2010 Annual Report
1a.Objectives (from AD-416)
Three of the most significant field diseases of sugarbeet in the U.S. are root rot, caused by Aphanomyces cochlioides; Rhizomania, caused by a fungal/viral complex; and wilt, caused by the sugarbeet cyst nematode, Heterodera schachtii, and concomitant infection by Fusarium fungi. The objectives of this project are to investigate methods to reproduce the field diseases of these pathogens in controlled environments, to develop qualitative and quantitative detection reagents and protocols for these organisms, and to determine genetic changes in viruses of the Rhizomania complex that condition heightened virulence to sugarbeet. Since the incorporation of natural genetic resistance into crops remains the most cost-effective strategy for disease control, an additional objective of the project is to obtain molecular genetic tags for disease resistance genes in sugarbeet in collaborative studies with ARS sugarbeet geneticists and pathologists.
1b.Approach (from AD-416)
Gradients of saturation across seedbeds will be tested as a means to evaluate sugarbeet varieties with known resistance to Aphanomyces cochlioides using stand loss as a measure of disease severity. Protocols for the inoculation of sugarbeet with Polymyxa betae will be modified to select for clonal isolates of the organism, an aspect lacking in past studies on this pathogen. Probe primers will be designed to perform in conjunction with specific primer sets in the development of real-time PCR (qPCR) methods for quantifying these pathogens in soil and plant samples. Disruptions (insertions) in the chromosomes of beet black scorch virus and beet necrotic yellow vein virus will be engineered in efforts to determine the role of virus genes in pathogen virulence. Plants typed for either resistance or susceptibility to the sugarbeet cyst nematode and Fusarium stalk blight will be subjected to DNA fingerprinting for generation of molecular markers linked to resistance genes.
Progress was made in all three Objectives and their Sub-objectives. In Sub-objective 1a, we made significant progress in the development of methods to reliably quantify pathogen levels in inoculated sugarbeet plants. The ability to reproduce sugarbeet diseases in controlled settings has important implications for understanding pathogen biology as well as future disease resistance screening strategies. Techniques to quantify pathogen levels relied on molecular technologies that have good lab-to-lab reproducibility and were shown to consistently quantify and discriminate pathogens from other microorganisms in soil and plant tissue. These technologies also address Subobjective 1b to develop protocols for the specific detection of important sugarbeet pathogens and vectors. Protocols and molecular tools were established to detect sugarbeet fungal and viral pathogens as well as vectors that transmit disease. The use of such technology allows for the detection of pathogens in the soil prior to disease onset, an important component of an integrated disease management strategy. Under Subobjective 2a, we made significant progress towards protein analysis of Cercospora beticola, the most economically important foliar pathogen of sugarbeet. By optimization of C. beticola growth conditions in vitro, we have made substantial gains in our efforts to induce pathogen protein production without the host plant present, thereby simplifying downstream protein identification. Techniques to confirm the biological role of pathogen proteins for sugarbeet colonization have confirmed that the developed protocols to induce protein production are appropriate. Under Subobjective 2b, we made significant progress in characterizing C. beticola mating type genes. Characterization of the mating type locus and mating type gene expression has important implications for population biology of this pathogen and the results suggest that sexual reproduction is likely in field populations and may explain the high levels of genetic diversity found in this species. Under Subobjective 3b, we have made significant progress towards determining efficacy for several fungicides using previously identified growth conditions that were optimized for Rhizoctonia root rot disease development in sugarbeet. These studies have identified new fungicide chemistries that can be added to the short list of fungicides that are currently used to combat this disease.
Characterization of the mating type locus. Cercospora beticola is the causal pathogen that is responsible for sugarbeet leaf spot; a disease costing sugarbeet producers millions of dollars annually. Field isolates of Cercospora beticola are well-known for high levels of variability, particularly with regard to morphological characteristics and fungicide resistance. Such variability typically only occurs in fungal species that reproduce sexually. Despite this, C. beticola is only known to reproduce asexually. ARS scientists at the Sugarbeet and Potato Research Unit in Fargo, ND have characterized the mating type genes of C. beticola, genes that are known to be required for sexual reproduction. Our results suggest that C. beticola mating type genes are still active and may play a role in sexual reproduction of this fungus. This information is useful for plant breeders when selecting parents for developing disease resistant sugarbeet and is useful for plant pathologists to help explain how C. beticola is able to gain resistance to fungicides.
Identification of fungicides to control Rhizoctonia root and crown rot of sugarbeet. Rhizoctonia root and crown rot is an increasing problem for sugarbeet growers in the United States. Currently, chemical control is the primary means to control this disease. However, only one fungicide is used in most sugarbeet growing areas, increasing the probability for fungicide resistance to develop in Rhizoctonia solani. Using conditions identified as optimal for disease development, ARS scientists at the Sugarbeet and Potato Research Unit in Fargo, ND screened several fungicides for efficacy to control this disease. The results have shown at least two additional fungicides are available that provide protection against this disease. The information is useful for growers through the expansion of fungicide possibilities to control this disease.
Weiland, J.J., Chung, K., Suttle, J.C. 2010. The Role of Cercosporin in the Virulence of Cercospora ssp. to Plant Hosts. In Lartey, R.T., Weiland, J.J., Panella, L., Crous, P.W., Windels, C.E., editors. Cercospora Leaf Spot of Sugar Beet and Related Species. St. Paul, MN: The American Phytopathological Society. p. 109-117.
Bolton, M.D., Panella, L.W., Campbell, L.G., Khan, M.F. 2010. Temperature, Moisture, and Fungicide Effects in Managing Rhizoctonia Root and Crown Rot of Sugar Beet. Phytopathology. 100(7):689-697.