Submitted to: Journal of Sugarbeet Research
Publication Type: Peer reviewed journal
Publication Acceptance Date: 6/10/2003
Publication Date: 6/1/2004
Citation: Dyer, A.T., Szabo, L.J., Windels, C.E. 2004. Characterization and spatial distribution of Aphanomyces in sugarbeet fields. Journal of Sugarbeet Research. 41:1-16. Interpretive Summary: Fungal root rot causes a significant perennial loss in sugarbeet yield in the United States and has increased in prevalence and severity over the last 10 years. The Red River Valley of Minnesota and North Dakota and west central Minnesota is the leading region in sugarbeet production in the U.S. In 2000, losses in sugarbeet yield and quality due to fungal root rot were conservatively estimated at $18 million in direct losses to producers. Control of this disease consists of partially resistant cultivars, seed treatment, early planting, control of weed hosts, and long rotations between sugarbeet crops. However, effective development and employment of control measures requires a better understanding of the fungal pathogen. Collections were made from soil samples from Minnesota, North Dakota, and Texas. Morphological and host range studies confirmed the identity of the sugarbeet root rot pathogen as Aphanomyces cochlioides. This work confirmed that this fungal pathogen is specific for sugarbeets but did not infect pea, oat, or tomato. A DNA marker was identified that was specific to the majority of samples from Texas and was not found in samples collected from North Dakota or Minnesota. Analysis of the spatial distribution indicated that in a North Dakota sugarbeet field this pathogen was evenly distributed, while in Minnesota and Texas this fungal pathogen tended to be aggregated in regions of the field. The results reported in this manuscript will be used by scientists and provide them with a better understanding of the fungal pathogen causing sugarbeet root rot. In addition, this work provides a DNA marker, which may be important in understanding differences in populations of this pathogen across fields and across the U.S.
Technical Abstract: To study phenotypes and spatial distribution of Aphanomyces in sugarbeet fields with histories of root rot, 80 soil cores were collected 1-m apart along a transect in one field each in Minnesota (MN), North Dakota (ND), and Texas (TX). Aphanomyces was isolated from 52, 84, and 49% of the soil cores collected in MN, ND, and TX, respectively. Random amplified polymorphic DNA (RAPD) analysis was completed on 41, 41, and 39 isolates from MN, ND, and TX, respectively, with A. eutieches as on outgroup. A single polymorphic (1,400 bp) RAPD product was identified specific to 56% of the Texas population. Oogonia of isolates from sugarbeet had a mean diameter of 26.1 micrometers compared to two isolates of A. eutieches that had a mean diameter of 26.1 micrometer. RAPD analysis, morphology, and pathogenicity were consistent with descriptions of a single species A. cochlioides. Isolates of Aphanomyces from sugarbeet were highly pathogenic on sugarbeet (Root rot index, 0-100 scale, ranged from 67-100, n=12) but not on pea, oat, and tomato. Isolates of A eutieches were pathogenic only on pea. Ordinal runs analysis detected aggregation of A. cochlioides at MN and TX (P=0.029 and P=0.002, respectively) while isolates at ND were uniform (P=0.89).