- Agronomic: potatoes
- Crop Production: application rate management
- Education and Training: demonstration, on-farm/ranch research, workshop
- Pest Management: allelopathy, field monitoring/scouting, genetic resistance, prevention
Marker-assisted selection was performed on 605 potato progeny using PCR to screen for the late blight resistant gene RB. 55% of the population was removed, leaving 275 potatoes with the RB gene for further evaluation in the field. Field days and presentations were given to Wisconsin potato growers that increased their knowledge of the concepts used in marker-assisted selection. Specifically, growers increased their knowledge about resistance genes, marker-assisted selection and how the RB gene will be beneficial to them in the future.
Potato late blight, a disease caused by the oomycete pathogen Phytophthora infestans, is one of the world's most devastating plant diseases. None of the currently grown potato cultivars in the United States has adequate resistance to late blight. In major epidemic years fungicides have to be applied weekly to control the disease. The cost for fungicide is easily in the range of $4-8 million per season in Wisconsin alone plus at least $4 million to cover the cost of application (Stevenson, 2003, The Badger Common’tater 55 (9): 10-11). $70-140 million is spent for fungicide against late blight in the U.S. Such heavy fungicide applications lead to significantly less profitable potato production and also have a negative environmental impact. Utilization of late blight resistant varieties is the most cost-effective and environmentally friendly method to control late blight. Therefore, it is appropriate that we use faster breeding methods to produce a potato that will have adequate protection against late blight and have a positive impact on the environment.
At least 11 resistance (R) genes originated from Solanum demissum were incorporated into numerous potato varieties. However, these R genes confer race-specific resistance. Potato cultivars possessing such R genes are not resistant to all isolates of the pathogen, only those containing the specific avirulence gene recognized by P. infestans. Race-specific R genes often only provide short-lived resistance in the field as new virulent races of the pathogen rapidly overcome the resistance. A wild diploid potato species, Solanum bulbocastanum, is highly resistant to all races of P. infestans even under intense disease pressure (Helgeson et al. 1998). Somatic hybrids between cultivated potato and S. bulbocastanum were developed (Helgeson et al. 1998). Backcrossed progenies derived from the somatic hybrids showed durable and high level resistance in the field. A major resistance gene, RB, was mapped to chromosome 8 of S. bulbocastanum (Naess et al. 2000). The RB gene has recently been cloned and transgenic Katahdin (a highly susceptible variety) plants containing the RB gene showed broad-spectrum resistance against various strains of P. infestans in both greenhouse and field tests (Song et al. 2003). These results demonstrate that we should be able to develop potato varieties with high level of late blight resistance by introgressing the RB gene using a conventional breeding approach, such as marker-assisted selection.
A few of the somatic hybrids between S. bulbocastanum and potato are female fertile and have been used in the Wisconsin Potato Breeding Program. Backcrossed progenies between the somatic hybrids and elite breeding lines have been generated and used as parents. Each year approximately 5,000 seedlings are generated from these crosses. These seedlings have been pooled into the main breeding populations for selection. Large backcross three populations have also been created and are currently being evaluated for agronomic traits in order to be used as parents in further generations. Elite backcross three parents will be important for reducing the amount of linkage drag associated with the S. bulbocastanum genetic background. The major disadvantage of selection within the breeding lines created using the backcross material is not being able to select for disease resistance. We are unable to inoculate in the field, thus the only measure of late blight resistance is obtained during years of natural epidemics. Due to the large size of early breeding generations, it is also not possible to evaluate all lines.
A genetic marker has therefore been developed to easily track the presence of the RB allele in each individual potato and its subsequent progeny in the next generation. A 100% correlation was found between the presence of this marker and the late blight resistance phenotype. DNA is isolated from tuber shoots or plant leaves and a simple PCR reaction is performed. Hundreds of lines can be screened within one week, as apposed to the limiting field screens. Selection has been performed on populations known to be resistant (tests were performed with a detached leaf assay), as well as on more advanced field generations that have only undergone selection based upon agronomic performance (Colton et. al. 2006). It has been determined that the RB gene is transferred to the next generation at a rate of thirty percent, which is consistent with a monogenic allele being transferred in a tetraploid state.
First, this proposals aim is to continue selections upon the backcross three population, as well as the remaining S. bulbocastanum–related breeding lines that have been accumulated in our breeding program. We will analyze the ~500 single hills selected in the 2004 season. We will discard all of the clones that are negative for the marker. The future evaluation and selection of the marker-positive materials will be solely based on their agronomic performance. This will facilitate faster selection of environmentally friendly, RB-containing potatoes and help move these potatoes to the farmer in a more efficient manner.
Second, an initiative to inform the farmer about current molecular breeding methods will be undertaken. A field day with surveys will be conducted. Information and demonstrations will be used to illustrate the effectiveness of marker-assisted selection and the benefits of potatoes with the RB gene will be highlighted. Markers have been being developed for selections since the 1980’s, but few growers understand the value of this technique. Transgenics will be discussed and how these differ from marker-assisted selection, and how they are similar and are tracked using the same methods.
The short-term results that will be measurable by the end of this project will be a change in the potato grower’s knowledge regarding marker-assisted breeding. Growers will be informed of the marker-assisted selection process used to screen out only RB potatoes in the first generation of selection. A demonstration of how this narrows down the genetic base for the next generation will be performed. Somatic-hybrid development and subsequent breeding line development will be addressed. Growers will also become aware of the differences between somatic hybrids and transgenic or genetically modified potatoes.
Intermediate results will be a general understanding of how marker-assisted breeding benefits the grower. They will have an understanding that by using this form of breeding, cultivars with important characteristics will be available faster. Growers will have a better understanding of what type of resistance is offered by the RB gene and that less fungicide will need to be used.
Long-term outcomes from this project will be potato varieties containing RB-gene mediated late blight resistance.