Heterodera glycines Ichinohe - Soybean Cyst Nematode

BACKGROUND

The soybean cyst nematode, Heterodera glycines was first reported from Japan in 1916. Studies at that time showed that the nematode had been present in Japan since 1881. The soybean cyst nematode was first reported in the United States in 1954 and in Canada in 1987. Soybean cyst nematodes are completely sedentary except for a small amount of movement by juveniles and adult males which may amount to only a few centimetres. As a result, a new infestation of soybean cyst nematode is usually caused by the movement of cysts from infested fields to non-infested fields, by means such as machinery, birds or wind.

HOSTS

Over 1,100 species of plants are reported as potential hosts for the soybean cyst nematode. Many species merely allow the nematode to survive until a more susceptible host becomes available. Soybean, Glycine max, is the only major economic crop severely affected. Other highly susceptible species are beans (Phaseolus), bush clover (Lespedeza), vetch (Vicia) and ornamental lupines (Lupinus).

DISTRIBUTION

• Africa: Egypt
• Asia: China (Hebei, Hubei, Heilongjiang, Henan, Jiangsu, Liaoning), Indonesia (Java), Korean peninsula, Japan, Taiwan (unconfirmed), Russia (Amur District in the Far East).
• North America: Canada (Ontario), USA (Alabama, Arkansas, Delaware, Florida, Georgia, Illinois, Indiana, Iowa, Kansas, Kentucky, Louisiana, Maryland, Minnesota, Michigan, Mississippi, Missouri, Nebraska, New Jersey, North Carolina, North Dakota, Ohio, Oklahoma, Pennsylvania, South Carolina, South Dakota, Tennessee, Texas, Virginia and Wisconsin).
• South America: Argentina (unconfirmed), Brazil (unconfirmed), Chile, Columbia, Ecuador.

BIOLOGY

The life cycle of the soybean cyst nematode consists of three stages- egg, larva and adults. Soybean cyst nematode eggs do not rely on the presence of soybean plants to stimulate hatching, though larval emergence is enhanced by the presence of soybean plants. Once hatched, the larvae undergo three additional larval stages the first occurring within the egg. The second stage, the infective stage, 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 will feed for 9 - 12 days, developing into sausage- shaped individuals. As eggs develop within the female, her body becomes pear-shaped, with a short anterior neck and a prominent posterior vulva. As more eggs develop the females become more lemon-shaped 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 fertilization the female extrudes fifty or sixty eggs in a jelly-like mass. The remainder are retained. The extruded eggs hatch immediately to begin a second generation of larvae which can re-infect the soybean plant within the same season (in Ontario, under ideal field conditions, 3-4 generations may occur per growing season). When the females die, their body gradually darkens to a brown colour and hardens to form the protective layer around the eggs - the cyst. A new cyst may contain as many as 500 eggs. The cysts are very durable and can persist in the soil for months. Soil temperature plays a key role in the duration of the life cycle. Optimum temperatures are reported to be 23-28°C.

There are at least 16 morphologically similar, but physiologically different, races of SCN recognized. Field populations of SCN may be a mixture of several races. In Ontario, Race 3 is said to be active.

DETECTION & IDENTIFICATION

SYMPTOMS:

Infested sections of fields will appear as round or oval shaped areas containing yellow, stunted plants. The most severe damage occurs toward the centre of the infested area, with less severe damage at the margins. These symptoms appear about two months after planting or by the time plants are six to twelve inches in height. (Note that host plant symptoms are not a reliable means of identification since symptoms of attack by Heterodera species may resemble mineral deficiency or water stress). Less visible symptoms which are also indicative of Soybean cyst nematode are: wilting, dark discoloured areas on the roots, root swellings, injury to the root tips, a reduced or discoloured and an extremely hairy root system. On heavily infested plants the cysts on the roots are clearly visible with the naked eye. In Ontario, field losses due to soybean cyst nematode can reach 20% depending on soil type, soybean variety planted and other stresses, like water deficit and disease.

IDENTIFICATION:

The immature larva is microscopic, transparent, slender, cylindrical, non-segmented and worm-like. The second stage or juvenile larva is 450 um in length, with a strong stylet for puncturing cell walls. By the third larval stage the sex of the nematode may be identified. A mature male larva may measure 1.3 mm by 0.30 mm. The adult female has a lemon shaped body described as 0.47 to 0.79 mm in length and 0.21 to 0.58 mm wide. The female body has a short anterior neck and a prominent vulva. The mature cyst is brown in colour and about the size of small grain of sand.

Text: Plant Pest Surveillance Unit.