Disease Incidence and Losses in 2006
The spread of soybean rust northward through states along the Atlantic Coast began on soybeans in Alabama, Georgia, and Florida. The disease was first reported in South Carolina on 21 August, North Carolina on 14 September, and Virginia on 9 October. The epidemic of 2006 was far reaching in that disease outbreaks occurred on soybeans as far north as Illinois and Indiana and east to Virginia (Fig. 1).
No significant losses of yield were expected as a result of soybean rust in Virginia due to low incidence and late appearance of disease. Essentially, all soybean fields were either at or beyond R6 (full seed stage) when the disease was detected. Nematodes had the greatest impact on yield based on diagnostic tests performed in the plant disease clinic at the Tidewater Agricultural Research and Extension Center (AREC) and field observations in Eastern Virginia (Table 1). Soybean cyst, southern and northern root-knot and stubby root nematodes accounted for the greatest losses of yield. Leaf spot diseases (frogeye leaf spot, anthracnose,Cercospora blight) showed low incidence in 2006 as a result of dry weather stress in July and August. Soybean yields averaged 31 bu/A in 2006 on 500,000 acres.
|Table 1. Estimated loss in yield as a result of soybean diseases in 2006.|
|Disease||Causal agent(s)||Percent loss|
|Downy mildew||Peronospora manshurica||Trace|
|Soybean rust||Phakopsora pachyrhizi||0.0|
|Frogeye leaf spot||Cercospora sojina||0.4|
|Phytophthora root & stem rot|
|Pod & stem blight||Diaporthe phaseolorum var. sojae||0.1|
|Stem canker||Diaporthe phaseolorum var. caulivora||Trace|
|Sudden death syndrome||Fusarium solani f.sp. glycines||Trace|
|Sclerotinia stem rot||Sclerotinia sclerotiorum and S. minor||0|
|Southern blight||Sclerotium rolfsii||0.1|
|Root & lower stem rot||Rhizoctonia solani||Trace|
|Purple seed stain||Cercospora kikuchii||0.1|
|Cercospora blight||Cercospora kikuchii||0.6|
|Brown spot||Septoria glycines||0.2|
|Red crown rot||Cylindrocladium parasiticum||0.2|
|Brown stem rot||Phialophora gregata||0.1|
|Charcoal rot||Macrophomina phaseolina||Trace|
|Viruses SMV,||PeMoV, BPMV, etc.||0.1|
|Bacterial pustule||Xanthomonas campestris pv. glycines||Trace|
|Bacterial blight||Pseudomonas syringae pv. glycinea||0.2|
|Southern root knot nematode||Meloidogyne incognita||1.8|
|Soybean cyst nematode||Heterodera glycines||2.2|
|Total loss (%)||8.1*|
|* The loss estimate equals 1.366 million bushels based on production of 15.5 million bushels in 2006. At a value of $6.00/bu, the loss in revenue at the farm gate was $8.2 million in 2006.|
Ten regional sentinel plots and 40 commercial fields were scouted from flowering up to beginning senescence for early detection of soybean rust in 2006. A total of 363 samples of 100 leaflets were processed by microscopic examination; 212 sentinel plot and commercial samples at the Tidewater AREC, 97 sentinel plot and commercial samples at the Eastern Shore AREC, and 54 sentinel plot samples by the Virginia Tech Department of Plant Pathology, Physiology, and Weed Science (PPWS) in Blacksburg. Sentinel plots were located at the Tidewater AREC (Suffolk), Southampton County (Courtland), Eastern Shore AREC (Painter), Northampton County, Shenandoah County, Southern Piedmont AREC (Blackstone), New Kent County, Northern Piedmont AREC (Orange), Eastern Virginia AREC (Warsaw), and Virginia Tech – Kentland Farm (Blacksburg).
Leaf samples were collected and shipped overnight by site cooperators to the Tidewater AREC, Eastern Shore AREC, or the department of PPWS for processing. Upon receipt, the samples were placed in moist chambers at room temperature (70º to 77°F), incubated for 3 to 5 days, and examined under a stereomicroscope for pustules of soybean rust.
Weekly examinations of leaf samples from 10 sentinel plots and field scouting in 40 commercial fields resulted in detection of the first outbreaks of soybean rust in Chesapeake on 9 October and in the sentinel plot at Suffolk on 10 October. Thereafter, intensive scouting up to 15 November confirmed incidence of the disease in a total of 18 counties (Suffolk, Chesapeake, Virginia Beach, Isle of Wight, Southampton, Greensville, Brunswick, Mecklenburg, Sussex, Surry, Prince George, King and Queen, New Kent, James City, Gloucester, Middlesex, Accomack, and Northampton). These findings represented the first report of soybean rust, caused by Phakopsora pachyrhizi, in Virginia. Photographs of leaflets were taken to illustrate the small size of lesions and the need for a microscope to find rust pustules (uredinia) and spores for disease detection (Fig. 2).
Further confirmation of positive samples with pustules on leaflets was obtained by ELISA and PCR tests in the laboratory. ELISA tests were run on 164 samples and PCR tests were run on six samples. The initial finds of soybean rust on 9 October were also confirmed by submitting leaf samples to the USDA Animal Plant Health Inspection Service lab in Rockville, Md.
Five spore traps were monitored weekly for early detection of airborne rust spores moving into Virginia by wind currents. Spore traps were located at the Tidewater AREC (Suffolk), Eastern Virginia AREC (Warsaw), Northern Piedmont AREC (Orange), Eastern Shore AREC (Painter) and Virginia Tech (Blacksburg). A freshly greased microscope slide was placed in each trap weekly from 12 June up to 10 October. Slides were replaced weekly and sent to John Rupe (University of Arkansas) for microscopic examination and count of rust spores. Table 2 summarizes the trapping dates when rust spores were found on slides at specific locations, and Table 3 provides a monthly summary of findings in 2006. The viability of spores could not be determined, nor could the identity be positively confirmed as P. pachyrhizi. However, the morphological traits of spores counted did conform to spores of the soybean rust fungus.
|Table 2. Occurrence of rust spores resembling Phakopsora pachyrhizi in spore traps, 2006|
|Trap location||No. positive rust spores||Trap dates|
|TAREC, Suffolk||3||Jul 3 – 10|
|NPAREC, Orange||1||Jul 10 – 17|
|ESAREC, Painter||1||Aug 3 – 10|
|Virginia Tech, Blacksburg||4||Aug 7 – 14|
|ESAREC, Painter||2||Aug 14 – 21|
|NPAREC, Orange||1||Aug 14 – 21|
|EVAREC, Warsaw||2||Aug 14 - 21|
|ESAREC, Painter||1||Aug 17 – 24|
|NPAREC, Orange||1||Sep 15 – 25|
|NPAREC, Orange||3||Sep 25 – Oct 2|
|Table 3. Monthly summary of rust spores believed to be Phakopsora pachyrhizi in spore traps, 2006|
|Month||Slides submitted||Slides w/rust spores||Total spores|
Seasonal Air Temperatures and Rainfall in 2006
Much of eastern Virginia experienced below-normal temperatures in May and above-normal temperatures in August and September. July and August were especially dry as a result of widely scattered thundershowers in eastern Virginia. These conditions caused moderate to severe drought stress in some fields especially in August when maximum temperatures ranged from 90° to 100ºF on 19 of 31 days. Overall, dry weather and above normal temperatures in July and August were believed to be responsible for reduced yields and the late appearance of soybean rust in South Carolina, North Carolina, and Virginia in 2006. Tropical Storm Ernesto brought heavy rainfall (8.04 in.) as it passed through eastern Virginia on 31 August and 2 September. This event was a major contributor to ending the drought in August and increasing total rainfall above normal for September and the entire growing season.
The optimum temperature range for leaf infection and development of rust pustules is 60° to 77ºF. Infection can occur at temperatures as low as 59º and as high as 84ºF, but will require a longer period of time at these temperatures. In addition to a favorable temperature, the fungus requires moisture (leaf wetness or > 95% relative humidity (RH) for spore germination and infection of leaflets. Only 11, 30, and 31 August were considered favorable for infection according to hourly measurements of air temperature and RH, and daily rainfall at the Tidewater AREC. Weather data from this site indicated that 24 of 30 days in September and 23 days of 31 days in October provided favorable conditions for the disease.
Table 4 summarizes seasonal temperatures at locations where fungicide trials were conducted on soybeans in 2006. All locations, except Warsaw, reported above normal rainfall for the period from May through October. Weather data in Suffolk, Capron, and Skippers were obtained from the Peanut/Cotton InfoNet (www.ipm.vt.edu/infonet). The Virginia Agricultural Experiment Station Mesonet (http://www.ahnrit.vt.edu/ProjectPortfolio/WebApplications/index.html) collected weather data at the Eastern Shore AREC at Painter and the Eastern Shore AREC at Warsaw. Normal rainfall records were obtained from annual reports by the Virginia Agricultural Statistics Service.
Plots were 30 feet long and 12 feet wide. Row spacing ranged from 12 to 36 inches, depending upon location. A randomized complete block design was used with four or five replications of treatments. Fungicides were applied with either a CO2-pressurized backpack sprayer in a 6-foot spray swath, or a Lee Spider sprayer in a 12-foot spray swath. Both sprayers were equipped with 8002VS nozzles spaced 18 inches apart and delivered a volume of 16.5 gallons per acre at 30 psi. Disease and yield data were collected from the central 4.75-foot by 30-foot long section in each plot. Standard practices for soybean production were followed after planting each trial. Plots were harvested with a self-propelled, small-plot combine. Samples of 100 seeds from each plot were weighed and seed numbers with purple seed stain, Phomopsis seed decay, and other diseases were recorded.
Tidewater AREC, Trial 106 (Phipps and Partridge). The field site was planted to S57-P1 on 25 May. The soil type was Nansemond fine sandy loam that was planted to soybean in 2005 and corn in 2004. Plots were eight 30-foot rows spaced 18 inches apart. Roundup Ultra Max at 22 fluid ounces per acre was used on 10 July for weed control. All treatments were applied using a Lee Spider sprayer. The timing of fungicide application was designed to evaluate one spray at R3 (21 August). Plots were harvested on 6 November. None of the treatments caused symptoms of chemical injury on leaves, stems, or pods. Brown spot, frogeye leaf spot and bacterial blight occurred at low levels and were not believed to reduce yield (Table 5). Cercospora blight was the most likely disease to reduce yield based on the percentages of leaf area with symptoms up to 10 October when leaflets began to show senescence. Soybean rust was not detected at the test site. Sprays of Headline, Absolute, Quilt with crop oil, Quadris with crop oil, and Stratego with Induce provided the most effective control of Cercospora blight and significantly delayed defoliation. The same treatments also significantly reduced the incidence of purple seed stain.
Tidewater AREC, Trial 206, Suffolk (Phipps and Partridge). The variety, planting date, cultural practices, and location of this trial were the same as the previous trial. All fungicides were applied with a Lee Spider sprayer in a single application at beginning seed (R3) on 21 August. Headline, Absolute, and Stratego were the most effective treatments in control of Cercospora blight (Table 6). Soybean rust was not detected in the trial. The same treatments suppressed the rate of defoliation significantly according to ratings on 10 October. None of the treatments caused visible evidence of plant injury. Portions of the trial were harvested on 27 November, 30 November, and 7 December due to delays caused by wet weather. None of the fungicide treatments significantly increased yield.
Tidewater AREC, Trial 306, Suffolk (Phipps and Partridge). The field site was planted to S57-P1 on 24 May. The soil type was Dragston fine sandy loam that was planted to corn in 2005. Plots were eight 30-foot rows spaced 18 inches apart. Roundup Ultra Max at 22 fluid ounces per acre was applied for weed control. All treatments were applied using a Lee Spider sprayer at R3 on 21 August. Plots were harvested on 20 November with a small-plot combine. None of the treatments caused symptoms of chemical injury on leaves, stems or pods. Soybean rust was not detected in the trial. Brown spot and frogeye leaf spot occurred at low levels throughout the growing season and were not believed to reduce yield (Table 7). Cercospora blight was the most likely disease to suppress yield in the untreated check based on percentages of leaf area with symptoms of disease on 10 October. All fungicide treatments resulted in significant suppression of Cercospora blight. Treatments with Quadris 6 fluid ounces per acre and Quilt 14 fluid ounces per acre + Quadris 1.5 fluid ounces per acre significantly reduced levels of defoliation on 10 October. A tank mix of Headline 4.7 fluid ounces + Folicur 3.1 fluid ounces per acre was the only treatment to increase yield significantly compared to the untreated check. All treatments resulted in significant reductions in the incidence of purple seed stain.
Tidewater AREC, Trial 406, Suffolk (Phipps and Partridge). The variety, planting date, cultural practices, and location of this trial were the same as the previous trial. All fungicides were applied with a Lee Spider sprayer in a single application at beginning seed (R3) on 21 August. Plots were harvested on 20 November with a small-plot combine. None of the treatments caused symptoms of chemical injury to leaves, stems or pods. Soybean rust was not detected in the trial. All fungicide treatments resulted in significant suppression of Cercospora blight. Application of Enable 7.1 fluid ounces per acre w/crop oil on 25 July followed by an application of Headline 7.1 fluid ounces per acre on 8 August or 11 August were significantly superior to other treatments in suppression of Cercospora blight (Table 8). All fungicide treatments showed significantly lower defoliation than the untreated check on 10 October. The highest level of leaf health and retention was observed in plots treated with Enable 7.1 fluid ounces per acre with crop oil on 25 July followed by an application of Headline 7.1 fluid ounces per acre on 8 August. Treatments with Enable followed by Headline resulted in the greatest reduction in purple seed stain.
Southampton County, Trial 506, Foxhill Farms (Phipps and Partridge). The field site was planted to cotton in 2004 and 2005. Seed of DP 5634RR were planted in rows spaced 36 inches apart on 28 May with Temik 15G 5 pounds per acre in furrow. Standard practices for production of glyphosate-resistant soybeans were followed after planting. Plots were 12 feet wide by 30 feet long and treatments were replicated in four randomized complete blocks. A single application of treatments was applied with a backpack sprayer at beginning pod stage (R3) on 17 August. Low levels of frogeye leaf spot, brown spot, and Cercospora blight were observed in disease ratings on 10 September (Table 9). Observations at beginning senescence of foliage showed moderate levels of Cercospora blight on foliage in untreated plots on 11 October. All fungicide treatments reduced Cercospora blight. The greatest level of disease suppression was observed in plots treated with Headline 6 fluid ounces per acre, Absolute 5 fluid ounces per acre, Stratego 10 fluid ounces per acre with Induce, and Quilt 14 fluid ounces per acre with crop oil. These same treatments also showed the lowest level of defoliation and fewer Cercospora lesions on stems and pods. No soybean rust was detected in the field. Domark 5 fluid ounces per acre and Absolute 5 fluid ounces per acre were the only treatments that increased yields significantly compared to the untreated check according to orthogonal contrasts. The incidence of purple seed stain was suppressed significantly by all treatments except the triazole fungicides (Domark, Laredo, Folicur).
Greensville County, Trial 606, Hawkins Farm (Phipps and Partridge). Soil at the field site was Emporia loamy fine sand planted to soybean in 2005 and cotton in 2004 and 2003. Seed of Pioneer 95B96RR were planted in rows spaced 12 inches apart on 18 May. Standard practices for production of glyphosate-resistant soybeans were followed after planting. Plots were 12 feet wide by 30 feet long and treatments were replicated in four randomized complete blocks. A single application of treatments was made with a backpack sprayer at beginning pod stage (R3) on 17 August. Low levels of frogeye leaf spot, brown spot, and Cercospora blight were observed on 11 September (not shown). Only the incidence of Cercospora blight increased to levels with potential for causing a loss of yield according to ratings on 11 October. Untreated plots showed significantly greater defoliation than fungicide treated plots on 11 October (Table 10) and 19 October (Fig. 4). Soybean rust was detected in the field at low levels of incidence on 23 October. Treatments with Absolute 5 fluid ounces per acre and Headline 6 fluid ounces per acre provided the best protection against defoliation. Soybeans were harvested on 7 December. The treatment with Absolute increased yield an average of 8.3 bushels per acre above the yield of untreated plots, but the response to treatment was significant only at P=0.1295 according to a comparison by orthogonal contrast. No significant differences in purple seed stain or Phomopsis seed blight were detected in harvested grain.
Eastern Shore AREC, Painter (Rideout and Waldenmaier).
The trials were conducted on a Bojac fine sandy loam soil (organic matter <1%). Standard practices for weed and insect control were followed in both trials. Conventional-tillage, full-season soybeans (cultivar S39N4RR) were planted on 26 May and no-till double-cropped soybeans (cultivar V622NRR) were planted on 19 July following wheat. Emergence in the double-cropped trial was affected by heavy rainfall throughout the growing season. Both trials were also inundated with 9 inches of rainfall and strong winds from Tropical Storm Ernesto on 1 September. Plots consisted of two 30-foot rows spaced 2.5 feet apart bordered by two nontreated rows. Treatments were arranged in a randomized complete block design with five (full-season trial) or four replications (double-cropped trial). Treatments were applied with a CO2pressurized backpack sprayer which delivered 20 gallons per acre at 42 psi. The spray boom had four 8002VS nozzles spaced 18 inches apart. Treatments were applied to the full-season soybeans on 7 August when soybeans were at reproductive stage R3 and no-till soybeans on 8 October at stage R5. Soybean harvest was delayed due to excessive rainfall through November and the early part of December. Soybeans were harvested and weighed on 11 December in fullseason plots and 12 December in double-cropped plots. A 100-seed sample was collected from each plot during harvest to assess seed weight and percent discolored seeds.
Full-season Soybean Trial – In the full season trial, canopy growth was greater than average throughout the summer. Large canopy growth coupled with excessive moisture promoted development of downy mildew; however, no significant differences in disease development according to treatment were observed. No other significant diseases were noted in this trial. Winds from Tropical Storm Ernesto bent the stems and removed some foliage. Frequent rainfall throughout the fall delayed harvest for nearly two months. No differences in yields according to treatment were observed (Table 11). Similarly, no differences in 100-seed weight or percent discolored seed were noted.
Double-cropped Soybean Trial – This trial was planted late to favor development of soybean rust. Seedling emergence was negatively impacted by excessive rainfall throughout August and September. Excessive rainfall suppressed plant development throughout the growing season. Trace amounts of soybean rust were detected in this trial on November 1, just prior to a killing frost. Disease severity was not sufficient to constitute an assessment. Yields in this trial were poor and no significant differences were detected according to treatments (Table 11). No differences were observed in 100-seed weights or percent discolored seed.
Eastern Virginia AREC, Warsaw (Stromberg). Summary for 2006 Soybean Fungicide Trials
|Soybean cultivar Vigoro V48N5RR|
|Full-season soybeans planted on 6 June|
Python 0.9 oz/A + Dual 1.0 pint/A PPI
RoundUp 1 qt/A postemergence on 25 July
|Fertilizer: 0-60-60 per acre|
|Insecticide: Warrior T 3.84 oz/A on 21 August for corn ear worm and stink bugs|
|Treatments 2-9 applied at R1-2 on 24 July|
|Treatments 10-17 applied at R3 on 2 August|
|2nd applications, treatments 11, 14, 15, 16, and 17 applied on 23 August|
|Disease ratings: upper and lower canopy.|
Eastern Virginia AREC, Warsaw (Stromberg). Summary for DC-Soybean Fungicide Trial 2006 – DOUBLE CROPPED
|Cultivar: Vigoro V48N5RR|
|Planted on 12 July 2006 no-tillage into wheat stubble|
|Herbicides: Gramoxone 1 pint/A on 10 July 2006 (burndown).|
|Dual 1 pint/A + RoundUp 1 qt/A on 23 August 2006|
|Insecticide: Warrior T 3.84 oz/A on 21 August 2006 for corn ear worms and stinkbug|
|Treatments 2-9 applied at R1 on 15 August 2006|
|Treatments 10-17 applied at R3on 28 August 2006|
|2nd applications 11, 14, 15, 16, and 17 applied on 11 September 2006|
|Disease ratings: upper and lower canopy.|
Summary: Soybean Rust Incidence and the Response of Soybeans to Foliar Fungicides in 2006
- Ten regional sentinel plots and ca. 40 commercial fields were scouted from flowering to senescence for detection of soybean rust in 2006.
- Sentinel plots were located at the Tidewater AREC (Suffolk), Southampton County (Courtland), Eastern Shore AREC (Painter), Northampton County, Shenandoah County, Southern Piedmont AREC (Blackstone), New Kent County, Northern Piedmont AREC (Orange), Eastern Virginia AREC (Warsaw), and Virginia Tech – Kentland Farm (Blacksburg).
- The first outbreak of soybean rust, caused by Phakopsora pachyrhizi, was in Chesapeake on 9 October and Suffolk on 10 October; thereafter, the disease was confirmed in 18 counties and cities in Eastern Virginia. No loss of yield to soybean rust was expected since the disease appeared when soybeans were at growth stage R6 (full pod) or later.
- High temperatures and below normal rainfall until the arrival of tropical storm Ernesto on 30 August suppressed yield and were unfavorable for soybean rust; thereafter, cooler temperatures and frequent rainfall in September and October were favorable for disease spread and development.
- Cercospora blight, caused by Cercospora kikuchii, was the primary foliar disease that appeared to be responsible for yield losses of soybeans in 2006.
- Fungicide treatments with Headline, Absolute, Quilt, and Stratego provided superior control of Cercospora blight. Significant yield increases (P<0.05 or 0.10) were obtained with sprays of Absolute, Domark, and Headline either alone or Headline tank mixed with Folicur or Caramba.
Contacting the Authors
Patrick M. Phipps, Professor of Plant Pathology Virginia Tech, Tidewater AREC, 6321 Holland Rd., Suffolk, VA 23437 (Email: email@example.com; Telephone: 757-657-6450, Ext. 413)
Darcy E. Partridge, former Research Associate, Virginia Tech, Tidewater AREC current Research Associate, University of Florida, Jay Research Facility (Email: firstname.lastname@example.org; Telephone: 850-995-3720, ext. 107)
Erik L. Stromberg, Professor of Plant Pathology Virginia Tech, Dept. Plant Pathology, Physiology and Weed Science, Blacksburg, VA 24061 (Email: email@example.com; Phone: 540-231-6361)
Steve Rideout, Assistant Professor of Plant Pathology Virginia Tech, Eastern Shore AREC, 33446 Research Drive, Painter, VA 23420 (Email: firstname.lastname@example.org; Phone: 757-414-0724)
David Holshouser, Associate Professor of Crop and Soil Environmental Sciences Virginia Tech, Tidewater AREC, 6321 Holland Rd., Suffolk, VA 23437 (Email: email@example.com; Telephone: 757-657-6450, Ext. 412)
Robert Pitman, Superintendent Virginia Tech, Eastern Virginia AREC, 2229 Menokin Rd., Warsaw, VA 22572 (Email: firstname.lastname@example.org; Telephone: 804-435-1965)
AcknowledgmentThis research was supported by the Virginia Agricultural Experiment Station, the Virginia Soybean Board, and the Virginia Department of Agriculture and Consumer Services. The authors thank Steve Byrum, Barron Keeling, Ed Hobbs, and Christine Waldenmaier for technical assistance in field trials and data processing.
Virginia Cooperative Extension materials are available for public use, reprint, or citation without further permission, provided the use includes credit to the author and to Virginia Cooperative Extension, Virginia Tech, and Virginia State University.
Issued in furtherance of Cooperative Extension work, Virginia Polytechnic Institute and State University, Virginia State University, and the U.S. Department of Agriculture cooperating. Edwin J. Jones, Director, Virginia Cooperative Extension, Virginia Tech, Blacksburg; M. Ray McKinnie, Administrator, 1890 Extension Program, Virginia State University, Petersburg.
May 1, 2009