HG Type Test

Contributed by Greg Tylka, Iowa State University and Terry Niblack, University of Illinois

An HG test in progress. Step 1. Soybean lines with different resistant genes are inoculated with nematodes and allowed to grow for 30 days.

Step 2. The soybean roots are washed over a sieve to collect the females.

Step 3. The number of females that have developed on each soybean line are counted, and compared with the number on a standard variety. Photo credit: Terry Niblack, University of Illinois

Background of the HG Type Test

The soybean cyst nematode (SCN) is a widespread and serious pest of soybeans throughout the United States and Canada. The microscopic worm is long-lived in the soil, can develop and reproduce quickly on susceptible soybean varieties, and is capable of causing significant yield loss even in years with ideal growing conditions.

Fortunately, soybean varieties have been developed that are resistant to SCN. These resistant varieties suppress 90% or more of the development of most SCN populations, resulting in a significant increase in soybean yields in SCN-infested fields. However, soon after the release of resistant varieties, scientists discovered some SCN populations that were capable of reproducing at elevated levels on resistant soybean varieties. 
Consequently, a race test system was developed in 1970 to assess the abilities of SCN populations to reproduce on resistant soybean varieties.

Much has been learned in the past several decades about development of SCN on resistant soybean varieties. It is apparent from this new knowledge that a change in how we describe the abilities of a SCN population to reproduce on resistant soybean varieties is warranted. A new system, called the HG Type test (HG for Heterodera glycines, the scientific name for soybean cyst nematode) has been developed and adopted by agronomists, plant pathologists, and soybean breeders. Following are answers to some common questions about the new SCN HG Type test.

What is a SCN HG Type test?

A HG Type test is a greenhouse test performed on a SCN population isolated from a field to determine how well the SCN population can develop on soybean lines that were used as sources of resistance for SCN-resistant soybean varieties. 

Why not a SCN race test?

The HG Type test is similar to a SCN race test, but includes only soybean lines that are sources of resistance in available SCN-resistant soybean varieties. It is much easier to understand than the race test. Once the HG Type test system has been used for a while, it should be easy to remember what an HG Type designation means in relation to the resistance possessed by available SCN-resistant soybean varieties.

For example, if the HG Type of a SCN population in your field has the number 2 in its designation, you will come to recognize that the number 2 corresponds to PI 88788, the most commonly available source of resistance in soybean varieties in the Midwest. The number 2 means the same thing whether the population is an HG Type 2, an HG Type 2.3.7, or an HG Type 1.2.6.

Who needs an HG Type test?

Soybean growers who have experienced sub-par performance from SCN-resistant soybean varieties in SCN-infested fields should consider having an HG Type test performed. Also, soybean growers who farm in an SCN-infested area that has had resistant soybeans grown numerous times in the past might consider having an HG Type test performed. 

How is a HG Type test conducted?

To determine the HG Type of a SCN population, we put the nematodes on soybean lines with different genes for SCN resistance in the greenhouse under controlled conditions (Figure 1). These conditions are similar to those under which resistant soybean varieties are developed.

After 30 days, enough time for SCN females to develop, we count the numbers of females that form on the roots of the various resistant soybean lines (Figures 2 and 3). We compare these numbers to the number of females that formed on a standard susceptible soybean variety. Finally, we note which resistant soybean lines show elevated development by the SCN population. “Elevated development” means that a resistant line has 10% or more of the number of females that developed on the susceptible variety. 

How do I interpret the results of a HG Type test?

The number or numbers in the HG Type designation correspond directly to sources of resistance used in available SCN-resistant soybean cultivars.

For example, a SCN population of HG Type 1.2 indicates that the nematode population has elevated development on Peking (line #1) and PI88788 (line #2). Either or both lines have been used to breed some SCN-resistant soybean varieties. A grower with a field infested with an HG Type 1.2 might not want to plant SCN-resistant varieties that contain resistance from Peking or PI88788, if possible. Facilities that provide SCN HG Type testing should also offer assistance in interpreting the results of the test.
Examples of HG Type Testing»

How do I interpret descriptions of public and private SCN-resistant varieties?

Growers should be aware that the traditional way that SCN-resistant varieties are labeled is somewhat misleading. For example, an SCN-resistant variety with resistance from PI 88788 may be labeled as resistant to SCN race 3, when in fact it might also be resistant to as many as seven additional SCN races. In addition, this variety also might be vulnerable to elevated development by as many as eight other SCN races. Unfortunately, none of this management-type information is conveyed in the labeling. 

With the HG Type designation, we label the nematodes, not the varieties. For example, if a grower’s SCN population is an HG Type 2, the name clearly indicates that the nematode exhibited elevated development on PI 88788 (line #2). That makes it more likely that the nematodes could develop on any SCN-resistant variety that obtained its SCN resistance from PI 88788, and it likely would be in the grower’s best interest to use a SCN-resistant variety that obtained its SCN resistance genes from a source other than PI 88788, if possible.

Examples of HG Type Testing»
Where can I get a SCN HG Type test performed?»