A soy plant introduction (PI) has showed resistance to soy aphid in the US state of Ohio, which may help in the development of new resistant cultivars.

The soy PI, labelled PI 243540, contains a single dominant gene (Rag2) that allows the plant to resist soy aphid feeding and colonisation.

Researchers are now focusing to develop a soy cultivar that the soy aphid won't colonise. Rouf Mian, a USDA-ARS molecular geneticist, and his team found that when given a choice between PI 243540 and a susceptible soy variety, the soy aphid would choose the latter. After four weeks of the choice test, PI 243540 showed no feeding damage and only a few soy aphids were found on the plant.

That is significant as aphid reproduces rapidly and population can reach into thousands after only two weeks, said Mian.

Mian said impacts from soy aphid feeding and disease symptoms become evident by four weeks, but PI 243540 remained healthy.

Researchers also found that when PI 243540 was the only feeding choice, the aphids would either leave or populations would die off.

Mian is currently backcrossing PI 243540 with two food soy cultivars, transferring the Rag2 gene into susceptible soy varieties and tests their resistance to the aphid. Researchers are seeing promising results and hope to have breeding lines available for release by the end of 2010 if agronomic characteristics, such as yield and disease resistance hold up.

Aside from PI 243540, Mian has identified other soy PIs that show moderate resistance to the aphid. Mian hopes to use the findings to develop partial-resistant soy cultivars, whereby multiple genes rather than a single, dominant gene define resistance.

Single genes are easy to transfer but the resistance lasts for only about 10 years, while partial-resistant varieties hold up much longer because it's more difficult to overcome multiple resistant genes rather than just one, according to Mian.

The soy aphid is a sapsucker that can greatly damage untreated soy fields. The insect has also been known to transmit numerous viruses, including soy mosaic virus, soy dwarf virus, and alfalfa mosaic virus not only to soy but also to a number of vegetable crops.