October 12, 2007

Everyone reading this article does not need to use a resistant soybean variety in all their fields. If you have a history of one or both of the problems mentioned below and it is affecting a major area in the field, then make this a criteria in variety selection this year. For soybean cyst nematode our threshold for using a resistant variety in the field is any detection of it in the field. When selecting a resistant variety, do not overlook the other agronomic traits you would normally consider such as chlorosis, maturity or emergence rating. There are enough resistant varieties on the market that you should be able to find varieties with the total genetic package to meet your needs. Unlike other variety traits for which you pay a "tech fee," disease resistance generally does not cost any more than comparable susceptible varieties. There are also good varieties with tolerance or resistance to disease problems not mentioned here, but these are currently the two most important in Nebraska.

Soybean Cyst Nematode (SCN)

Over the last few years we have seen an increase in the number of fields with SCN. Producers are encouraged to use the SCN free sample program being funded by the Nebraska Soybean Board to determine if they have SCN. If you have a history of SCN and you have been using resistant varieties, be sure to rotate varieties in the field and if at all possible, rotate the source of resistance.

Resistant Varieties: SCN-resistant soybeans are a very effective management tool. Check with seed companies for SCN-resistant cultivars that perform best in your area. SCN populations are not the same in regard to their ability to parasitize soybeans and likewise resistant soybean cultivars are not equally resistant to all SCN populations. This is the basis of the "race" system of the past and the current HG (Heterodera glycines)-Type system. As with many diseases, resistance does not imply immunity to SCN. It implies the ability of a resistant variety to reduce the reproduction (cyst formation) of SCN on its roots. Resistance is rarely complete.

Currently, there are three main genetic sources for resistance to SCN: PI88788, Peking (PI54840), and Hartwig (PI437654). Of the three sources of resistance commonly available, PI88788 is in the vast majority of SCN-resistant soybean varieties. A relatively new source of resistance is found in the CystX® varieties. CystX is a selection from the Hartwig source of resistance and is being marketed as complete resistance. However, since some research has shown some reproduction of some SCN populations on CystX, it is recommended that this not be the only source of resistance used in a field. It should be included in rotation with other resistant soybean varieties. Hartwig resistance has been known for many years, but the variety Hartwig has low yields. Researchers at Purdue University separated the SCN resistance genes from the genes that caused poor agronomic performance and included the Hartwig SCN resistance source into high yielding lines. This is not a genetically modified line of soybean, but a product of the use of molecular techniques to identify a specific trait and incorporate it with traditional breeding methods.

Phytophthora Root and Stem Rot

We have seen this disease become very active in the last two years. Early planting combined with wet conditions, create a situation very conducive for Phytophthora if it's present.

Phytophthora root and stem rot on soybean.

Resistant Varieties: Use of resistant varieties is the most effective way to manage Phytophthora root and stem rot of soybean. Genetic resistance in the host is expressed in terms of Rps (Resistant to Phytophthora sojae) genes. The race-specific genes are complete resistance to a specific race of P. sojae and genes are denoted as Rps 1a, 1b, 1c, 1d, 1k, 3, 6, 7. The pathogen exists in races or biotypes that interact with these genes. A race is identified by its interaction with the eight known Rps genes.

Soybean varieties are marketed on the basis of their genetic make-up in relation to the predominant Phytophthora sojae races in a given area. The predominant races in Nebraska are 3 and 33. The most widely available resistance genes in the Midwest are 1c and 1k, commonly referred to as "c" or "k" in seed company literature. Gene 1c protects against races 1, 3, 8, 9, 13, 23, 28, 41, and 44 where gene 1k protects against races 1, 3, 4, 5, 8, 9, 13, 14, 18, 23, 43, and 44. Gene 3 is the only gene that protects against 99% of the races that occur in Nebraska. Growers should consult local seed company representatives to request varieties with different Rps genes than marketed in a specific area.

The other parameter on which soybean varieties are rated for P. sojae is partial resistance (also called field resistance or tolerance). Soybean varieties with high levels of partial resistance can become infected with P. sojae but the symptoms are not as severe as highly susceptible varieties. In field research trials conducted in Nebraska, good partial resistance performed as well as varieties with resistance genes and partial resistance. In fields where the P. sojae biotype is aggressive against the resistance genes available in commercial varieties, this is the only choice for management with genetics. If possible, a combination of good partial resistance and an Rps gene are recommended. Partial resistance alone will not be as effective during early growth stages or under high disease pressure.

Producers should keep records on which fields they identify with Phytophthora so that they can select soybean varieties with resistance and/or tolerance to the disease. This disease will not affect corn. Fields with a long-term history of Phytophthora may require a different resistance gene if you are noticing Phytophthora killed plants when you grow resistant varieties. In addition, fields with a history of Phytophthora will require additional levels of Mefenoxam chemistry above the standard rate. See extension publication entitled, Management of Phytophthora Diseases of Soybean (NF02-518).

Loren J. Giesler
Extension Plant Pathologist