John D. Mueller

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

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.

Site-Specific Detection and Management of Nematodes

Nematode distribution varies significantly throughout a field and is highly correlated to soil texture and other edaphic factors. Field-wide application results in nematicides being applied to areas without nematodes and the application of sub-effective levels in areas with high nematode densities. Efforts to use grid maps as a guide to site-specific application have proven to be too expensive to be cost effective. Recently, the availability of GPS –GIS has allowed the use of soil electrical conductivity systems to rapidly and inexpensively develop cost effective soil texture maps. These maps are used to project where nematodes are likely to occur within a field. Variable-rate application systems for granular and fumigant nematicides have been developed and tied via software to soil texture maps providing a mechanism for the effective delivery of nematicides in a site-specific , variable-rate manner in individual fields. Efforts in South Carolina, Georgia, and Arkansas are further developing this system and refining our knowledge of how soil texture and other edaphic factors affect the distribution of cotton nematodes .

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.

Importance of Fungicide Seed Treatment and Environment on Seedling Diseases of Cotton

The importance of fungicide seed treatments on cotton was examined using a series of standardized fungicide trials from 1993 to 2004. Fungicide seed treatments increased stands over those from seed not treated with fungicides in 119 of 211 trials. Metalaxyl increased stands compared to nontreated seed in 40 of 119 trials having significant fungicide responses, demonstrating the importance of Pythium spp. on stand establishment. Similarly, PCNB seed treatment increased stands compared to nontreated seed for 44 of 119 trials with a significant response, indicating the importance of Rhizoctonia solani in stand losses. Benefits from the use of newer seed treatment chemistries, azoxystrobin and triazoles, were demonstrated by comparison with a historic standard seed treatment, carboxin + PCNB + metalaxyl. Little to no stand improvement was found when minimal soil temperatures averaged 25°C the first 3 days after planting. Stand losses due to seedling pathogens increased dramatically as minimal soil temperatures decreased to 12°C and rainfall increased. The importance of Pythium increased dramatically as minimal soil temperature decreased and rainfall increased, while the importance of R. solani was not affected greatly by planting environment. These multi-year data support the widespread use of seed treatment fungicides for the control of the seedling disease complex on cotton.

Spatial Distribution of Reniform Nematode in Cotton as Influenced by Soil Texture and Crop Rotations

Reniform nematode (RN) is an important pest in cotton production. Knowledge of the distribution patterns of RN is essential for selecting sampling strategies and for site-specific management. A 3-year study was conducted in two fields in South Carolina with the purpose of characterizing the distribution of RN using a fine-scale sampling scheme in plots representing different soil textures (field 1), and using a large-scale arbitrary sampling scheme (field 2). Horizontal distribution data showed an aggregated pattern of RN densities at planting and after harvest in both fields each year, with patches ranging from 8 to 12 m. However, a significant neighborhood structure was only detected when suitable hosts (cotton and soybean) were planted. Correlations between RN densities and percent sand and silt were detected, showing nematode densities peaked when sand content was around 60% and declined when sand content increased above 60 to 65%. When fewer samples were taken in the field with more uniform sand content, percentage of sand was a less reliable predictor of RN densities. Vertical sampling showed the highest numbers of RN were found at 15 to 30 cm deep after cotton, but were deeper after a nonhost crop. Understanding distribution patterns of RN within a field may improve the effectiveness of management practices.

High genetic diversity and geographic subdivision of three lance nematode species (Hoplolaimus spp.) in the United States.

Lance nematodes (Hoplolaimus spp.) feed on the roots of a wide range of plants, some of which are agronomic crops. Morphometric values of amphimictic lance nematode species overlap considerably, and useful morphological characters for their discrimination require high magnification and significant diagnostic time. Given their morphological similarity, these Hoplolaimus species provide an interesting model to investigate hidden diversity in crop agroecosystems. In this scenario, H. galeatus may have been over-reported and the related species that are morphologically similar could be more widespread in the United States that has been recognized thus far. The main objectives of this study were to delimit Hoplolaimus galeatus and morphologically similar species using morphology, phylogeny, and a barcoding approach, and to estimate the genetic diversity and population structure of the species found. Molecular analyses were performed using sequences of the cytochrome c oxidase subunit 1 (Cox1) and the internal transcribed spacer (ITS1) on 23 populations. Four morphospecies were identified: H. galeatus, H. magnistylus, H. concaudajuvencus, and H. stephanus, along with a currently undescribed species. Pronounced genetic structure correlated with geographic origin was found for all species, except for H. galeatus. Hoplolaimus galeatus also exhibited low genetic diversity and the shortest genetic distances among populations. In contrast, H. stephanus, the species with the fewest reports from agricultural soils, was the most common and diverse species found. Results of this project may lead to better delimitation of lance nematode species in the United States by contributing to the understanding the diversity within this group.

Distribution of Hoplolaimus Species in Soybean Fields in South Carolina and North Carolina

Hoplolaimus columbus is an important nematode pest of soybean in South Carolina and North Carolina. Tolerant cultivars are available for the management of this plant-parasitic nematode; however, variation in the response of soybean cultivars to H. columbus populations has been observed. This variation may be due to the presence of different species or high genetic diversity of H. columbus populations. The objective of this study was to identify the Hoplolaimus spp. present in fields representing the main soybean-growing regions in South Carolina and North Carolina and to examine the genetic variability of these populations. In South Carolina, the only species found associated with soybean was H. columbus but, in North Carolina, H. stephanus was the dominant species. The two species were never found together. Genetic variability analyses of a mitochondrial and a nuclear marker showed that only one haplotype was shared by the H. columbus populations. H. stephanus showed higher genetic variability, with private haplotypes per sampling location. Knowledge of the distribution and genetic variability of these two Hoplolaimus spp. is valuable to growers to determine potentially damaging infestations of these plant-parasitic nematodes in soybean fields.