T. L. Kirkpatrick

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

Influence of Meloidogyne incognita and Thielaviopsis basicola populations on early-season disease development and cotton growth.

Microplot studies were used to examine the effect of various population densities of Meloidogyne incognita and Thielaviopsis basicola on cotton-plant development and disease severity. Plots were infested with 0, 20, or 100 T. basicola chlamydospores/g and 0, 5, or 10 M. incognita eggs and juveniles/cm3 of soil in a factorial arrangement in 1997 and 1998. Combinations of M. incognita and T. basicola reduced plant survival in both years compared to the noninfested control, except in 1998 for the high rate of T. basicola over all nematode rates. Plant height-to-node ratios were reduced by pathogen combinations compared to the noninfested control or to either pathogen alone. Plant dry weight was reduced by M. incognita in 1998 and the high rate of T. basicola in 1997. Root necrosis was increased by increasing rates of T. basicola in 1997 and by M. incognita over all rates of T. basicola in both years. Colonization of root tissue by T. basicola was increased by the low inoculum density of the nematode at 20 CFU/g soil in 1997 and 100 CFU/g in 1998. Nematode reproduction with the high M. incognita treatment rate was reduced in both years of the study by the high T. basicola rate. This study suggests the importance of population level of each pathogen to the severity of disease and confirms the potential of this disease interaction to impact cotton production.

Effect of Temperature on and Histopathology of the Interaction Between Meloidogyne incognita and Thielaviopsis basicola on Cotton

ABSTRACT Controlled environments were used to study the relationship between the root-knot nematode (Meloidogyne incognita) and Thielaviopsis basicola on cotton. Temperature treatments were continuous 20, 24, and 28 degrees C or two cyclic linear regimes with ranges of 14 to 32 or 18 to 28 degrees C over 24 h. Cotton seeds were planted in fumigated soil infested with T. basicola, M. incognita, or both. After 42 days, pathogen effects on plant growth and pathogen development were evaluated. Histology was conducted on roots collected 14, 28, and 42 days after planting in the continuous 24 degrees C treatment. Reductions in plant height-to-node ratio and total fresh weight were observed for soils infested with both pathogens compared with the control or with soils infested with either pathogen, except for M. incognita-infested soil at 28 degrees C. T. basicola reduced root galling and reproduction of the nematode at all temperatures. Vascular discoloration caused by T. basicola was greater in the presence of M. incognita compared with that by T. basicola alone. At 2 and 4 weeks, histological studies showed that plants grown in all T. basicola-infested soils contained chlamydospore chains on the root surface and in cortical cells. The fungus was not observed inside the vascular cylinder. Roots from 4-week-old plants from soils infested with T. basicola and M. incognita showed fungal sporulation in vascular tissue and localized necrosis of vascular tissue adjacent to the nematodes. At 6 weeks, plants grown in soil infested with T. basicola alone exhibited no remaining cortical tissue and no evidence of vascular colonization by the fungus. Six-week-old plants grown in T. basicola + M. incognita-infested soils exhibited extensive vascular necrosis and sporulation within vascular tissue. These studies suggest that coinfection expands the temperature ranges at which the pathogens are able to cause plant damage. Further, M. incognita greatly increases the access of T. basicola to vascular tissue.

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 .

Efficacy of Seed Treatment Chemicals for Black Root Rot, Caused by Thielaviopsis basicola, on Cotton

The efficacy of triazole and host resistance-inducing seed treatment chemicals was examined for black root rot on cotton caused by Thielaviopsis basicola in both artificially and naturally infested soils with and without nematodes. In naturally infested soil, myclobutanil was effective in reducing root and hypocotyl discoloration over a wide range of soil population densities. Treatments containing high rates (42 g a.i./100 kg seed) of myclobutanil provided greater reductions in disease than low rates (21 g a.i./100 kg seed) in some experiments. Acibenzolar-S-methyl applied to the seed reduced black root rot or colonization by T. basicola on seedlings in artificially infested soils. Rates of acibenzolar-S-methyl did not differ in efficacy. In controlled studies, root colonization by T. basicola was significantly lower when seeds were treated with both myclobutanil and acibenzolar-S-methyl than with either chemical alone. In naturally infested soil under low (24 CFU/g soil) and high (154 CFU/g soil) populations of T. basicola, a combination of myclobutanil and acibenzolar-S-methyl at the high rate resulted in the lowest root discoloration and colonization. The nematicide seed treatment abamectin improved the control of black root rot in the presence of Meloidogyne incognita. The semi-selective medium TB-CEN allowed the importance of T. basicola to be evaluated in the presence of other pathogens that contribute to the seedling disease complex on cotton by quantifying the isolation frequency and percent colonization of T. basicola.

Conventional PCR Detection and Real-Time PCR Quantification of Reniform Nematodes

Reniform nematode (Rotylenchulus reniformis) is a relatively recent introduction into the continental United States that can cause major yield losses on a variety of important crops including cotton and soybeans. DNA sequences from the internal transcribed spacer (ITS) region of this nematode were used to design primers for conventional and real-time PCR, as well as a TaqMan probe. These primers amplified DNA of reniform nematode isolates from a wide geographic range but did not detect genetically related species or other pathogenic nematodes found in production fields including Meloidogyne incognita and Heterodera glycines. Both SYBR green and TaqMan assays reliably quantified as little as 100 fg of reniform nematode DNA, and could be used to quantify as few as five reniform nematodes. An inexpensive and rapid DNA extraction protocol for high throughput diagnostic assays is described.

Effects of Nocturnal Soil Temperatures and Meloidogyne incognita Densities on Cotton Seedling Growth and the Interaction with Thielaviopsis basicola

Controlled studies were conducted to evaluate the effects of soil temperatures typical of field conditions during the first 6 weeks of the growing season in Arkansas and different population densities of Meloidogyne incognita on damage to cotton (Gossypium hirsutum) seedlings associated with the interaction between M. incognita and Thielaviopsis basicola. Treatments consisted of varying nocturnal temperatures that approximated the temperatures that occurred during the 2001, 2002, and 2003 growing seasons in southeastern Arkansas. Nocturnal temperatures in the study were as follows: high, the first week at 15°C, followed by 3 weeks at 17°C, 1 week at 21°C, and 1 week at 17°C (approximating the 2002 season); medium, 3 weeks at 15°C and 3 weeks at 19°C (approximating the 2003 season); and low, 1 week at 15°C, 1 week at 13°C, 2 weeks at 17°C, 1 week at 15°C, and 1 week at 17°C (approximating the 2001 season). Pathogen population densities were either 0 or 100 chlamydospores of T. basicola per gram of soil and 0, 2,000, 4,000, or 8,000 eggs of M. incognita per 500 cm3of soil. Plant height and fresh top weight increased with an increase in nocturnal temperature across treatments. There were significant reductions in plant growth and development with T. basicola, but not with M. incognita, at these nocturnal temperatures, but decreased plant height and weight were seen where both pathogens were present in comparison with either pathogen alone. Trends of increased disease associated with T. basicola were observed with increasing inoculum rates of M. incognita, indicating that the interaction between T. basicola and M. incognita occurs even at soil temperatures below the minimum temperature reported as necessary for damage from M. incognita.

Soil Texture Influence on Meloidogyne incognita and Thielaviopsis basicola and Their Interaction on Cotton

Microplots were used to evaluate the impact of soil texture on Meloidogyne incognita, Thielaviopsis basicola, and their interaction on cotton. A native silt loam soil (48% sand) and four different artificial soil textures produced by mixing native soil with sand (53, 70, 74, and 87% sand) were studied. Each soil texture was infested with 0, 4, or 8 M. incognita eggs and 0 or 20 T. basicola chlamydospore chains per gram of soil in a factorial treatment arrangement. Plots were watered when soil moisture fell below -10 joules/kg for the first 21 days and -30 joules/kg from 22 days to harvest. Plant growth was suppressed early in the season and midseason by T. basicola. M. incognita suppressed plant growth and delayed plant development late in the season across all soil textures. Cotton yield was lower in the presence of either T. basicola or M. incognita. An interaction between M. incognita and T. basicola, which decreased plant growth and yield, occurred in 2006 when neither pathogen caused substantial plant damage. Plant growth, development, and yield were lowest in soils with >74% sand. Root colonization by T. basicola and fungal reproduction and survival decreased in soil having 87% sand. M. incognita generally caused more galling and reproduction in soils as sand content increased. Root galling severity and M. incognita reproduction were suppressed by the presence of T. basicola in soil at sand contents lower than 87%. Soil texture had a greater impact on T. basicola than on M. incognita in this study.

Spatial econometric approaches to developing site-specific nematode management strategies in cotton production

Root-knot nematode infestations tend to be spatially clustered within agricultural fields and result in varied crop yield penalties. Site-specific nematode management could provide the opportunity for producers to maximize profit while maintaining acceptable yield and reducing over-use of chemical nematicides. This paper determined the potential for site-specific nematicide application by using spatial econometric analyses of on-farm experimental data to estimate cotton yield response functions with respect to environmental factors and treatment applications. The results suggest that yield response to nematicide application differs by soil texture. Post-treatment of the nematode population at maximum flowering stage and the sand content of soil were significant factors in explaining the variation in yield. When explicitly modeled, the neighboring plot effects affected yield estimates considerably. The results provide practical recommendations for the effective control of nematodes by site-specific management.

Effects of Meloidogyne incognita and Thielaviopsis basicola on Cotton Growth and Root Morphology

Effects of the root-knot nematode Meloidogyne incognita and the fungal pathogen Thielaviopsis basicola on cotton seedling growth and root morphology were evaluated in controlled environmental experiments. Four pathogen treatments, including noninfested soil, soil infested with M. incognita, soil infested with T. basicola, and soil infested with both pathogens were evaluated at soil bulk densities (BDs) of 1.25 and 1.50 g/cm(3). Plant growth and the morphology of the root systems were evaluated 44 days after planting. Infestation with M. incognita and T. basicola together significantly reduced seedling emergence, number of stem nodes, and root system volume compared with either pathogen alone. Either M. incognita or T. basicola reduced plant height, root fresh weight, top dry weight; root parameters total root length, surface area, and links; and root topological parameters magnitude, altitude, and exterior path length. M. incognita infection increased root radius. Root colonization by T. basicola increased with the presence of M. incognita at the lower soil BD. In contrast to previous research with Pythium spp., root topological indices (TIs) were similar with all of the treatments. Root TIs were near 1.92, indicating a herringbone (less branching) root architectural structure. Studying root architecture using a topological model offers an additional approach to evaluating fungi and nematodes and their interactions for soilborne-pathogen systems.