Compendium of Potato Diseases - (PDF, 101 mb) - USAID

infested areas of North America severity of the problem ias
declined.
Selected References
ANI)ERSSON, S. 1967. Investigationson the occurrenceand behavior
of Ditvlenchus destructor in Sweder. Nematologica 13:406-A 16.
FAU.KNER, L. R.. and H. M. DARIJNG. 1961. Pathological
histology, hosts, and culture of the potato rot nematode.
Phytopathology 51:778-786.
SMART, G.C.. Jr, and H. N1.)ARi.ING. 1963. Pathogenic variation
and nutritional requirements (f Dityilenchus destructor.
Phytopathoiogy 53:374-381.
THORNE, (. 1961. Pages 138-148 in: G. Thorne, ed. Principles of
Nematology. McGraw-Hill. Inc., New York. 553 pp.
Stubby-Root Nematodes
Stubby-root nematodes have a very wide host range in the
temperate regions, and they transmit virus in many different
types of plants.
Symptoms
No diagnostic aboveground symptoms exist except stunting.
Roots cease elongation, resulting in numerous stunted "stubby
roots" which show little or no necrosis, discoloration, or other
injury symptoms.
Causal Organism
Paratrichordorus pachyvderm us, P.christiei, and Trichodorus
primiivusattack potatoes.
Disease Cycle
Eggs are deposite! in the soil. Immature and adult forms
migrate through the soil and feed superficially on roots without
becoming embedded in the plant tissues. When soil temperature
is 15-20' C. the life cycle is completed in about 45 days. These
nematodes most frequently occur in light, sandy soils, although
they have also been reported in other soil types.
Histopathology
Feeding activity of the stubby-root nematode occurs
principally at root tips. Epidermal and outermost cortical cells
are punctured: as feeding proceeds, the protoplast shrinks from
the cell wall. After 5-10 sec, the nematode moves on to another
cell. Feeding activity is followed by a loss of meristematic
activity. Parasitized roots lack a root cap and a region of
elongation. Differentiation of protoxylem elements occurs
almost at the root apex. Apparently, therefore, new cell
production is halted but differentiation of existing cells
continues.
Epdemiology
In the Netherlands, nine species of stubby-root nematodes
transmit tobacco rattle virus. A close relationship exists
between populations of Trichodorus and the virus is')Iates.
Tobacco rattle virus is probably not readily spread for long
Aphids
distances by virus-infected plant material because the strain of
the virus would probably not be suitable to the nematode
population of the new location. Activity of stubby-root
nematodes, as determined by virus spread, is affected by soil
moisture, type, and temperature. Greatest activity occurs in
sandy soil at 15' C with 16.7% moisture; as soil moisture
decreases, activity decreases. Very little more is known about
the influence of the environment on these nematodes.
Control
1)Soil fumiga!nts have been used to control Trichodorusspp.
and thereby reduce spread of tobacco rattle virus.
2) Not :nough is known about the host range of the nematode
and the virus, advise rotation as a control measure.
Selected References
HEWITT, W.B.,D.J. RASKI, and A.C.GOHEEN. 1958. Nematode
vector of soil-borne fanleaf virus of grapevines. Phytopathology
48:586-595.
RASKI, D.J.,and W. B.HEWIT. 1963. Plant-parasitic nematodesas
vectors of plant viruses. Phytopathology 53:39-47.
ROHDE, R.A.,and W. R.JENKINS. 1957. Host range of a species of
Trichodorus and its host-parasite relationships on tomato.
Phytopathology 47:295-298.
VAN HOOF. H. A. 1968. Transmission of tobacco rattle virus by
Trichodorusspecies. Nematologica 14:20-24.
Nematicides
Control of nematodes in soil can be achieved through use of
nematicides, of which only a limited number are presently
available. All should be considered potentially hazardous, and
some are difficult to apply. They must be applied correctly and
under suitable environmental conditions in order to obtain their
full nematicidal potential.
Dispersion through the soil and activity of most soil-applied
nematicides is enhanced when soil tilth, moisture, and temperature
are in the proper range. Dosage level and technique of
application will depend on the nematicide, soil type, rate of
control desired, and economic considerations. Nematicides may
be gases, liquids, or granular solids. A well-qualified, experienced
person should be consulted before applications of
nematicide are attempted.
Some nematicides are fumigants, which volatilize in soil and
become gases that move through soil. Others are nonfumigants,
which depend upon external forces such as soil water for
movement. Another category of nematicides consists of those
that move systemically in the plant and can be applied to foliage.
Most currently available ner.aticides are either halogenated
hydrocarbons, organic phosphates, or carbamate compounds.
Some are phytotoxic. All are toxic to humans. Therefore,
caution must be exercised in their use. Label directio,!s must be
carefully read and strictly followed.
(Prepared by W. F. Mai, B. B. Brodie, M. B. Harrison,
and P. Jatala)
Several potato virus diseases are transmitted by aphids, and Others, although seldom establishing colonies on potatoes, are
identification of the vector involved is often necessary. Those vectors of some nonpersistently transmitted potato viruses
that commonly colonize potato (Table I1)can be easily (Table i11).M)'zuspersicaeisthemost efficient aphid vector and
identified by morphological characteristics (Figs. 105 and 106) is found worldwide.
that are visible to the naked eye or visible when magnified by a Aphid species differ not only in morphology and ability to
hand lens (Table IV). These aphids may transmit both transmit potato viruses but also in form (morph), life cycle, and
circulative (persistent) and stylttborne (nonpersistent) viruses, behavior, depending on the environment (temperature, relative
101