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

immature females in the white or yellow stage erupt through the
root epidermis. Yield losses vary according to nematode
densities, and complete economic crop failure can result when
densities are high. Potato cyst nematodes may increasc
incidence of Verticillium wilt and bacterial wilt (brown rot).
and mate with the females. Fertilized females increase in size to
become subspherical. Mature females measure between 0.5 and
0.8 mm in length and greatly vary in size, probably due to the
type of host and amount of nutrition during their development.
Eggs are produced and retained within the female. The female
Causal Organisms
G. rostochiensis and G. pal/ida become round cysts upon
maturity. Cysts are light to dark brown with an irregular pattern
of subsurface punctuations over most of the body.
Cysts of G. rostochiensis differ from those of G. pallida by a
greater average anal-vulval distance, 60 compared with 44 pm,
and a greater average number of cuticular ridges between anus
and vulva, 21.6 compared with 12.2. G. rostochif'nsis females
develop through a golden yellow phase before turning brown.
hence tihe common name. golden nematode: G. pallida females
are white or cream before turning brown (Fig. 99A, Plates 81
and 82). I.engths of larval body, stylet, and tail are usually
longer in G.palla than in G.rostocliensis.
Races are differentiated by ability to multiply on resistant
cultivars of S. tuberosum ssp. antligena, S. mulidissectui, S.
verm'i, or S.kurtzianum.
Disease Cycle
In the spring, over 50('i of the second stage larvae inside eggs
within a cyst are stimulated to hatch. They enter the host plant
roots, feed, and develop through a series of three molts. The
females enlarge and rupture the root tissue but remain attached
to the root by their heads and protruding necks, which stay
inserted in root tissue. Mature wormlike males leave the root
cuticle darkens and hardens, becoming the cyst, which may
contain as many as 500 eggs (Figs. 9913 and I00A). Cysts remain
in the soil when the crop is harvested.
M ultiplication rate and sex ratio are influenced by population
density of the nematode and host crop. An ample food supply
favors a multiplication rate up to 60-fold. When food is limited
and the population islarge ( 100 eggs pergram of soil) nematode
density may decline.
Histopathology
Glohodera spp. are stimulated to hatch by exudates from
plant roots. Second stage larvae usually enter the root hair zone.
As larvae move through cortex cells of potato roots, feeding
may cause some limited necrosis in susceptible cultivars.
The female feeds near the vascular cylinder, resulting in
multinucleate units called syncytia (giant cells) near the
nematode's head. Syncytia are formed by incorporation of
adjacent cells following cell-wall dissolution, which begins in the
cortex. Cell walls ofa column of cells toward the vascular tissue
are then dissolved. In the vascular cylinder, syncytia are limited
by lignified xylem, so incorporation of new cells proceeds
parallel to the root axis (Fig. 10013).
Syncytia may be formed in the cortex, endodermis. pericycle,
and parenchyma of the central vascular strand. Cytoplasm of
syncytia becomes dense and granular in structure.
A
Fig. 99. Potato cyst nematode (Globodera rostochiensis). A,immature swollen females attached to potato roots. B, diagrammatic life
cycle of potato cyst nematodes: 1,cyst showing enclosed eggs; 2, enlarged egg showing enclosed, coiled larva; 3, larvae entering root; 4
and 5, swollen females feeding in root; 6, mature female breaking through root surface. (Courtesy W. F. Mai, B. B. Brodie, and M.B.
Harrison)
B
.
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