Role of Pesticides in Produce Production, Preservation, Quality, and Safety 361
naturally occurring volatile terpene in the apple fruit that can be oxidized to a variety
of products (conjugated trienes). These oxidation products cause injury to the cell
membranes, eventually leading to cell death in the outermost cell layers of the fruit.
11.2.6 BACTERICIDES
Numerous bacterial pathogens can cause serious plant diseases; however, their chem-
ical control by bactericides is rather limited and the main effort concentrates on their
prevention. This consists, among other methods, of killing pests that serve as their
vectors or facilitate bacterial colonization by providing entry points. Thus, many
other pesticide groups play an important, indirect role in the control of bacterial
infections. Moreover, some fungicides (e.g., copper compounds or dodine) also
possess bactericidal properties.
One of the basic roles of bactericides in produce production lies in soil steril-
ization as a control of soil-borne diseases using compounds such as 8-hydroxyquin-
oline sulfate and the pre- or postharvest fumigants (mainly in greenhouses) methyl
bromide and formaldehyde [5]. A rather unique bactericide used for soil treatment
is nitrapyrin which controls Nitrosomonas spp., the bacteria oxidizing ammonium
ions in the soil. Thus, nitrapyrin acts as a nitrification inhibitor, preventing loss of
nitrogen in soil [70].
Isolated instances of the use of bactericides in produce production comprise
cases in which bacterial diseases would cause serious economic losses, such as death
of fruit trees or extensive damage in orchards (that is, if a chemical control is
available). Important examples include the treatment of fire blight (caused by
Erwinia amylovora) on pear and apple trees by copper compounds and the antibiotics
streptomycin and oxytetracycline, or the application of oxytetracycline (in addition
to the copper compounds or dodine) as a control of bacterial leaf spot on peaches
and nectarines caused by Xanthomonas campestris pv. pruni [4].
The postharvest protection of produce against bacterial spoilage usually involves
means other than treatment with antibiotics (see Chapter 11) because their use for
this purpose is not accepted in many countries [68]. Generally, fungi constitute more
serious spoilage microorganisms than bacteria. This is particularly true for fruits, in
which the relatively acidic conditions tend to suppress bacterial growth. On the other
hand, vegetables with a higher pH (e.g., cole vegetables) may suffer significant losses
from bacterial infection. Mechanisms of bacterial infiltration and spoilage of fruits
and vegetables (including examples of key spoilage bacteria) and related topics are
discussed elsewhere (Chapters 13 through 18).
11.2.7 HERBICIDES
Herbicides are designed to kill weeds and other plants that grow where they are not
wanted. From approximately 250,000 species of plants worldwide, about 8000
species are considered weeds, and 200 to 250 of them pose a serious concern in
agriculture [71]. In addition to chemical treatment with herbicides, basic methods
of weed control include mechanical operations (burial, cultivation, or mowing) and
crop rotation.