Biotype 4 induced susceptibility of Rag1+Rag2 soybean

Induced susceptibility was first observed with biotype 1 soybean aphids. The effect was further evaluated and feeding facilitation and obviation of resistance effects were later observed on Rag1 soybean. Feeding facilitation occurrs when biotype 1 soybean aphid populations improve the host suitability for subsequent biotype 1 populations on either susceptible or resistant soybean. Obviation of resistance occurrs when biotype 2 populations improve host suitability for subsequent biotype 1 populations on Rag1 soybean. Obviation of resistance resulted in populations of biotype 1 and biotype 4 being indistinguishable on Rag1 soybean. To date, no study has evaluated the potential for biotype 4 soybean aphids to obviate the resistance of Rag1+Rag2 soybean for biotype 1 soybean aphids. The purpose of this study was to explore the potential that induced susceptibility was not unique to Rag1 soybean. To do this, we used a susceptible (IA3027) and a Rag1+Rag2 (IA3027RA12) soybean cultivars. Inducer populations were either no soybean aphids, biotype 1 or biotype 4 soybean aphids and were placed onto the plant and allowed to feed for 24 hours. The response populations were either biotype 1 or biotype 4 soybean aphid. We observed that biotype 4 soybean aphids improved the host suitablity of Rag1+Rag2 for biotype 1 soybean aphids. These results suggest that induced susceptibility, specifically obviation of resistance are not unique to individual biotypes and Rag gene combinations.