468 Produce Degradation: Reaction Pathways and their Prevention
enzymatic degradation by capable phytopathogens is a weakness. Other forms of
defense may consist of constitutively produced biochemicals, or responses that are
activated upon pathogen attack, such as hypersensitivity responses and induction of
phytoalexins and/or pathogenesis-related proteins (PRP). Induced resistance
responses can, in some cases, persist and act as a form of “immunization” of the
plant against subsequent pathogen attack in the field and/or in storage (Tuzun and
Kloepper, 1995; Niemira et al., 1996). Disease-resistant varieties have been bred to
possess systemic and/or inducible defenses against a particular pathogen. This vari-
etal resistance may be of the broad/horizontal type (somewhat resistant against all
races/genotypes of a particular pathogen), or it may be narrow/vertical (completely
resistant to certain races/genotypes of the pathogen, completely susceptible to oth-
ers). Unfortunately, the biochemical details of disease resistance are often only
poorly understood, and the ability of one plant organ (e.g., leaves) to resist pathogen
attack does not necessarily correlate with the disease resistance of another
(e.g., tubers) (Niemira et al., 1999).
FIGURE 15.1The plant pathology “disease triangle.” Disease occurs when a compatible
pathogen and host are together under the proper environmental conditions.