HortFACT
Identification
Infested plants show yellowing and wilting symptoms typical of many soil-borne root pathogens. The presence of the nematode is most easily confirmed in the field by the appearance of small, round, white to golden-yellow cysts on the roots of infested plants. These are 200-500 (µm* in diameter; they can be seen by the naked eye, but are best viewed with a hand lens. They are readily dislodged from roots, so plants must be lifted and handled with care. Heavily infested roots may be shortened and excessively branched.
Pathotypes
Pathotypes are sub-groups of potato cyst nematode which may be distinguished by their ability or inability to reproduce on potato hosts with different genetic backgrounds. At least eight pathotypes are recognised in Europe, and more are known to occur in South America. Recently, recognition of certain morphological and physiological differences led to the regrouping of pathotypes into two species: Heterodera rostochiensis (golden nematode), which has a long, golden-yellow phase during cyst development; and Heterodera pallida (pallid nematode) which has a long, creamy-white cyst phase. A change of generic name from Heterodera to Globodera is also being considered.
Host plants
Potato and tomato are the most economically important hosts. Other Solanum species (e.g., poroporo and tomato weed) are hosts, particularly tuber-forming varieties.
Damage
For any one set of growing conditions there is a critical cyst level in the soil above which yield losses become significant, and there is a maximum nematode density that can be supported by the host. At population levels approaching maximum density plants may die, but more often plants survive and yield is affected. The actual tuber quality is not directly affected, but the size of tubers may be reduced as a result of poor plant nutrition and root damage caused by the nematode.
Distribution
Potato cyst nematode originated from the Andean highlands of South America, where tuber forming Solanum species evolved. It has been spread by transport of potatoes to many potato growing areas of the world (Europe; British Isles; Central, South, and North America; Mediterranean basin; Scandinavia; India; South Africa; the USSR; and the Pacific basin). It is found mostly in temperate regions, but it also occurs in coastal areas of the tropics.
Potato cyst nematode was first identified in New Zealand in 1972, from the intensive potato producing Pukekohe district. It was found in Canterbury in 1975, and in isolated pockets of Otago and Southland in 1976.
The resistant brown cyst stage is the body of the dead female, containing 100-500 transparent, oval eggs (length 60 Fm), inside each of which is a dormant second-stage juvenile. Cysts are dislodged from host-plant roots at harvest, and are scattered through the soil, where they may remain viable for many years in the absence of a host.
Exudations from the roots of a host plant stimulate up to 90% of viable eggs in a cyst to hatch. Second-stage juveniles (length approximately 500 elm) leave the cyst, migrate through the soil, and enter roots of the host plant. Once inside the root, juveniles become sedentary and cause the host to form giant feeding cells (transfer cells) on which they feed using their stylet [mouth spear]. At this stage sex is determined by nutritional status: those juveniles which induce sufficiently large giant cells become females, the rest become males. Second-stage juveniles moult soon after root penetration, and the main feeding phase lasts several weeks as third and fourth stage juveniles (length 400-500 Fm).
A final moult produces adults. The bodies of adult females gradually swell until they burst through the side of the root. Initially white, they become cream or yellow on maturity (see Pathotypes above) and brown on death (cyst formation). Adult males are vermiform, or worm-shaped (length approximately 1200 ,am); they leave the root and are attracted to and mate with young adult females. Both sexes are capable of mating several times.
The generation interval (egg to egg) of potato cyst nematode and the rate of increase of populations depends on the soil environment and the condition of the host plant (10-20 weeks under Pukekohe conditions). There is normally only one generation per crop.
*1 µm is 1 micrometre = one thousandth of a millimetre.
M.A. FOOT and F.H. WOOD