Pale cyst nematode - Globodera pallida
Both species of the Potato Cyst Nematode are obligate parasites of certain members of the family Solanaceae. Potatoes (Solanum spp.) are the most important host plant followed by tomatoes (Lycopersicum esculentum) and eggplant (S. melongena). Altogether some 90 species of the genus Solanum are known to be hosts.
- Africa: Algeria, Egypt, Libya, Morocco, Sierra Leone, South Africa, Tunisia
- Asia: India, Japan, Pakistan, Philippines, Sri Lanka
- Central America and Caribbean: Costa Rica, Panama
- Europe: All countries except Turkey.
- Middle East: Lebanon
- North America: Canada (BC - G. rostochiensis, NFLD - G. rostochiensis & G. pallida, QC - G. rostochiensis), USA (NY - G. rostochiensis, ID - G. pallida) and Mexico.
- South America: Argentina, Bolivia, Brazil (Reported by EPPO refuted by Brazil), Chile, Colombia, Ecuador, Peru, Venezuela
The cyst (Figure 1), the protected resting stage of this group of nematodes, contains eggs, which are stimulated to hatch when they are in proximity to the roots of a host plant. Once hatched, the larvae will undergo three additional larval stages (the first occurred within the egg), the second stage, the infective stage (Figure 2), occurs within the soil while the third and fourth stages (maturing stages) occur within the plant root.
After entering the host root, usually at the growing point, the larvae become sedentary and sausage shaped. The females eventually becoming "sac-like" with their posteriors protruding from the root. Fully mature females can be seen as tiny white embedded objects along the host's roots. When the males are fully developed they migrate back into the soil . The free roaming males fertilize the embedded females. After copulation the males die.
Fertilized females begin to swell as the eggs develop within their bodies. When the females die their body wall gradually hardens and darkens to form a protective layer around the eggs - the cyst. G. rostochiensis females progress from white to a golden yellow colour before darkening into the cyst (Figure 3).
Females of G. pallida do not pass through the golden phase but remains white to cream colour until gradually darkening. A new cyst may contain as many as 500 eggs and persist in the soil for more than 20 years. There is a reduction in the number of viable eggs within a cyst over time. Mortality appears to be greatest under cultivation due to the higher temperatures in arable land).
Detection and identification
Host plant symptoms are not a reliable means of identification since symptoms of attack by Globodera species are not specific. Patches of poor growth may occur generally in the crop, sometimes with yellowing, wilting or death of foliage. Potato tubers may be small.
Mild infestation symptoms suggest the plants may be under stress from water or mineral deficiency. On heavily infested plants the cysts on the roots are clearly visible with the naked eye (Figure 4). No tuber symptoms have been reported.
Second stage juveniles are vermiform, about 470 um long and with a strong stylet for puncturing cell walls. The tail is pointed. Adult males are similar in appearance, about 1200 um long with a short bluntly rounded tail. Copulatory spicules are located close to the tail. Adult females are virtually spherical, approximately 450 um in diameter with a projecting neck containing the oesophagus and associated glands.
The cyst, when found in soil samples, is a medium brown to a dark brown, the colour of polished oak furniture. Most mature cysts are spherical, with a prominent stalk. The stalk is the head and neck. The cysts of other species of cyst nematode are often shaped very much like a lemon with a prominent stalk and calyx.
Synonym: Heterodera rostochiensis Wollenweber
Synonyms: Heterodera pallida Stone / H. rostochiensis Wollenweber
Cysts (Photo: Dr. A. Morgan Golden, USDA, Beltsville MD)
Cysts and females (Photo: Dr. A. Morgan Golden, USDA, Beltsville MD)
Infected potato roots (Photo: Agriculture and Agri-Food Canada, Research Branch)
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