State of knowledge on breeding for durable resistance to soybean rust disease in the developing world

Abstract

Soybean (Glycine max L.) is increasingly playing an important nutritive role in the food and feed industry in most developing countries. However the crop is currently threatened by Soybean rust disease (SRD) caused by Phakopsora pachyrhizi. This review provides general information on soybean rust, worldwide disease threats and case studies of widely used resistance breeding approaches. Currently, soybean rust is known to have spread from Asia to most developing countries in Africa and South America through continental movement of urediniospores. At least four major genes have been deployed to control soybean rust in the orient, where the disease has been known for a long time, and in other areas of the tropics. However, genetic resistance has broken down due to several virulent races of soybean rust that are now prevalent. More resistant sources have been identified in Uganda and Brazil and are being used in various breeding programmes. Durable resistance is considered the most cost effective and sustainable means of controlling SRD in areas where it has become endemic. Durable resistance is theoretically effective against all races of the soybean rust pathogen, which is an important consideration as soybean rust is a complex of races, with multiple virulence factors. Different mechanisms can be used by breeders to control SRD: vertical and horizontal resistance, and tolerance. However, the utilization of each depends on the germplasm available to the breeder; resources and facilities available; time required to release the variety; soybean rust races present; and the targeted longevity of the resistance. This review on approaches to breeding for resistance is complemented by suggested future research areas for breeders to improve the durability of resistance to SRD. These are conveniently classified into short, medium and long-term strategies. Emphasis is also put on pre-breeding through exploration of wild relatives of soybean to broaden the range of resistance sources. Various breeding strategies, ranging from conventional to molecular techniques, are suggested, including gene pyramiding, multi-line formation, combining ability studies, and molecular techniques such as marker assisted selection (MAS) and genetic transformation.