Which HG Types are found in Minnesota?
The frequency distribution of HG Types (percentage of fields with an HG Type) varies in different regions in the United States. In two previous surveys conducted in 1998 and 2002, SCN populations in most Minnesota fields were HG Type 0 or 7 (Table 2), which have a low level of virulence on the current commercial resistant varieties. The frequency of virulent populations in the state may change over time in response to planting SCN-resistant soybean varieties.
Table 2. Percentage of SCN populations from Minnesota with Female Index more than 10 on the indicator soybean lines.
Fig. 10. Relationship between reproduction potential (Female Index) of SCN on 'PI 88788', 'Freeborn', and 'Peking' after the use of the SCN-resistant soybean Freeborn for various years during 1996-2007 in a Minnesota field infested with an original population of HG Type 0 (race 3).
In a field plot experiment, SCN reproduction potential on the resistant soybean variety Freeborn and its resistance source PI 88788 increased with increasing years of growing the variety (Fig. 10). After 5 years, the population changed from the original HG Type 0 (race 3) to a population that was able to overcome the resistance of PI 88788 (FI > 10; HG Type 2.5.7). After 10 years, Freeborn that had been moderately resistant (FI ˜ 15) to the original population became susceptible (FI > 60) to the resulting SCN population.
Such a change of virulence phenotypes may occur in other fields where resistant varieties have been planted for a number of years. Across Minnesota, the percentage of virulent populations on the resistance source lines PI 88788 and Peking increased dramatically from 2002 to 2008 (Table 2). Approximately 20% of fields in southern and central Minnesota have SCN populations with FI on PI 88788 more than 30, to which PI 88788 varieties are no longer effective. In a few fields (about 2%), the SCN FI are high (>30) on both PI 88788 and Peking. The results of these studies convey a warning that more soybean varieties with alternative sources of resistance are needed for effective long-term management of the nematode in the state.