S. R. Koenning

Soybean disease loss estimates for the United States from 1996 to 1998

Soybean disease loss estimates were compiled for the 1996 to 1998 harvested crops from all soybean-producing states in the United States. Scientists from each state provided estimates of losses based on field surveys, information from field workers and university extension staff, and research plot data. Total yield losses caused by soybean cyst [Heterodera glycines Ichinohe] in the United States were greater than those caused by any other disease. Next in importance were phytophthora root and stem rot [Phytophthora sojae (Kaufman & Gerdemann)], brown stem rot [Phialophora gregata (Allington & Chamberlain) Gams], sclerotinia stem rot [Sclerotinia sclerotiorum (Lib.) de Bary], and seedling diseases. Yield loss estimates due to particular diseases varied by region and among years. The estimated soybean yield losses to diseases in the United States were 10.9 × 106 t in 1996, 11.9 × 106 t in 1997, and 14.0 × 106 t in 1998.

Plant-Parasitic Nematodes Attacking Cotton in the United States Old and Emerging Production Challenges

Cotton is the most important fiber crop in the world, and current U.S. lint production accounts for nearly one quarter of the world supply. The unique role of cotton in world and American history is profound. Primitive cottons have been used in Africa, Asia, and the Americas for millennia. Domestic and international demand for cotton fiber contributed greatly to the westward expansion of the United States, the American Civil War, and the industrial revolution (81). The land area devoted to cotton production in the United States peaked in 1926 with approximately 18 million hectares (Fig. 1). The advent of mechanized farming equipment and the availability of effective, relatively low-cost fertilizers, pesticides, and improved cotton cultivars after World War II allowed the production of significantly greater yields per unit of land area, and hectarage declined. U.S. production of cotton lint in the past 5 years has varied from 3.0 × 10 to 4.4 × 10 kg produced on about 5 million hectares (147). Additionally, cotton seed is a valuable source of vegetable oil and protein used in animal feed, with production of 4.9 × 10 to 5.9 × 10 kg of cotton seed annually. Since World War II, cotton cultivation was increasingly dependent on inputs of chemical pesticides for weed and insect control. Historically, the cotton boll weevil, Anthonomus grandis Boheman, was the most costly pest of cotton in the United States. The combination of crop loss due to this insect directly and the expense for insecticides that was incurred by cotton growers attempting to control it amounted to several billion dollars annually until recently (130). The successful establishment of the Boll Weevil Eradication Program coordinated by the U.S. Department of Agriculture in many states in the eastern half of the country has resulted in a reduction in insecticide usage, improved profitability for growers, and has led to a resurgence of cotton production in the Southeast (37). In addition, the current widespread use of transgenic cotton cultivars with resistance to herbicides and/or insects also has greatly reduced the need for inputs of pesticides. Currently, 71% of cotton grown in the United States is herbicide resistant, resistant to lepidopteran insects, or has resistance to both (3). Reductions in pest pressure from weeds and insects as a result of the deployment of transgenic resistance and the boll weevil eradication program have

Impact of crop rotation and tillage system on Heterodera glycines population density and soybean yield.

The long-term effects of no-till planting practices and rotation on the population dynamics of the soybean cyst nematode (Heterodera glycines) and soybean yield were investigated in field experiments over a period of 8 yr. The experiment was a 2 X 4 factorial, comparing no-till vs. conventional tillage practices in four cropping patterns (continuous soybean, a 1-yr rotation of corn and soybean, a rotation of 2 yr of corn followed by soybean, and a corn-wheat/soybean double-cropping system). Treatments were arranged so that each combination occurred every year after 1986. Soybean after 1 yr of corn had higher yields (P = 0.0001) than soybean after soybean. Two years of corn between soybean crops resulted in soybean yields higher than those after 1 yr of corn in only 2 out of 6 yr. The yields of soybean in the corn, wheat/soybean double-cropping system, however, were generally similar to monoculture soybean. No-till practices had positive or no effects on soybean yield early in the study, but yields of no-till soybean were lower (P = 0.01) than conventionally tilled soybean after several years because weed pressure was greater in no-till plots. Population densities of H. glycines were greater (P < 0.10) in conventionally tilled plots than in no-till plots in 1988 and 1990-1992. Numbers of H. glycines fluctuated in an unpredictable manner from year to year, possibly because of unidentified biological control agents or excessive moisture in certain years. H. glycines population densities declined in a predictable manner when a nonhost was planted

Root-Parasitic Nematodes Enhance Soil Microbial Activities and Nitrogen Mineralization

Obligate root-parasitic nematodes can affect soil microbes positively by enhancing C and nutrient leakage from roots but negatively by restricting total root growth. However, it is unclear how the resulting changes in C availability affect soil microbial activities and N cycling. In a microplot experiment, effects of root-parasitic reniform nematodes (Rotylenchulus reniformis) on soil microbial biomass and activities were examined in six different soils planted with cotton. Rotylenchulus reniformis was introduced at 900 nematodes kg−1 soil in May 2000 prior to seeding cotton. In 2001, soil samples were collected in May before cotton was seeded and in November at the final harvest. Extractable C and N were consistently higher in the R. reniformis treatments than in the non-nematode controls across the six different soils. Nematode inoculation significantly reduced microbial biomass C, but increased microbial biomass N, leading to marked decreases in microbial biomass C:N ratios. Soil microbial respiration and net N mineralization rates were also consistently higher in the nematode treatments than in the controls. However, soil types did not have a significant impact on the effects of nematodes on these microbial parameters. These findings indicate that nematode infection of plant roots may enhance microbial activities and the turnover of soil microbial biomass, facilitating soil N cycling. The present study provides the first evidence about the direct role of root-feeding nematodes in enhancing soil N mineralization.

Effects of cropping systems on population density of Heterodera glycines and soybean yield

We evaluated the effects of soybean planting date and maturity group (MG) on final population density of Heterodera glycines and yield in cultivars susceptible to H, glycines grown in monoculture and in rotations with 1 or 2 yr of nonhosts. Population density of eggs and eggs plus second-stage juveniles of H. glycines declined to barely detectable levels after 2 yr of nonhost culture. Population densities of this nematode were consistently greater (P=0.05) for an MG VII cultivar than an MG V cultivar. Planting date had variable effects on final population density of H. glycines: early planting resulted in the highest nematode numbers in some years, whereas late planting was associated with significantly greater (P=0.05) population densities in other years

A Coordinated Effort to Manage Soybean Rust in North America: A Success Story in Soybean Disease Monitoring

Existing crop monitoring programs determine the incidence and distribution of plant diseases and pathogens and assess the damage caused within a crop production region. These programs have traditionally used observed or predicted disease and pathogen data and environmental information to prescribe management practices that minimize crop loss. Monitoring programs are especially important for crops with broad geographic distribution or for diseases that can cause rapid and great economic losses. Successful monitoring programs have been developed for several plant diseases, including downy mildew of cucurbits, Fusarium head blight of wheat, potato late blight, and rusts of cereal crops. A recent example of a successful disease-monitoring program for an economically important crop is the soybean rust (SBR) monitoring effort within North America. SBR, caused by the fungus Phakopsora pachyrhizi, was first identified in the continental United States in November 2004. SBR causes moderate to severe yield losses globally. The fungus produces foliar lesions on soybean (Glycine max) and other legume hosts. P. pachyrhizi diverts nutrients from the host to its own growth and reproduction. The lesions also reduce photosynthetic area. Uredinia rupture the host epidermis and diminish stomatal regulation of transpiration to cause tissue desiccation and premature defoliation. Severe soybean yield losses can occur if plants defoliate during the mid-reproductive growth stages. The rapid response to the threat of SBR in North America resulted in an unprecedented amount of information dissemination and the development of a real-time, publicly available monitoring and prediction system known as the Soybean Rust-Pest Information Platform for Extension and Education (SBR-PIPE). The objectives of this article are (i) to highlight the successful response effort to SBR in North America, and (ii) to introduce researchers to the quantity and type of data generated by SBR-PIPE. Data from this system may now be used to answer questions about the biology, ecology, and epidemiology of an important pathogen and disease of soybean.

Effects of Rate and Time of Application of Poultry Litter on Hoplolaimus columbus on Cotton

Field experiments were conducted to evaluate the effect of soil-incorporated poultry litter on the population dynamics of Hoplolaimus columbus and cotton lint yield. Rates of poultry litter applied varied from 0.0 to 27.0 t/ha and were applied in December, February, or March. Time of application did not influence population densities of this nematode or cotton yield. The rate of poultry litter applied was negatively related to the population density of H. columbus at midseason, but not at other sampling dates. The lower midseason levels of this nematode corresponded with increases in cotton lint yield in all experiments. Cotton yield increases generally were linear with respect to the rate of litter applied, although the highest rates of litter applied did not always result in the greatest cotton yield. Poultry litter can be used effectively to supply nutrients to the crop and suppress damaging levels of H. columbus. Optimal rates of litter application were from 6.0 to 13.4 t/ha. Application of poultry litter at these rates, however, may exceed nutrient levels required for best management practices.

Increased Occurrence of Target Spot of Soybean Caused by Corynespora cassiicola in the Southeastern United States

Target spot of soybean (Glycine max (L.) Merr.) caused by Corynespora cassiicola (Berk. & Curt.), although found in most soybean-growing countries, is considered to be a disease of limited importance (1) and has never been reported to cause soybean yield loss in the southeastern United States (2,3). Soybean plants submitted to the North Carolina Plant Disease and Insect Clinic (NCPDIC) in August 2004 from Beaufort, Robeson, Wilson, and Johnston counties, NC had symptoms consistent with target spot. Symptoms consisted of roughly circular, necrotic leaf lesions from minute to 11 mm in diameter, though typically approximately 4 to 5 mm in diameter, and with a yellow margin. Large lesions occasionally exhibited a zonate pattern often associated with this disease. Microscopic examination of the lesions revealed the presence of spores (conidia) typical of C. cassiicola (1). Conidia were mostly three to five septate with a central hilum at the base and ranged in size from 7 to 22 wide × 39 to 520 μm long. Three commercial soybean fields near Blackville, SC (Barnwell County) were severely affected by this disease and it caused premature defoliation. Nineteen of twenty-seven maturity group VII and VIII genotypes in the 2004 Clemson University soybean variety trial near Blackville, SC had visible symptoms of target spot. Heavy rainfall associated with hurricanes during September 2004 probably enhanced the incidence of this disease, and yield suppression due to target spot was estimated at 20 to 40% in some fields. In 2005, 20 of 161 soybean samples submitted to the NCPDIC or collected in surveys from 16 counties were positive for target spot on the basis of microscopic examination. Target spot also was diagnosed in six counties (Baldwin, DeKalb, Elmore, Fayette, Macon, and Pickens) in Alabama and in four additional counties (Bamberg, Hampton, Orange-burg, and Calhoun) in South Carolina in 2005. Records from the NCPDIC indicate that target spot had not been diagnosed on soybean in North Carolina since 1981. The large increase in incidence of target spot in the southeast may be related to changes in weather patterns, changes in pathogen virulence, and/or the introduction of more susceptible host genotypes. References: (1) J. B. Sinclair. Target spot. Page 27 in: Compendium of Soybean Diseases. G. L. Hartman et al. eds. The American Phytopathological Society, St. Paul, MN, 1999. (2) J. A. Wrather et al. Plant Dis. 79:1076. 1995. (3) J. A. Wrather et al. On-line publication. doi:10.1094/PHP-2003-0325-01-RV. Plant Health Progress, 2003.

Resistance of Soybean Cultivars to Field Populations of Heterodera glycines in North Carolina

The soybean cyst nematode (SCN), Heterodera glycines, is the most important pathogen of soybean, Glycine max, in North Carolina. Cultural practices are the most effective means of managing this pathogen because a majority of cultivars are susceptible to the races of this nematode that predominate in the state. Resistant and susceptible cultivars were evaluated in 14 H. glycines-infested fields from 1992 to 1999. Resistance in cvs. Hartwig and Delsoy 5710, and line S92-1603 derived from plant introduction (PI) 437654, was highly effective against all populations of H. glycines evaluated in these experiments. Numbers of cysts (cysts and white females) per three plants 28 days after planting and final egg population densities (Pf) were lower than on other cultivars evaluated. Cultivars with SCN resistance derived from PI 90763 were moderately resistant in many of the test fields, but cultivars with Peking-derived resistance were effective at only two locations. Some cultivars with resistance derived from PI 88788 were highly to moderately resistant to races 9 or 14 of SCN, but were not consistently effective against other populations. Hartwig and Delsoy 5710 had low SCN reproductive factors (Rf = egg density at harvest/mean egg density at planting for site) of 0.16 and 0.23 compared with an Rf of 1.9 and 2.19 on the susceptible cvs. Essex and Hutcheson, respectively. In contrast, the Rf on cultivars derived from Peking generally was greater than on susceptible cultivars. Resistant cvs. Hartwig and Delsoy 5710 generally yielded more than susceptible cultivars or cultivars derived from other sources of resistance. The initial inoculum level (Pi) was negatively correlated with soybean seed yield, but cysts 28 days after planting proved to be better at predicting seed yield than Pi. Due to the genetic diversity of H. glycines populations with regard to the ability to parasitize resistant cultivars, cultivars with resistance derived from PI 437654 or other genotypes are needed to manage this nematode in North Carolina.

Cotton Tolerance to Hoplolaimus columbus and Impact on Population Densities

Glyphosate-tolerant transgenic-cotton cultivars were evaluated for tolerance to Hoplolaimus columbus in field experiments conducted from 2001 to 2003. The studies were arranged in a split-plot design that included treatment with 1,3-dichloropropene at 42 liter/ha to establish fumigated versus nonfumigated subplots with cultivars as whole plots. Cotton cultivars were divided by relative maturity into two separate but adjacent experiments in order to facilitate cotton defoliation, with 10 early-maturity and 5 late-maturity cultivars. Fumigation was effective in suppressing H. columbus population densities and increased cotton lint yield. The cultivar-fumigation interaction was significant for early-season cotton cultivars but not for late-season cultivars. A tolerance index ([yield of nontreated/yield of treated] × 100) was used to compare cultivar differences. Both groups of cultivars expressed significant levels of tolerance to H. columbus, but late-season cultivars tended to yield more than early-season cultivars in infested fields.