Horticulture Principles and Practices

7.7.4 APPARENT RESISTANCE
Certain plants known to be susceptible may not be adversely affected by pathogens
under certain conditions. Two kinds of apparent resistance are known—escape and
tolerance.
Escape
Plants may not show symptoms of disease because one of the three critical ingredients
in the disease triangle may be missing (e.g., required temperature may not be present or
moisture may be inadequate). In some plants, the young tissue may be susceptible to
pathogens and an older one may not. For example, powdery mildew (caused by
Phytium), bacterial, and viral infections affect younger tissue more severely than older
tissue. By manipulating the cultural environment (e.g., spacing, planting date, pH, fertility,
and moisture), susceptible plants may escape diseases.
Tolerance
Plants are said to be tolerant when they are able to grow, develop, and be fairly productive
even when infected with a pathogen. Most plant viral infections produce this kind of
resistance, in which the pathogen (virus) does not kill the host but causes reduced productivity
or performance.
Virulence
The degree of
pathogenicity of a
pathogen.
7.7.5 GENETIC BASIS OF DISEASE INCIDENCE
As previously discussed, a virulent pathogen, a susceptible host, and an appropriate
environment are all required for disease to occur. The virulence of a pathogen and the
resistance of a host have genetic bases. For each gene that confers resistance in the host,
there is a corresponding gene in the pathogen that confers virulence to the pathogen and
vice versa, called the gene-for-gene concept of genetics of disease resistance and susceptibility.
Generally (exceptions occur) in the host (e.g., plants), the genes for resistance
are dominant (R) and the genes for susceptibility or lack of resistance are
recessive (r). However, in the pathogen, the genes for avirulence (inability to infect) are
usually dominant (A) and those for virulence are recessive (a). Therefore, when two
cultivars, one carrying the gene R (for resistance to a certain pathogen) and the other
carrying the gene r (for susceptibility) are inoculated with two races of the pathogen of
interest, one of which carries the gene A (for avirulence against R) and the other race
carrying the gene r (for virulence against R), the progeny will have the genotypes summarized
in Table 7–4.
TABLE 7–4
The Genetics of Disease Resistance in Plants
Resistance or Susceptibility Genes in the Plant
Virulent or Avirulent
Genes in the Pathogen R (resistant) dominant r (susceptible) recessive
A (avirulent) dominant AR () Ar (+)
a (virulent) recessive aR (+) ar (+)
Where is incompatible (resistant) reaction (no infection) and + is compatible (susceptible) reaction (infection develops).
AR is resistant because the plant (host) has a certain gene for resistance (R) against which the pathogen has no specific virulence (A)
gene. This does not mean other virulence genes do not occur.
Ar is susceptible due to lack of genes for resistance in the host and hence susceptible to other virulence genes from the pathogen. aR
host has the resistance gene, but the pathogen has a virulence gene that can attack it.
ar is susceptible because the plant is susceptible and the pathogen is virulent.
238 Chapter 7 Biological Enemies of Horticultural Plants