Nacobbus aberrans
(Thorne, 1935) Thorne & Allen, 1944
False root-knot nematode or Rosary potato nematode
Photo Gallery- Scottsbluff, NE sugarbeet

Nematodes in the genus Nacobbus produce galls that are similar in appearance to those caused by root-knot nematodes.  If diagnosis is primarily based on field symptoms, it is often erroneously assumed that crop damage caused by Nacobbus species is due to Meloidogyne species.  For this reason, Nacobbus species are referred to as false root-knot nematodes.

The false root-knot nematode is a root feeder pest of field and vegetable crops in North (US included) and South America.  In the US the false root-knot nematode parasitizes sugarbeet and other field and vegetable crops, but it does not infect potato.  In Mexico and South America there are populations of this nematode which cause serious damage on potato.
These populations from South America can be introduced with contaminated soil debris and also with infected potato tubers, edible root material and rooted propagative material.

Life cycle and histopathology
First stage juveniles of Nacobbus develop and molt within eggs that are laid in a gelatinous matrix outside the female's body.  Embryonic development is greatly affected by temperature.  Second stage juveniles hatch from the eggs.  Second, third, and fourth stage juveniles, as well as young females are capable of penetrating roots and, as they migrate and feed in the roots, they cause cavities that extend from the cortex into the stele.  Maturing females become sedentary, and establish themselves with their heads near the vascular tissue, where they stimulate cell division, partial dissolution of cell walls, and fusion of cell protoplasts.  A spindle-shaped synctium is formed, and these specialized cells with dense cytoplasm become the feeding site for the female.  Around this feeding site a gall is formed by proliferation of cortical and vascular tissue.  Numerous starch granules accumulate at the feeding site.  Galls tend to be rounded or bead-like and sometimes numerous small rootlets develop around the galls.  The time required for N. aberrans to complete a life cycle is primarily temperature dependent, but also may be influenced by the nematode population and host species.

Distribution and principal hosts
In 1935, Thorne described the first species of Nacobbus from specimens he observed parasitizing Atriplex confertifolia, a native shrub known as shadscale, that he collected in the desert foothills near Utah Lake, Utah.  Since that time, Nacobbus aberrans has been reported on many important crop plants in Argentina, Bolivia, Chile, Ecuador, Great Britain, India, Mexico, Netherlands, Peru, United States, and the U.S.S.R.

In the United States, Nacobbus species are parasites of sugarbeet.  Because of its long life cycle and the limited fecundity of females, N. aberrans is considered to be less aggressive on sugarbeet than the sugarbeet cyst nematode (H. schachtii) or the root-knot nematode (M. hapla).  Nevertheless, with high populations of the false root-knot nematode, the sugarbeet storage root is distorted and deformed, and the top growth of this plant is greatly reduced.  Surveys in the early 1950's indicated approximately 1/3 of the sugarbeet fields in western Nebraska were infested with N. aberrans.  This nematode has also been found on sugarbeet in limited areas in Colorado, Kansas, Montana, South Dakota, and Wyoming.

In South America, in the highlands of Argentina, Bolivia, Chile, Ecuador, and Peru N. aberrans is a serious problem on potato.  These populations are known to have a rather wide host range, and are adapted to a variety of soil types.

Regulatory considerations
Nacobbus aberrans populations, which parasitize sugarbeet populations in the western US, do not parasitize potato, whereas many populations from South America are capable of parasitizing potato and sugarbeet.  Where high infestations of N. aberrans occur in South America, potato yield losses of 55% to 90% have been reported.  The ability of South American populations to adapt to many hosts and diverse conditions increases the risk of these populations becoming established, should they be introduced into noninfested areas.  Special efforts should be made to prevent populations of N. aberrans that parasitize potatoes from entering the United States.

Information taken from Florida Dept. of Agriculture and Consumer Service, Plant Industry Div.
Nematology Circular # 119, June 1985
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DNA Sequences Obtained
Specimen: Collected:
SB-SB-1-07 Scottsbluff, NE
SB-SB-1-09 Scottsbluff, NE