroot pathogen and fungi causing vascular

Plant pathogenic plasmodiophoromycetes, however, such as the clubroot pathogen and fungi causing vascular wilts, produce spores within the host tissues, and these spores are not released outward until the host dies and disintegrates. Parasitic higher plants produce their seeds on aerial branches, and some nematodes lay their eggs at or near the surface of the host plant.

Bacteria reproduce between or, in xylem- or phloem-inhabiting bacteria, within host cells, generally inside the host plant; they come to the host surface only through wounds, cracks, stomata, and so on. Viruses, viroids, mollicutes, protozoa, and fastidious bacteria reproduce only inside cells and apparently do not reach or exist on the surface of the host plant.