352 Produce Degradation: Reaction Pathways and their Prevention
plant, which may lead to the same adverse consequences as infestation by leaf-eating
or damaging insects.
In the U.S., the important examples of mite pests on fruit trees and other crops
include the two-spotted spider mite (Tetranychus urticae), the European red mite
(Panonychus ulmi), the Pacific spider mite (T. pacificus), and the McDaniel spider
mite (T. mcdanieli) [4]. In the case of citrus fruits, the citrus rust mite (Phyllocoptruta
oleivora) and some other mites may feed not only on foliage but also on stems and
fruits, causing epidermal cells to die [46]. When rust mite injury occurs prior to fruit
maturity, the destruction of epidermal cells will generally result in smaller fruit.
Further growth leads to breaking up of the dead epidermis and a wound periderm forms
over the newly formed epidermis (“russeting”). Rust mite injury to mature fruit causes
epidermal cells to die and become a brownish-black color with no periderm formation.
This condition is known as “bronzing.” Primary effects of fruit damage caused by rust
and some other epidermis-destroying mites are a reduction in citrus fruit grade and size,
increased water loss, fruit drop, and reduced juice quality.
11.2.3 NEMATICIDES
Nematicides are pesticides intended for killing nematodes, which are microscopic,
unsegmented eelworms, most of which live in the soil and feed on plant roots. From
about 3000 known nematodes, almost one-third commonly attack crop plants [2].
Nematodes may feed on plant tissues from the inside (endoparasitic nematodes) or
from the outside (ectoparasitic nematodes), and their feeding activity may interfere
with water and nutrient uptake or stimulate physiological changes that disrupt healthy
plant functions, leading to reduced plant vigor (decrease in growth and productivity)
or even death if the infestation is enormous. Nematode-feeding on roots, tubers, and
bulbs intended for consumption (such is the case with carrots, potatoes, or onions)
translates directly to their lower quality and susceptibility to microbial spoilage.
Furthermore, nematodes may serve as vectors of serious plant diseases.
Nematodes can overwinter in a dormant state in the soil. They are not able to
travel through the soil to a large extent but are rapidly spread by running water and
contaminated equipment, seeds, transplants, sets, and bulbs. Thus, the practices for
effective nematode prevention mainly include the use of nematode-free seeds, uncon-
taminated water for irrigation, and nematicides prior to planting to eliminate their
spread by farm equipment. Preplanting treatment mainly involves soil fumigation,
which is usually done to combat a combination of problems associated with nema-
todes, weed seeds, and soil-dwelling insects and fungi.
Fumigants are vapor-active chemicals, and thus the fumigation process basically
requires an enclosed space; otherwise, the active ingredient will be lost to the
atmosphere. For soil treatment, the surface of the soil must be covered with a plastic
tarp [1]. The most widely applied soil fumigants effective as nematicides include
metam-sodium, methyl bromide, chloropicrin, and 1,3-dichloropropene [4]. Metam-
sodium is a very broad-spectrum, water-dispersible, crystalline compound that is
generally applied through irrigation systems. In soil, it slowly decomposes to the
highly toxic fumigating agent methyl isothiocyanate [5], which reacts with nucleo-
philic centers, such as thiol groups, in vital nematode or other pest enzymes [1].