Horticulture Principles and Practices

2. Instead of excessive growth, the plant may be stunted (as occurs with most viral
infections) or plant organs may be only partially developed. This response is
described as underdevelopment of tissue.
3. To contain the infection, the cells around the point of attack may die so that the
organism cannot spread any further. This response of tissue death is called necrosis
(hypersensitive reaction). Diseases such as leaf spots, blights, cankers (e.g., anthracnose),
and decays (e.g., soft rot) are symptoms of diseases involving tissue death.
Dissemination
Agents of dissemination are air, water, insects, other animals, and humans. Fungal spores
are usually spread by air currents. Bacteria and nematodes are often disseminated by
water. Viral infections are transmitted by vectors. Pathogens are carried on farm tools and
equipment and on the feet or bodies of animals.
Overwintering or Oversummering
Annual plants die after a season, infection or no infection. The pathogen must therefore
be able to survive in the absence of its host. Similarly, pathogens of perennial plants must
survive during the period of dormancy while adverse weather prevails. For example,
nematodes overwinter as eggs in plant debris. Some fungi inhabit the soil.
7.7.2 DEFENSE AGAINST PATHOGENS BY PLANTS
Plants have structural and biochemical means of combating an invasion by pathogens.
Structural and biochemical defense mechanisms involve the use of physical barriers or
the secretion of chemicals to block entry of pathogens into plant hosts. Plants differ in
the types of mechanisms they possess.
Structural Defense
Preexisting Defense Mechanisms When a pathogen lands on the surface of the plant
and attempts to penetrate the tissue, the plant’s first means of defense is its epidermis. Some
plants have deposits of wax in the epidermal cells or a thick cuticle over the epidermal layer,
either of which is impenetrable to pathogens. Some leaves have a waxy, water-repellent surface
or a pubescent surface that makes it difficult for water to settle to provide the moisture
certain pathogens such as fungi and bacteria require to grow or multiply. The presence of
such a tough material severely limits direct penetration of pathogens, thus reducing infection
because the remaining avenues for entry are the natural openings (stomata).
Induced Defense Structures If the pathogen gains entrance into the tissue, the
presence of the alien matter may induce the formation of a cork layer for containment of
the invasion. In some cases, especially in stone fruits, an abscission layer may be induced
to form in active leaves. This layer causes a permanent break between the site of infection
and the rest of the leaf, consequently forestalling the advancement of the infection.
In some plants, an injury (e.g., resulting from penetration) induces gums to be formed
around the point of infection, thus limiting its spread. Another induced structure is tissue
necrosis, a hypersensitive reaction to invasion. A cell that is penetrated by the hyphae
of fungi, for example, soon experiences degeneration of the nucleus, followed ultimately
by death. Death of tissue means the fungus has no access to nutrients, leading to the
eventual death of an obligate parasite.
Biochemical Defense
Plants under attack by pathogens may secrete certain metabolites to ward off attack.
Preexisting Mechanisms Certain plants under normal conditions exude chemicals
onto the plant surface (stem, root, or leaves). Tomato, for example, is known to release
236 Chapter 7 Biological Enemies of Horticultural Plants