Jason R. Brantner

Variability in sensitivity to metalaxyl in vitro, pathogenicity, and control of Pythium spp. on sugar beet

Pythium ultimum var. sporangiiferum (76 isolates) and P. aphanidermatum (21 isolates) cultured from diseased sugar beet seedlings in Minnesota and North Dakota were tested for sensitivity to metalaxyl, pathogenicity on sugar beet, and disease control by metalaxyl seed treatment. Sensitivity to metalaxyl (effective concentration causing 50% growth inhibition [EC50]) was determined by linear growth on corn meal agar amended with 0, 0.01, 0.1, 1, 10, and 100 μg a.i. metalaxyl ml-1 after 48 h in the dark at 21 ± 1°C. Variation among isolates was significant (P = 0.05) within and between species, and EC50 values averaged 0.16 (range: 0.05 to 1.30 μg ml-1) for P. ultimum var. sporangiiferum and 2.06 (range: 1.19 to 3.12 μg ml-1) for P. aphanidermatum. In pathogenicity tests on sugar beet, most isolates of P. ultimum var. sporangiiferum (72 of 76) and all of P. aphanidermatum significantly (P = 0.05) decreased final stands compared to the noninoculated control. There was no correlation between aggressiveness in the absence of metalaxyl and in vitro sensitivity to metalaxyl. When Pythium-infested soil was planted with seed treated with metalaxyl at the standard (0.625 g a.i. kg-1) or half rate, some isolates that were least sensitive to metalaxyl in vitro resulted in a significant (P = 0.05) reduction in disease control. These results may explain, at least in part, why producers do not attain expected stands when they plant metalaxyl-treated sugar beet seed.

First Report of Fusarium oxysporum Causing Yellows on Sugar Beet in the Red River Valley of Minnesota and North Dakota

In 2002, somel sugar beet (Beta vulgaris L.) fields in the Red River Valley (RRV) of Minnesota and North Dakota had symptoms characteristic of Fusarium yellows (4). In 2004, ≈5% of fields in the RRV had symptomatic plants. Interveinal yellowing of older leaves typically began in mid-July and as the disease progressed, younger leaves turned yellow. Sometimes, one side of the leaf was yellow or necrotic while the other side remained green. As leaves died, they remained attached to the crown. Transverse sections of roots revealed a light gray-brown discoloration of the vascular tissue but no external rotting of roots. Isolations from 35 symptomatic roots collected in eight fields yielded 25 isolates identified as F. oxysporum (from single conidia grown on homemade potato dextrose agar and carnation leaf agar) (3). Pathogenicity was determined by dipping roots of 5-week-old sugar beet plants (cv. ACH 9363) in a suspension of 104 conidia per ml for 8 min (12 isolates, 10 to 12 plants per isolate). Plants were planted in Cone-tainers (3.8 cm diameter × 21 cm; Stuewe and Sons, Inc. Corvallis, OR) containing sterile soil. Three known cultures of F. oxysporum Schlecht. emend. Snyd. & Hans. f. sp. betae Stewart (= F. conglutinans var. betae Stewart [4]) also were included (13 and 216c from L. Hanson, USDA-ARS, Fort Collins, CO; 0-1122 from The Pennyslvania State University Fusarium Research Center). The control was sterile water. Plants were placed in a greenhouse at 24 to 27°C with natural light supplemented with illumination from high-pressure sodium-vapor lamps for 16 h daily and lightly fertilized biweekly to avoid chlorosis from nutrient deficiency. After 6 to 7 weeks, plants were rated for disease on a 0 to 4 scale: 0 = no disease; 1 = slight to extreme plant stunting, leaves may be wilted; 2 = chlorotic leaves, some with necrosis at margins; 3 = tap root dried and brown to black in color, leaves dying; and 4 = plant dead (1). The experiment was repeated. Disease severity differed between trials, but all isolates of F. oxysporum and F. oxysporum f. sp. betae resulted in disease ratings statistically (P < 0.05) greater than that of the water control. In Trial 1, isolates of F. oxysporum averaged a rating of 2.1 (range of 1.8 to 3.3) and F. oxysporum f. sp. betae averaged 2.1 (range of 2.0 to 2.2) compared with 0.1 for the water control. One isolate of F. oxysporum had a statistically higher rating than did the cultures of F. oxysporum f. sp. betae. In Trial 2, isolates of F. oxysporum averaged a rating of 3.3 (range of 2.7 to 3.7) and F. oxysporum f. sp. betae averaged 3.1 (range of 2.7 to 3.4) compared with 0.2 for the water control. Cultures of F. oxysporum (8 of 12) resulted in ratings statistically higher than that of the least pathogenic culture of F. oxysporum f. sp. betae. Cultures of F. oxysporum and F. oxysporum f. sp. betae recovered from inoculated plants were identical to those used to inoculate plants. To our knowledge, this is the first report of F. oxysporum f. sp. betae on sugar beet in the Red River Valley of Minnesota and North Dakota. The disease has been reported in California, Colorado, Montana, Nebraska, Oregon, Texas, and Wyoming (1,2). References: (1) R. A. Cramer et al. J. Phytopathol. 151:352, 2003. (2) G. A. Fisher and J. S. Gerik. Phytopathology 84:1098, 1994. (3) P. E. Nelson et al. Fusarium Species: An illustrated Manual for Identification. The Pennsylvania State University Press. University Park, 1983. (4) D. Stewart. Phytopathology 21:59, 1931.

Comparison of fungal brown spot severity to incidence of seedborne Bipolaris oryzae and B. sorokiniana and infected floral sites on cultivated wild rice.

The incidence of Bipolaris oryzae and B. sorokiniana in cultivated wild rice seed collected from two fields in Minnesota during 1991 and 1992 was related to disease severity on leaves. The site of Bipolaris spp. on the seed was primarily the awns. Caryopses were not infected under the conditions of this study. This is the first evidence for B. oryzae and B. sorokiniana being seedborne on cultivated wild rice and located primarily in the external tissues of the seed

Fungicide registration and a small niche market : A case history of hymexazol seed treatment and the U.S. sugar beet industry

The United States ranks among the top four sugar producers worldwide, and sugar beet (Beta vulgaris L.) plays a major role in the sweetener industry. Sugar beet was planted on approximately 553,100 ha (1.37 million acres) in 2006 (33). The greatest volume of production occurs in the Red River Valley (RRV) of Minnesota and North Dakota and in southern Minnesota. In 2005, this region planted 302,000 ha (746,000 acres) of sugar beet, which accounted for over half of the hectares sown (58%) and metric tons of roots produced (51%) in the United States (33); total economic impact of the crop exceeded

Aggressiveness of Rhizoctonia solani AG 2-2 on sugar beet and rotation crops

3 billion (4). Three regions, including nine additional states, comprise the remainder of the production areas (Fig. 1). They include the Far West (California, Idaho, Oregon, Washington), Great Plains (Colorado, Montana, Nebraska, Wyoming), and Great Lakes (Michigan; Ohio stopped production in 2005), which produced 25, 13, and 12% of the total metric tons of roots harvested in 2005, respectively (33). Overall, the annual impact of producing and processing sugar beet in the United States contributes

IN-FURROW FUNGICIDE AND STARTER FERTILIZER APPLICATION EFFECTS ON SUGARBEET STAND ESTABLISHMENT

4.5 billion to the economy and over 79,000 full-time equivalent jobs (30).

Efficacy of in-furrow and post-emergence fungicides in controlling Rhizoctonia on sugar beet

Rhizoctonia crown and root rot (RCRR) of sugarbeet, caused by the soilborne fungus Rhizoctonia solani, is increasing in prevalence in the United States, Europe, and other countries (1,2). The fungus is composed of 13 genetically isolated populations called anastomosis groups or AGs (8). The primary population attacking sugarbeet is R. solani AG 2-2, which is further divided into the intraspecific groups (ISGs) AG 2-2 IV and AG 2-2 IIIB. Both ISGs occur in Minnesota and North Dakota (1) and produce identical symptoms of RRCR on sugarbeet. The ISGs of AG 2-2 are identified by growth on culture media at 95 0 F; AG 2-2 IIIB grows at this temperature but AG 2-2 IV does not (8). According to the literature, AG 2-2 IIIB is more aggressive and has a wider host range (e.g., bean crops, corn) than AG 2-2 IV (3-6, 8).

Spent lime effects on yield, quality, and Aphanomyces root rot of sugar beet

Rhizoctonia crown and root rot (RCRR), caused by Rhizoctonia solani AG 2-2, is a common root disease on sugarbeet. Postemergence applications of azoxystrobin typically are used for disease control, but in-furrow (I-F) fungicides provide excellent control without application timing concerns. Stand establishment problems, however, are a concern, especially when applied in combination with starter fertilizer. The objective of this research was to evaluate the effect on sugarbeet stand and sucrose yield of fungicides applied in-furrow either down the drip tube or by t-band, alone and in combination with 3 GPA 10-34-0 starter fertilizer at two planting dates in both 2011 and 2012. The experiment was set up in a randomized block design with 4 replicates. Plots were six rows (22-inch row spacing) by 35 ft and seed was sown at 4.5-inch seed spacing. Fungicides (azoxystrobin, pyraclostrobin, and penthiopyrad) were applied in-furrow, alone and in combination with starter fertilizer (6 GPA total volume), by two methods: 1) down the drip tube at 18 psi and 2) in a t-band (Teejet 400067E nozzle at 30 psi) positioned directly behind the disc openers. Application of starter fertilizer was always down the drip tube. A no-fungicide control with and without starter fertilizer was also included. Planting dates one and two were May 16 and 25, respectively, in 2011 and April 19 and May 8, respectively, in 2012. All trials were sown at the University of Minnesota, Northwest Research and Outreach Center in Crookston. Sugarbeet stand data were collected from one to six wk after planting in 2011 and two to six wk after planting in 2012. The center two rows of each plot were harvested September 26 for all trials and data were collected for RCRR, number of harvested roots, yield, and quality. Data were subjected to analysis of variance for interactions of starter fertilizer x fungicide x fungicide application method, fungicide x application method, fungicide x starter fertilizer and main effects of starter fertilizer, fungicide, and fungicide application method. In both trials in 2011, there were significant fungicide x application method interactions for most stand counts, but no significant starter fertilizer x fungicide x fungicide application method or starter x fungicide interactions. Azoxystrobin applied by t-band and pyraclostrobin applied down the drip tube reduced stands compared to no fungicide for both planting dates in 2011 and penthiopyrad applied down the drip tube reduced stands compared to no fungicide in the second planting date of 2011. In the first planting date of 2012, there was a significant three-way interaction (starter fertilizer x fungicide x fungicide application method) for most stand count dates. Azoxystrobin and pyraclostrobin applied by t-band reduced stands when starter fertilizer was also applied compared to the same fungicides applied down the drip tube or the same fungicides without starter. Vertisan reduced stand slightly when applied with starter compared to without starter regardless of application method. In the second planting date in 2012, conditions were very dry, and interactions and main effects were rare. There was a significant main effect of starter fertilizer application on stand data in all four trials. Stands at four weeks after planting in each trial with no starter and starter, respectively, averaged 201 and 189, 160 and 148, 191 and 162, and 102 and 84 per 100 ft of row. Main effects and their interactions were not significant for RCRR or most harvest parameters in the four trials. Overall, impact of starter fertilizer was more consistent in reducing stands than application of in-furrow fungicides, which varied by application method.

Early-season Application of Azoxystrobin to Sugarbeet for Control of Rhizoctonia solani AG 4 and AG 2-2

Rhizoctonia damping-off and crown and root rot (RCRR) caused by Rhizoctonia solani AG 2-2 are increasing on sugarbeet in Minnesota and North Dakota. This soil-borne fungal pathogen can cause disease throughout the growing season and reduces stands and sucrose yield and quality. Several control options, when combined, help to reduce disease and include planting partially resistant varieties, cultural practices (e.g., early planting, rotation with cereal crops), and application of fungicides.

PREVIOUS CROP INFLUENCES RHIZOCTONIA ON SUGARBEET

Aphanomyces cochlioides (= A. cochlioides) is a serious economic pathogen and infests over 50% of acres planted to sugarbeet in the Red River Valley (RRV) and most acres in southern Minnesota. When soil is warm and wet, A. cochlioides causes damping-off of seedlings and root rot of older plants. Storage of diseased roots in piles contributes to additional losses. A. cochlioides persists in soil for years. Consequently, growing sugarbeet requires all available control options including early planting of resistant varieties treated with the fungicide Tachigaren and various cultural practices (e.g., cultivation and improved drainage) to avoid or lessen infections by A. cochlioides. However, when inoculum levels of the pathogen are high and soil is wet, implementation of these measures is inadequate for economic yields and fields often are abandoned or yield poorly. This chronic situation has generated interest in finding effective, alternative methods to control A. cochlioides.