Horticulture Principles and Practices

(a)
(b)
FIGURE 7–15 (a) Slug and (b) snail. (Source: Photos provided courtesy of Oklahoma Cooperative Extension Service, Oklahoma State University)
Pathogenicity
The capability of a
pathogen to cause disease.
7.7.1 DISEASE FACTORS
234 Chapter 7 Biological Enemies of Horticultural Plants
7.7 HOW DISEASE OCCURS
Every organism has a life cycle during which it reproduces, develops, grows, and dies. Disease
conditions involve the interaction between the life cycles of the host and the pathogen. To
understand the nature of disease, it is important to distinguish between parasitism and -
pathogenicity. An organism that lives in or on another for the purpose of deriving food is
called a parasite, and the relationship between the two individuals (parasite and host) is called
parasitism. A parasite deprives its host of food, making the latter less vigorous and less
productive. This one-sided relationship may not go beyond depriving the host of nutrients.
In certain cases, the relationship is mutually beneficial (symbiotic), as occurs in the root
nodules of legumes where Rhizobia reside. The bacteria derive nutrients from the legumes
and in return fix atmospheric nitrogen into usable form for the plant. Sometimes the activities
of the parasites coupled with the reaction of the host result in abnormal physiological
activities in the host and physical degeneration of cells and tissue. This condition describes
the state of pathogenicity, and the organism associated with it is called a pathogen.
Pathogens differ in the plant types, parts, tissues, and organs they can successfully
attack and grow on. Obligate parasites tend to be host specific (limited to one species),
whereas nonobligate parasites can produce disease symptoms on a variety of plants. The
variety of plants a pathogen can grow on is called its host range. For disease to develop,
three ingredients must be present: (1) a pathogen (causal agent), (2) a susceptible host
(plant on which the pathogen can grow), and (3) a favorable environment. This grouping
constitutes the disease triangle (Figure 7–16). Disease will not occur if one of these components
is not present. Each component can occur to a variable extent, which affects the
degree of disease development. For example, the pathogen may be present in small or
large numbers, be a highly virulent or less virulent race, and be in active or dormant stage.
The host may be genetically resistant to the pathogen, or it may be too young or too old.
The environment may be too dry or too wet, too cold or too warm, or very favorable.
Disease development occurs in stages. The chain of events leading to the development
and perpetuation of the disease is called the disease cycle. The primary events in a
disease cycle include inoculation, penetration, infection, dissemination, and overwintering
or oversummering.
Inoculation
The coming into contact of host and pathogen is called inoculation. The pathogen (or any
part of it) that can initiate infection is called the inoculum. Examples are spores and sclerotia
(in fungi) or the whole organism in bacteria, viruses, and mycoplasmas. Since an