Robert M. Harveson, Extension Plant Pathologist University of Nebraska, Panhandle REC, Scottsbluff
The sugar beet, which arose in Northern Europe as a product of breeding research, has long been notorious for acquiring pathogens via native plants from areas into which it was introduced. Due to its relatively recent origin, this includes some of the most important early diseases encountered in Europe such as the cyst nematode and Rhizoctonia root rot. In North America, we are now aware of a specific nematode disease (false root-knot) that was never documented prior to the arrival of the sugar beet in the Western Hemisphere. In fact it has yet to be identified from anywhere else in the world where sugar beets are produced. .
False Root-Knot Nematode
The false root-knot nematode (Nacobbus aberrans) has also been referred to in the literature as the Nebraska root galling nematode, Cobb’s root galling nematode, and the potato rosary nematode. Nematodes in the genus Nacobbus produce galls on the roots of their hosts that are similar in appearance to those caused by several species of the root-knot nematodes (Meloidogyne spp), which provides the origin of the pathogen’s common name. If diagnoses were based on symptoms alone, disease could easily be mistakenly attributed to Meloidogyne, thus these nematodes are familiarly known as false root-knot nematodes. The pathogen was named in honor of Nathan Augustus Cobb = Nacobbus, the pioneering USDA pathologist who is commonly referred to as the Father of American Nematology.
New Nematode Pathogen in North America
Nathan Cobb was apparently the first worker to observe and record specimens of Nacobbus from Colorado in 1918, although he mistakenly thought them to be Heterodera schactii (sugar beet cyst nematode). We now know that the first species of Nacobbus was actually discovered and described by Gerald Thorne, but named Anguillulina aberrans in 1935 in non-agricultural land from shadscale (Atriplex confertifolia), a native desert shrub, in the foot hills of the Great Basin near Utah Lake, Utah. The genus Nacobbus was later created and proposed in 1944, based on a new species found in southern California, and called N. dorsalis. Anguillulina aberrans was eventually placed in the genus Nacobbus, thereby becoming N. aberrans and distinguished from N. dorsalis by several key morphological features.
New Sugar Beet Disease in North America
The false root-knot nematode was first isolated from sugar beets in 1949, near Mitchell, NE by UNL pathologist Max Schuster, but the disease symptoms had been recognized and mistaken for infection by the root-knot nematode for many years prior to that. The most conspicuous symptoms of N. aberrans on sugar beets are galls or swellings with a proliferation of lateral branches formed on mature taproots (Figure 1). These lateral roots may be studded with galls, giving them a beaded appearance, with numerous small rootlets emerging from galls.
After thorough investigations of its distribution, host range, life cycle, and pathology were completed the sugar beet pathogen from Nebraska was interpreted as being distinct from the two species previously described (N. dorsalis and N. aberrans) and named N. batatiformis based on the shape of the mature female. The female’s body was tapered at both ends at maturity resembling a sweet potato (Ipomoea batatas), (Figure 2). However, this species was later synonomized with N. aberrans and it retained that name.
Distribution
A regional survey from the late 1950’s determined that although disease incidence was not as widespread compared to the sugar beet cyst nematode, the false root-knot pathogen was still identified from 16 counties in 6 states throughout the Great Plains, including Colorado, Kansas, Montana, Nebraska, South Dakota, and Wyoming. Infestation of sugar beet fields by this nematode is still limited to the high plains east of the Rocky Mountains. Another earlier survey was conducted in western Nebraska and found the nematode infesting one third (32%) of 125 sugar beet fields monitored over two years.
It is still considered a potentially serious pest in this region due to its widespread presence and because of the potential yield reductions when infection does occur. Presently, Nebraska and Wyoming are the states most affected by this pest, with losses in sugar beets crops of 10-20% losses being documented.
Host Range
In the United States, N. aberrans is primarily an economic problem on sugar beet, but the pathogen can also affect numerous other hosts such as carrot, pea, lettuce, tomato, and species in the mustard (broccoli, cabbage, rutabaga, radish, and turnip), and cucurbit (pumpkin, cucumber) families. Fortunately, most field crops grown in the central high plains under irrigation other than sugar beet are not susceptible, including potatoes, corn, dry beans, sunflower, alfalfa, wheat and barley. Because of the broad host range, the disease can also be problematic for city residents with backyard gardens, particularly with root vegetables like beets, turnips, radishes and carrots.
The most commonly observed non-economic hosts for the nematode in western Nebraska were determined to be three species of cacti, including two species of the prickly pear, and the spiny star cactus. All of these species are native to this region and other locations throughout western North America. Colonies isolated from cacti and sugar beet resulted in reciprocal infections and the morphology and reproduction of the colonies were identical. The false root-knot nematode has also found commonly infecting native weed species within fields including various mustards, common lambsquarters, and Kochia, in addition to the introduced species common purslane, puncturevine, and Russian thistle.
Taxonomic Confusion
The taxonomic history of N. aberrans has been very controversial. Although most nematologists today recognize two primary species, N. aberrans and N. dorsalis, it is now becoming increasingly evident that N. aberrans is in dire need of revision as its species differences are not fully resolved. However, at present, it is still generally accepted that the false root-knot nematode can be separated into at least three pathotypes or groups, based on different host preferences and DNA characteristics. These pathotypes include a strain from the highland Andean regions of Argentina, Bolivia, Chile, and Peru that attacks potatoes but not peppers. A bean strain from Mexico has also been observed that damages beans, tomatoes, and peppers, but not sugar beet or potato. Lastly, the sugar beet strain previously discussed has been well characterized biologically and documented from the Great Plains of the United States. It infects sugar beets and many common vegetable crops, but not potato.
Conclusions
The ability of the North American sugar beet pathotype of N. aberrans to parasitize introduced host species makes it unique when compared to the two pathotypes from Mexico and South America that infect primarily the New World solanaceous (potato, peppers, and tomato) and leguminaceous (bean) crops.
We can also conclude that the genus Nacobbus (false root-knot nematode) is clearly distinct from the various Meloidogyne species (root-knot nematodes) with which it has often been confused, and is also unquestionably endemic to the Americas. It has been demonstrated to occur in diverse climatic areas of the Western Hemisphere, but is absent from any other locale in the world. The documentation of the sugar beet strain of N. aberrans (formerly N. batatiformis) on native cacti from uncultivated rangeland in Scotts Bluff, Sioux, and Morrill counties of the Nebraska Panhandle in the 1950s, provided strong additional evidence that this nematode was indigenous to this region, but then adopted the sugar beet as its preferred host after it was introduced into North America.