Introduction

The soybean cyst nematode (SCN), Heterodera glycines, is the most devastating pest to soybean crop yields in the U.S., targeting the roots of soybean and other legume plants. When infection is severe SCNs cause stunting, yellowing, impaired canopy development, and yield loss. The symptoms caused by SCNs can go easily unrecognized by farmers—in some cases there are no warning symptoms before a loss of 30% of the yield. Due to the slight stunting and yellowing, many farmers may mistake these symptoms as environmental problems when in fact they are SCNs. Another symptom of SCNs that may affect farmers' yields is stunted roots with fewer nitrogen-fixing nodules. Due to the fact that soybean cyst nematodes can only move a few centimeters in the soil by themselves, they mostly are spread via tillage or plant transplants. This area of infection will look patchy and nonuniform making diagnosis more difficult for farmers. They can be seen in the roots of summer soybean plants if the roots are taken out very carefully and gently washed with water. The egg masses should be seen as bright white or yellow "pearls" on the roots. The later the roots are pulled the harder it will be to diagnose due to the SCNs female dying and turning a much darker color, forming a "cyst". The best way to know if a field is infected by soybean cyst nematodes is to take a soil sample to a nematologist.

Biology

As the nematode feeds, it swells. The female swells so much that her posterior end bursts out of the root and she becomes visible to the naked eye. In contrast, the adult male regains a wormlike shape, and he leaves the root in order to find and fertilize the large females. The female continues to feed as she lays 200 to 400 eggs in a yellow gelatinous matrix, forming an egg sac which remains inside her. She then dies and her cuticle hardens forming a cyst. The eggs may hatch when conditions in the soil are favorable, the larvae developing inside the cyst and the biological cycle repeating itself. There are usually three generations in the year. In the autumn or in unfavorable conditions, the cysts containing dormant larvae may remain intact in the soil for several years. Although soybean is the primary host of SCN, other legumes can also serve as hosts.

Disease cycle

Soybean cyst nematodes are so devastating due to their life cycle being so efficient for multiplication. They have six life stages, which is the norm for all nematodes. These stages include: egg, juvenile (J1-J4), and adult and can complete multiple cycles within a single growing season. The SCNs makes its first molt while still inside the egg going from a juvenile 1 (J1) to a juvenile 2 (J2). When the soybean cyst nematode is a J2 it may then enter the root of the plant, usually just behind the root tip. Once the SCNs has gained entry into the root, it will then create a specialized feeding cell called a syncytium by penetrating the pericycle, endodermis, or cortex cell with its stylet in order to take in nutrients from the plant. These are cell masses with multiple nuclei that are formed due to cell wall dissolution and fusion. The J2 will then continue on in its development only when a syncytium cell is created. Female SCNs will remain there for the rest of their life, while males will leave the root after reaching adulthood. The males are attracted to a pheromone that is released by the female when her egg mass is ready for fertilization. When the eggs from the egg mass are fertilized, some of them will hatch within that same growing year to continue on with infection. Eggs will still remain inside the female, and when she dies and hardens into a "cyst", they will go into dormancy until the following growing season or until conditions are favorable. This cyst is very resilient against environmental conditions and can hold as many as 250 eggs.