Nematodes are minute, worm-like animals, able to move between soil particles, between folded leaves of plant buds, in the air spaces of leaves and stems or in plant tissues themselves. More than 68 species of plant parasitic nematodes belonging to 24 genera are associated with potato fields from different parts of the world. Discovery of root-knot nematode on potatoes in the U.S. dates back to 1889 when J.C. Neal reported Meloidogyne arenaria on a potato crop in Florida. H.J. Jenson discovered stubby-root nematode, Paratrichodorus allius, in 1963 from an Oregon field. Later it was demonstrated that P. allius transmit the Tobacco Rattle Virus that cause the corky ring-spot disease of potato.
The earliest record of lesion nematode Pratylenchus spp. on potatoes in the U.S. was by N.A. Cobb, who found that P. penetrans was causing pustules over the surface of potato tubers. In the United States, the potato-rot nematode, Ditylenchus destructor, was first found in Idaho in six farms in the vicinity of Aberdeen. In the year 2006, pale cyst nematode, Globodera pallida, was first detected in the U.S. in Idaho (Bingham County). Nematodes important to potato production in Idaho include root-knot nematodes (Meloidogyne spp.), stubby-root nematodes (Trichodorus and Paratrichodorus spp.), root-lesion nematodes (Pratylenchus spp.), stem nematode (Ditylenchus spp.) and pale cyst nematode (Globodera pallida).
ROOT-KNOT NEMATODES
Root-knot nematode (Meloidogyne spp.) has been recognized as a major nematode pest on potatoes and are frequently found in abundance, especially in sandy soils. They severely damage and cause significant losses in potatoes, in both warm and cool climates, depending upon the nematode species. Although there are several species of root-knot nematodes, four species commonly found on potato in the Unites States include M. chitwoodi, M. hapla, M. incognita and M. javanica. The two most common on potatoes in Idaho and eastern Oregon are the Columbia root-knot nematode (M. chitwoodi) and Northern root-knot nematode (M. hapla). M. chitwoodi was first described on potatoes in Quincy, Wash. M. hapla occurs in every southern state, the Pacific coast states and British Columbia. Damage of this species to potatoes is usually most severe following alfalfa crops and during years with high spring temperatures. M. incognita infests areas where the milder climates predominate. Infestations are known in the southern states, the southwest and extend up the Pacific coast to Oregon and the Atlantic coast to New Jersey. M. javanica are found mainly in the southern and southwestern states and California.
All species can attack potatoes and reports of root-knot nematode damage on potatoes have increased during the past several years. Second-stage juveniles of the nematode penetrate the tuber (fig. 1). Females (fig. 2) feeding in the tubers cause bumps on the surface (fig. 3), rendering these tubers unmarketable for either fresh packing or processing.
Root-knot nematodes have a wide host range leading to population increases when other susceptible crops are grown in rotation with potatoes. They cause field damage that is localized, usually in circles of various sizes, or spread throughout an entire field with plants becoming chlorotic and stunted. Damaged roots are not able to obtain soil nutrients and symptoms appear as nitrogen or micronutrient deficiencies. Plants may wilt easily, especially in warm weather, due to root damage even though soil moisture may be adequate. Root-knot nematode populations can increase dramatically when susceptible crops are grown in rotation with potatoes.
ROOT-LESION NEMATODES
Root-lesion nematodes (Pratylenchus spp.), migratory endo-parasites on potatoes are of concern to growers because they reduce yield indirectly by weakening and increasing stress on the plants and by making them more susceptible to fungal and bacterial diseases. There is also a positive correlation of root-lesion nematodes with the incidence of Verticillium wilt (early die).
Although more than 15 species of root-lesion nematodes are reported to parasitize potatoes, Pratylenchus neglectus is the predominate lesion nematode species in the state of Idaho. Two species of root-lesion nematode, Pratylenchus neglectus and Pratylenchus penetrans, can increase susceptibility of potato plants to the early die complex. P. penetrans interacts strongly with the fungus pathogen Verticillium dahliae, the main cause of potato early die (fig. 4). P. neglectus is not known to interact directly with Verticillium; however, high populations may be associated with other factors that reduce optimal growth, contribute to crop stress and increase the incidence and severity of early die.
All life cycle stages outside the egg may infect roots immediately behind the growing tips, causing reddish brown lesions around the root cortex. Lesions coalesce, turn black and are often invaded by soil micro-organisms, which can cause weakened root systems, reduced water and nutrient uptake, loss of plant vigor and ultimately yield reduction.
High populations of lesion nematodes cause areas of poor growth where plants are less vigorous, turn yellow and have stunted growth.
Damage is often caused by direct feeding and usually only cortical tissues are affected. In addition to directly causing damage to the crop, P. neglectus can contribute to Potato Early Dying (PED). Premature death and yield reduction of 30 percent (6-12 t/ha) due to PED have been documented in Idaho. Potato plants with PED are characterized by stunted growth, chlorotic foliage, deterioration of roots, premature senescence and reduced yields.
The primary cause of PED is the soil borne fungus Verticillium, but it can occur as a disease complex that includes V. dahliae or V. albo-atrum and the root-lesion nematode Pratylenchus penetrans. In Idaho, PED is associated with the soil-borne fungus Verticillium dahliae Kleb.
STUBBY-ROOT NEMATODES
Stubby-root nematodes (Trichodorus spp. and Paratrichodorus spp.) are migratory ectoparasites that cause deformation of tubers (fig. 5) and are found in sandy, moist, cool soils. Damage is profoundly influenced by soil moisture and is greater in wet seasons. These nematodes are important parasites of potatoes, not so much for the direct damage they cause but as vectors for tobacco rattle virus which they transmit to potatoes.
This virus causes a disease of potato tubers called corky ring spot. Rusty brown, irregularly shaped lesions that have a corky texture appears in the flesh of the tubers (fig. 6). Corky ring spot problems occur mostly in isolated sandy soil areas of southern Idaho. These nematodes have wide host ranges, making management with crop rotation difficult and relatively ineffective.
Stubby-root nematodes transmit TRV to cause corky ring spot symptoms. Virus infection blemishes tubers and renders them unmarketable. This disease is of major concern to growers in the Pacific Northwest as tuber injury may result in rejection of the harvest of an entire field. Distribution of P. allius in the United States is presently limited to Oregon, Washington and California while P. allius, P. minor and P. porosusare common in Idaho and Eastern Oregon. They have the ability to adopt to unfavorable conditions by migrating downward in the soil. Problems with stubby-root nematodes have now been consistently documented in eastern Idaho for several years and have a wide host range that includes cereal crops and potatoes. In due course, tubers exhibit deep cracks and shallow corky depression on their surface, which renders them unmarketable.
POTATO-ROT NEMATODE
Potato-rot nematode (Ditylenchus destructor) is not widely present in the U.S., but can cause devastating rot in stored tubers. High relative humidity is an essential factor for the establishment of this nematode. The effect of nematodes will manifest itself at harvest or storage when infected tubers will rot. Host range studies indicated that relative to the potato cv. Norland, snap bean was a good host, red clover and corn were intermediate hosts, alfalfa was a poor host, and oat was a non-host for nematodes grown in vitro in monoxenic cultures. D. destructor enters tubers through lenticles on the skin near eyes. Nematodes at first exist singly or in small numbers in the tissue just beneath the skin of the tubers, and small white lesions are present during early and mid-season tuber formation. Diseased tubers are readily recognized in advanced stages of infection when sunken, dark-colored pits mark the surface and the skin cracks (fig.7).
Such advanced symptoms are rarely observed during the growing season but are reported in tubers stored in field before lifting or stored in tubers. Subsurface tissue will develop a brown matted wool-like appearance. D. destructor mainly feeds on underground stem parts and is well able to parasitize growing and stored tubers under most climatic conditions.
Infection of D. destructor starts as small, whitish regions in the cortex, which can be detected by removing the peel. In severe infestation these lesions coalesce, the affected tissue darkens gradually through grayish to dark brown color.
The tuber skin is not attacked and may remain intact but papery thin over the lesions or may crack as a result of stress in the tuber. Affected tissues are soft and mealy unlike the hardness of tissues attacked by blight. On the other hand, lesions caused by D. dipsaci extend throughout the tuber results in the affected tissues being spongy and yellowish to brown in color.
PALE CYST NEMATODE
The pale cyst nematode, Globodera pallida, is a major pest of potato crops in cool-temperate areas. It primarily affects plants within the potato family including tomatoes, eggplants and some weeds. Potato cyst nematode infestations may be associated with patches of poor growth. Affected potato plants may exhibit yellowing, wilting or death of foliage-none of which has been observed in Idaho potato fields. If left unmanaged, nematodes can cause significant yield loss.
The PCN is widely distributed in many potato-growing regions throughout the world. In North America, the nematode is also known to be present on the Island of Newfoundland, Canada.