Role of Pesticides in Produce Production, Preservation, Quality, and Safety 353
Methyl bromide, chloropicrin, and 1,3-dichloropropene are volatile alkyl halides
that also attack nucleophilic sites (–OH, –SH, or –NH 2 groups) in important enzymes.
Due to the environmental concerns associated with the application of methyl bromide
(depletion of ozone), this fumigant is currently scheduled to be banned in the U.S.
by the year 2005, and its present use is under restriction [47].
Essentially, insects and nematodes have similar nervous systems, and thus many
insecticides, such as organophosphates and carbamates, can also act as nematicides.
Organophosphorus insecticides, though, tend to rapidly degrade in soil, and therefore
only the systemic ones (present in the plant roots), such as dimethoate or phorate,
are of some practical use [1]. An example of a specially developed and widely
applied organophoshorus nematicide is fenamiphos, which is a systemic compound
absorbed by roots and is sufficiently persistent in soil. From the carbamate group,
aldicarb, oxamyl, and carbofuran are the three most frequently used systemic car-
bamates for nematode control in the U.S. [4]. A recently introduced toxin produced
by fermentation of the fungus Myrothecium verrucaria has been also used on several
fruits and vegetables (e.g., grapes) to effectively control both juvenile and adult
nematodes and also egg/cyst hatching [4,5].
In the U.S., the most widespread and economically important nematode species
attacking all sorts of fruit and vegetable plants include the root-knot (Meloidogyne
spp.), root lesion (Pratylenchus spp.), dagger (Xiphinema americanum), stubby root
(Paratrucgidirys spp. and Trichodorus spp.), ring (Criconemella xenoplax), and sting
(Belonolaimus longicaudatus) nematodes [4].
Presumably the most damaging nematode infestation is that of potatoes [48].
Root-knot nematodes are the major nematode pests on potatoes; they are found in
abundance, especially in sandy soils. Females feeding in the tubers and the devel-
opment of live young cause enlargement or bumps in the outer layers of the tubers,
rendering them useless for either fresh or processing markets. Specific symptoms
caused by root-knot nematodes include swellings on the roots called “galls,” which
may contain one to several adult root-knot females [49]. These nematodes cause
field damage that is localized, usually in patches of various sizes, or that may be
spread throughout an entire field, and plants become chlorotic and stunted. Damaged
roots are not able to obtain soil nutrients; above ground, symptoms appear as nitrogen
or micronutrient deficiencies. Plants may wilt easily, especially in warm weather,
due to root damage, even though soil moisture may be adequate.
Other nematode pests on potatoes are the ectoparasitic root lesion and stubby
root nematodes [49]. The root lesion nematode reduces yield indirectly by weakening
the plants and increasing their stress, which causes the potato plants to be more
susceptible to fungal and bacterial diseases. Stubby root nematodes also do not cause
much direct damage, but they transmit the tobacco rattle virus. This virus causes a
disease of potato tubers called corky ringspot, resulting in rusty brown, irregularly
shaped lesions that have a corky texture.
11.2.4 MOLLUSCICIDES
Molluscicides control snails and slugs, both members of the mollusk phylum and
similar in structure and biology, except that snails have external spiral shells. The