There are two types of resistance, race specific resistance and partial resistance, that are incorporated into commercial varieties. These two types of resistance have very different appearance in the field. Both can be effective in limiting losses due to soybean pathogens. But each has its limitations.
This type of resistance is typically controlled by a single dominant gene. When the soybean has this single gene, the pathogen can no longer successfully infect the plant. However, pathogens are biological organisms with the capacity to adapt to previously resistant varieties. Over time, the number of individuals in the pathogen population that recognize the gene in the soybean will begin to build. When this happens, the soybean variety is no longer resistant, but is susceptible. The resistance gene acts like a lock on a door and once the pathogen has found the "key," it can infect and cause as much disease as if there were no lock on the door. Resistance genes in soybeans can be very effective and may last several years in a field, but once the pathogens can recognize them (have the key), the genes will no longer be effective.
To illustrate this point, Rps1a was deployed in commercial soybean varieties in Ohio in the 1960s. It was effective in limiting disease losses. It was not planted during the 1980s and 90s. In 1998, as a result of the shortage of Round-Up ready soybeans, some fields were planted with a soybean variety that had Rps1a. Following heavy rains, Phytophthora root rot developed extensively throughout the field. This Phytophthora population had found the "key" to the Rps1a gene.
|
| Figure 2. Specific Rps genes have an all-or-nothing type of resistance to P. sojae. A variety with an Rps gene is either resistant or susceptible to any given race of P. sojae. The plant on the left is resistant, and the plant on the right is dying. |
Races of plant pathogens are determined by the number of resistance genes that they can defeat. In any specific field, many different races exist. Table 1 illustrates the number of different races of Phytophthora sojae that were identified in one field in Ohio. The same situation exists for soybean cyst nematode, which also is controlled with specific resistance genes in soybeans.
| Table 1. Races of Phytophthora sojae Identified in One Field in Ohio. | |
| P. sojae Race | Commercial Rps Genes That Are Susceptible |
| 38 | 1a, 1b, 1c, 1d, 1k, 3a, 6, 7 |
| 4 | 1a, 1c, 7 |
| 46 | 1a, 1c, 3a, 7 |
| 21 | 1a, 3a, 7 |
| 3 | 1a, 7 |
| new | 1a, 1c, 1k, 7 |
| 7 | 1a, 3a, 6, 7 |
| 34 | 1a, 1k, 7 |
There is concern about the long-term effectiveness of resistance genes for the soybean cyst nematode (SCN) in that very few sources of resistance have been identified. Current resistance genes originate from only three soybean sources — PI88788, Peking, and Hartwig — that have been incorporated into high-yielding soybean varieties. In order to maintain their effectiveness, it will be necessary to rotate among the different resistance sources in fields that have populations of soybean cyst nematode. The reasoning behind this strategy is that by rotating resistance sources, we will avoid building up a population of soybean cyst nematode that can reproduce at high rates on a particular source of resistance. It is not advisable to utilize soybean varieties that have combined resistance sources for SCN. The strategy of rotating sources of resistance will only work for those fields where populations of SCN have not adapted to the sources of resistance (PI88788, Peking, or Hartwig).
Back | Forward | Table of Contents
All educational programs conducted by Ohio State University Extension are available to clientele on a nondiscriminatory basis without regard to race, color, creed, religion, sexual orientation, national origin, gender, age, disability or Vietnam-era veteran status.
Keith L. Smith, Associate Vice President for Ag. Adm. and Director, OSU Extension.
TDD No. 800-589-8292 (Ohio only) or 614-292-1868