Shift in Virulence of Soybean Cyst Nematode is Associated with Use of Resistance from PI 88788

The soybean cyst nematode (SCN) is the most economically important pathogen of soybean in the United States. Most of the SCN-resistant cultivars being grown in this region have resistance derived from a single source, Plant Introduction (PI) 88788. A survey conducted in 2005 showed that 83% of the soybean hectarage in Illinois is infested with SCN, with average population densities high enough to cause significant yield suppression (2,700 eggs/100 cm3 soil). Further characterization of these populations showed that 70% have adapted to PI 88788 at some level, reducing the effectiveness of using SCN-resistant cultivars as a crop management tool. Rotation with alternative sources of resistance is recommended as a means to slow the adaptation to PI 88788. Introduction Heterodera glycines Ichinohe, commonly known as the soybean cyst nematode (SCN), causes significant soybean yield suppression in many soybeanproducing countries (14) and is the most economically important pathogen of soybean in the United States (15). First identified in the US in North Carolina in 1954 (12), SCN has since been found in most soybean production areas of this country (5). In 1999, 82% of the soybean fields in Illinois were reported to be infested with SCN (13). The primary measurable effect of infection by SCN is yield reduction in the absence of symptoms (11) (Fig. 1), although chlorosis and stunting may occur if nematode population densities are high or infection is associated with one or more secondary problems such as nutrient deficiency or root rot (5) (Fig. 2). Fig. 1. SCN-resistant and –susceptible soybean cultivars growing in a replicated yield trial near Urbana, IL, in a field infested with ca. 10,000 SCN eggs/100 cm3 soil. Note the absence of obvious visible symptoms. Fig. 2. Visible symptoms on soybean plants heavily infested with soybean cyst nematode, and also exhibiting evidence of root rotting, in Champaign County, IL in 2007. 18 January 2008 Plant Health Progress The most effective and typical means of managing soybean in SCN-infested fields involve host resistance and nonhost rotation (7). Nonhost rotation, usually to maize in Illinois, effectively decreases SCN population densities (4) and has few economic disadvantages. The use of host resistance is more problematic, because complete resistance to SCN has not been found or engineered, and the nematode readily adapts to resistance (5). Genes for resistance to SCN may be found in numerous accessions in the USDA Soybean Germplasm Collection (1), seven of which have actually been used in breeding programs directed at SCN management (6). Of these seven, only Plant Introduction (PI) 548402 (sometimes referred to as ‘Peking’) and PI 88788 have been used to much extent in commercially-available soybean cultivars, although an increasing number of cultivars are available with resistance genes derived from PI 437654. These three PIs are used as indicator plants for SCN virulence profiling by means of the Illinois SCN Type test (5). In 1991, a survey was conducted in Illinois to determine the level of adaptation of SCN field populations to resistance in soybean (10). At that time, adaptation was measured by means of a race test (3,9), a greenhouse bioassay in which the nematode’s ability to develop on a set of four resistant soybean lines was compared with its ability to develop on a standard susceptible line, the cultivar Lee. The objective of the 1991 survey was to determine the percentage of SCN populations that could be described as "race 3," defined as those without the ability to develop on any of the four differentials – in other words, those that could be managed through the use of any SCN-resistant cultivar. By 2005, over 90% of the SCN-resistant cultivars available in Illinois (and surrounding states) were derived from a single source, PI 88788. We conducted a preliminary study of Illinois SCN Types in southern Illinois to determine whether a shift in virulence phenotypes had occurred, because cultivars with SCN resistance derived from PI 88788 have been used since the 1980s in that part of the state. Following the preliminary study, we conducted a survey of soybean fields throughout Illinois during 2005 and virulence phenotyping in 2005-2006 in order to determine whether the consistent use of this single source of resistance has resulted in an increase in the percentage of SCN populations that are adapted to PI 88788. This is important information for managing SCN, because if a population can parasitize PI88788, then it can parasitize any cultivar derived from PI 88788 (5). Preliminary Study of Virulence Phenotypes in Southern Illinois During 2003, soil samples were collected from arbitrarily-selected fields in southern Illinois in order to determine the distribution of southern root-knot nematode, Meloidogyne incognita. The samples were processed for SCN as well as M. incognita, and 28 of those found to be positive for SCN were subjected to Illinois SCN Type tests. The materials and methods were described previously (6), with the exception that the infestation level was 2,500 eggs/100 cm3 soil. The results (Table 1) showed that 82% of the SCN populations tested were able to develop on PI 88788, which demonstrated that adaptation to this source of resistance had occurred in southern Illinois, and encouraged us to survey the