Horticulture Principles and Practices

FIGURE 8–4 Selected examples
of biological control. (a) A
lacewing attacking prey.
(b) Tetrastichus gallerucae attacking
elm leaf beetle eggs.
(c) Female Aphytis piercing scale
insect with ovipositor. (d) Predaceous
midge larva eating aphid.
(e) Yellow jacket attacking a
caterpillar. (f) Flower fly larva eating
aphids. (Source: Photos provided
courtesy of Oklahoma Cooperative Extension
Service, Oklahoma State University)
(a)
(b)
(c)
(d)
(e)
(f)
certain soilborne diseases such as tomato wilts. The causal organisms of such diseases
need the host plant in order to thrive and consequently cannot persist in the soil if the
host is absent for about two to three years. Rotations are particularly effective in
controlling diseases and pests whose causal organisms do not travel long distances (such
as nematodes, weevils, certain wilts, and phytophthora).
8.8.2 SANITATION
Disease-causing organisms and insects remain in the field if infected plant debris is left
on the ground. Sometimes infected plant remains have to be incinerated to kill the
pathogens. Uninfected plant remains may harbor insects and disease organisms.
8.8.3 USE OF RESISTANT CULTIVARS
Plant breeders genetically manipulate the genotypes of plants to the advantage of
humans. Through scientific inquiry, some of the protective strategies of plants in the wild
have been discovered and studied. Some species resist certain diseases and pests because
they have genes that condition such characteristics. Through breeding, scientists are able
258 Chapter 8 Principles and Methods of Disease and Pest Control