Management of Meloidogyne incognita with Chemicals and Cultivars in Cotton in a Semi-Arid Environment.

Management of Meloidogyne incognita (root-knot nematode) in cotton in the United States was substantially affected by the decision to stop production of aldicarb by its principle manufacturer in 2011. The remaining commercially available tools to manage M. incognita included soil fumigation, nematicide seed treatments, postemergence nematicide application, and cultivars partially resistant to M. incognita. Small plot field studies were conducted on a total of nine sites from 2011-2013 to examine the effects of each of these tools alone or in combinations, on early season galling, late-season nematode density in soil, yield, and value ($/ha = lint value minus chemical costs/ha). The use of a partially resistant cultivar resulted in fewer galls/root system at 35 d after planting in eight of nine tests, lower root-knot nematode density late in the growing season for all test sites, higher lint yield in eight of nine sites, and higher value/ha in six of nine sites. Galls per root were reduced by aldicarb in three of nine sites and by 1,3-dichloropropene (1,3-D) in two of eight sites, relative to the nontreated control (no insecticide or nematicide treatment). Soil fumigation reduced M. incognita density late in the season in three of nine sites. Value/ha was not affected by chemical treatment in four of nine sites, but there was a cultivar × chemical interaction in four of nine sites. When value/ha was affected by chemical treatment, the nontreated control had a similar value to the treatment with the highest value/ha in seven of eight cultivar-site combinations. The next "best" value/ha were associated with seed treatment insecticide (STI) + oxamyl and aldicarb (similar value to the highest value/ha in six of eight cultivar-site combinations). The lowest valued treatment was STI + 1,3-D. In a semi-arid region, where rainfall was low during the spring for all three years, cultivars with partial resistance to M. incognita was the most profitable method of managing root-knot nematode in cotton.

The relationship between environmental variables and response of cotton to nematicides.

Nematicide/irrigation rate trials were conducted in Texas (TX) in 2009 and 2010 in cotton grown at three irrigation rates, where irrigation rate (base (B), B - 33%, B + 33%) was the main plot and treatment (untreated check, aldicarb, and nematicide seed treatment (NST) and NST + aldicarb) were the subplots. Aldicarb improved cotton lint yield with the base (medium) irrigation rate over the untreated check, but not at the B - 33% and B + 33% irrigation rates. In a second evaluation, 20 tests conducted over 7 yr at the same field in TX and 12 tests conducted over 6 yr at the same field in Alabama (AL) were examined for impact of environmental variables (EV) on the response to NST (containing thiodicarb or abamectin), aldicarb, a nontreated check (CK), insecticide seed treatment (TX only), and a combination of NST + aldicarb + oxamyl (NST/A/O, AL only) on root galls (TX only), early season nematode eggs (AL only), and yield (both sites). Galls/root system were lower with aldicarb-treated plots, than for the CK- or NST-treated plots. As water (irrigation plus rain) in May increased, galls/root system increased for CK or insecticide-only-treated plots, and decreased for NST- and aldicarb-treated plots, suggesting efficacy of nematicides was strongly improved by adequate soil moisture. Nematode reproduction was not affected by EV in either location, though yield was negatively affected by root-knot nematode eggs in AL at 60 d. Yield in both AL and TX was negatively related to temperature parameters and positively related to water parameters. With the addition of EV in TX, chemical treatments went from not significantly different in the absence of EV to aldicarb-treated plots having higher yields than nonnematicide-treated plots in the presence of EV. In AL, NST/A/O-treated plots yielded similar to aldicarb and better than CK or NST in the absence of EV and had significantly higher yields than all other treatments in the presence of most EV.

Effects of Crop Rotation, Cultivar, and Irrigation and Nitrogen Rate on Verticillium Wilt in Cotton.

A field experiment was conducted under center-pivot irrigation in four wedges, with one wedge in continuous cotton (CC) and three wedges in a rotation (ROT) with 2 years cotton and 1 year in sorghum. Three irrigation rates (base = 1.0B, 1.5B, and 0.5B) were applied during 2007 to 2009 on a susceptible (ST) and partially resistant (PR) cultivar. Nitrogen applied during the season was proportional to irrigation rate. In the ROT wedges, 0.5B, 1.0B, and 1.5B irrigation and nitrogen rates averaged 1, 3, and 9% incidence of wilt, respectively. Disease incidence in the CC wedge averaged 6, 18, and 34% wilt incidence for 0.5B, 1.0B, and 1.5B irrigation and nitrogen rates. In the ROT wedges, the ST cultivar returned $143/ha more than the PR cultivars at the 0.5B irrigation and nitrogen rate whereas, at the 1.0B and 1.5B rates, the PR cultivars averaged $121 and $350/ha more than the ST cultivar. There was no significant irrigation and nitrogen or cultivar effect in the CC wedge on net value; however, trends were similar to the ROT wedge. Overall, ROT returned $285/ha more than CC, PR cultivars returned $123/ha more than the ST cultivar, and 1.0B returned $271 and $296/ha more than 0.5B and 1.5B rates, respectively. Microsclerotia density of V. dahliae averaged 2/cm3 of soil in the ROT wedges and 23/cm3 of soil in the CC wedge. Crop rotation, avoiding excessive irrigation, and using a partially resistant cultivar all reduced incidence of Verticillium wilt and improved net returns.

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.

First Report of "Candidatus Liberibacter solanacearum" on Field Tomatoes in the United States.

In August 2008, 30% of tomato (Solanum lycopersicum) plants in plots in Lubbock County, Texas showed yellowing, lateral stem dieback, upward leaf curling, enlargement of stems, adventitious roots, and swollen nodes. Yellowing in leaves was similar to that seen with zebra chip disease (ZC) of potato that was confirmed in a potato field 112 km away in July 2008 and was associated with a 'Candidatus Liberibacter' species (1), similar to findings earlier in 2008 in New Zealand and California (2,3). Tissue from four symptomatic plants of cv. Spitfire and two of cv. Celebrity were collected and DNA was extracted from midribs and petioles with a FastDNA Spin Kit (Qbiogene, Inc., Carlsbad, CA,). PCR amplification was done with 16S rRNA gene primers OA2 and OI2c, which are specific for "Ca. Liberibacter solanacearum" from potato and tomato and amplify a 1.1-kb fragment of the 16S rRNA gene of this new species (1,3). Amplicons of 1.1 kb were obtained from all samples and these were sequenced in both orientations (McLab, San Francisco, CA). Sequences of the 16S rRNA gene were identical for both Spitfire and Celebrity and were submitted to the NCBI as GenBank Accession Nos. FJ939136 and FJ939137, respectively. On the basis of a BLAST search, sequence alignments revealed 99.9% identity with a new species of 'Ca. Liberibacter' from potato (EU884128 and EU884129) in Texas (1); 99.7% identity with the new species "Ca. Liberibacter solanacearum" described from potato and tomato (3) in New Zealand (EU849020 and EU834130, respectively) and from the potato psyllid Bactericera cockerelli in California (2) (EU812559, EU812556); 97% identity with 'Ca L. asiaticus' from citrus in Malaysia (EU224393) and 94% identity with both 'Ca. L. africanus' and 'Ca. L. americanus' from citrus (EU921620 and AY742824, respectively). A neighbor-joining cladogram constructed using the 16S rRNA gene fragments delineated four clusters corresponding to each species, and these sequences clustered with "Ca. L. solanacearum". A second PCR analysis was conducted with the CL514F/CL514R primer pair, which amplifies a sequence from the rplJ and rplL ribosomal protein genes of "Ca. L. solanacearum". The resulting 669-bp products were 100% identical to a sequence reported from tomato in Mexico (FJ498807). This sequence was submitted to NCBI (GU169328). ZC, a disease causing losses to the potato industry, is associated with a 'Candidatus Liberibacter' species (1-3) and was reported in Central America and Mexico in the 1990s, in Texas in 2000, and more recently in other states in the United States (4). In 2008, a "Ca. Liberibacter solanacearum" was detected on Capsicum annuum, S. betaceum, and Physalis peruviana in New Zealand (3). Several studies have shown that the potato psyllid, B. cockerelli, is a potential vector for this pathogen (2,4). To our knowledge, this is the first report of "Ca. Liberibacter solanacearum" in field tomatoes showing ZC-like foliar disease symptoms in the United States. References: (1). J. A. Abad et al. Plant Dis. 93:108, 2009 (2) A. K. Hansen et al. Appl. Environ. Microbiol. 74:5862, 2008. (3) L. W. Liefting et al. Plant Dis. 93:208, 2009. (4) G. A. Secor et al. Plant Dis. 93:574, 2009.

Effect of a Terminated Cover Crop and Aldicarb on Cotton Yield and Meloidogyne incognita Population Density.

Terminated small grain cover crops are valuable in light textured soils to reduce wind and rain erosion and for protection of young cotton seedlings. A three-year study was conducted to determine the impact of terminated small grain winter cover crops, which are hosts for Meloidogyne incognita, on cotton yield, root galling and nematode midseason population density. The small plot test consisted of the cover treatment as the main plots (winter fallow, oats, rye and wheat) and rate of aldicarb applied in-furrow at-plant (0, 0.59 and 0.84 kg a.i./ha) as subplots in a split-plot design with eight replications, arranged in a randomized complete block design. Roots of 10 cotton plants per plot were examined at approximately 35 days after planting. Root galling was affected by aldicarb rate (9.1, 3.8 and 3.4 galls/root system for 0, 0.59 and 0.84 kg aldicarb/ha), but not by cover crop. Soil samples were collected in mid-July and assayed for nematodes. The winter fallow plots had a lower density of M. incognita second-stage juveniles (J2) (transformed to Log(10) (J2 + 1)/500 cm(3) soil) than any of the cover crops (0.88, 1.58, 1.67 and 1.75 Log(10)(J2 + 1)/500 cm(3) soil for winter fallow, oats, rye and wheat, respectively). There were also fewer M. incognita eggs at midseason in the winter fallow (3,512, 7,953, 8,262 and 11,392 eggs/500 cm(3) soil for winter fallow, oats, rye and wheat, respectively). Yield (kg lint per ha) was increased by application of aldicarb (1,544, 1,710 and 1,697 for 0, 0.59 and 0.84 kg aldicarb/ha), but not by any cover crop treatments. These results were consistent over three years. The soil temperature at 15 cm depth, from when soils reached 18 degrees C to termination of the grass cover crop, averaged 9,588, 7,274 and 1,639 centigrade hours (with a minimum threshold of 10 degrees C), in 2005, 2006 and 2007, respectively. Under these conditions, potential reproduction of M. incognita on the cover crop did not result in a yield penalty.

Effect of Fumigation on Rotylenchulus reniformis Population Density Through Subsurface Drip Irrigation Located Every Other Furrow.

Plots naturally infested with Rotylenchulus reniformis were sampled in the spring of 2006 and 2007 at depths of 15 and 30 cm in the bed, furrow over the drip tape, and "dry" furrow, and at approximately 40 to 45 cm depth in the bed and dry furrow. Then, 1,3-dichloropropene (Telone EC) was injected into the subsurface drip irrigation at 46 kg a.i./ha, and 3 to 4 weeks later the plots were resampled and assayed for nematodes. The transformed values for nematode population density (IvLRr) before fumigation were higher at 30 and 40 cm depths than at a 15 cm depth. IvLRr before fumigation was higher in the soil over the drip lines than in the bed or dry furrow and was higher in the bed than the dry furrow. IvLRr was higher in the plots to be fumigated than the plots that were not to be fumigated for all depths and locations except at a 15 cm depth over the drip lines, where the values were similar. However, after fumigation, IvLRr was lower over the drip lines at a 30 cm depth in plots that were fumigated compared to samples in a similar location and depth that were not fumigated. There were no other location/depth combinations where the fumigation reduced IvLRr below that in the nonfumigated plots. Yield in 2006, which was a very hot and dry year, was predicted adequately (R(2) = 0.67) by a linear model based on the preplant population density of R. reniformis, with a very steep slope (-2.8 kg lint/ha per R. reniformis/100 cm(3) soil). However, no relationship between nematode density and yield was seen in 2007, which had cooler weather for most of the season. Yield was not significantly improved by fumigation through the drip irrigation system in either year compared to plots treated only with aldicarb (0.84 kg a.i./ha), indicating that the level of control with fumigation did not kill enough R. reniformis to be successful.

First Report of Sclerotinia minor Infecting Ipomoea hederacea and I. coccinea in Texas.

Sclerotinia blight, caused by the soilborne fungus Sclerotinia minor Jagger, is a major disease of peanut (Arachis hypogaea L.) in parts of west Texas. Previous reports have indicated that annual weed species may serve as collateral hosts for S. minor (2). Several Ipomoea spp. are commonly found in peanut fields throughout the region. In September of 2007, Ipomoea hederacea and I. coccinea plants with bleached, shredded stems, and signs of black sclerotia were collected from a field known to be infested with S. minor. Symptomatic stem sections were rinsed in tap water, surface disinfested in 0.5% sodium hypochlorite for 1 min, air dried, and plated on potato dextrose agar (PDA). Pure cultures of S. minor consisting of white, fluffy mycelia and small (<2 mm), black, irregular sclerotia were consistently recovered. Pathogenicity tests were conducted by wound-inoculating healthy I. hederacea and I. coccinea transplants (n = 3) with agar plugs obtained from the edges of actively growing S. minor cultures. Plants were incubated in a dew chamber at 20°C and 95% relative humidity for 5 days. Plants inoculated with sterile PDA plugs served as controls (n = 3). A similar test was conducted using the susceptible peanut cultivar Flavorunner 458. Characteristic symptoms of Sclerotinia blight (3) were observed on all inoculated weed and peanut plants; whereas, the controls remained healthy. Pathogenicity tests were repeated with similar results. Cultures of S. minor were obtained from all symptomatic tissues, fulfilling Koch's postulates. These results indicate that I. hederacea and I. coccinea are additional hosts of S. minor and that sclerotia produced on infected plants can significantly augment soil inoculum. S. minor has been observed to infect I. batatas seedlings in New Jersey (1); however, this to our knowledge is the first report of S. minor infecting Ipomoea spp. in Texas. Therefore, weed management should inevitability be a part of disease management strategies for the control of Sclerotinia blight in peanut. References: (1) Anonymous. Index of Plant Diseases in the United States. USDA Handb. No. 165, 1960. (2) J. E. Hollowell et al. Plant Dis. 87:197, 2003. (3) D. M. Porter and H. A. Melouk. Sclerotinia blight. Page 34 in: Compendium of Peanut Diseases. 2nd ed. N. Kokalis-Burelle et al., eds. The American Phytopathologicial Society, St. Paul, MN, 1997.

Assessing Heterodera glycines-Resistant and Susceptible Cultivar Yield Response.

The soybean cyst nematode Heterodera glycines (SCN) is of major economic importance and widely distributed throughout soybean production regions of the United States where different maturity groups with the same sources of SCN resistance are grown. The objective of this study was to assess SCN-resistant and -susceptible soybean yield responses in infested soils across the north-central region. In 1994 and 1995, eight SCN-resistant and eight SCN-susceptible public soybean cultivars representing maturity groups (MG) I to IV were planted in 63 fields, either infested or noninfested, in 10 states in the north-central United States. Soil samples were taken to determine initial SCN population density and race, and soil classification. Data were grouped for analysis by adaptation based on MG zones. Soybean yields were 658 to 3,840 kg/ha across the sites. Soybean cyst nematode-resistant cultivars yielded better at SCN-infested sites but lost this superiority to susceptible soybean cultivars at noninfested sites. Interactions were observed among initial SCN population density, cultivar, and location. This study showed that no region-wide predictive equations could be developed for yield loss based on initial nematode populations in the soil and that yield loss due to SCN in our region was greatly confounded by other stress factors, which included temperature and moisture extremes.

Evaluation of Several Approaches to Manage Meloidogyne incognita and Cotton Seedling Disease Complexes in the High Plains of Texas.

Field experiments were conducted for control of the southern root-knot nematode (Meloidogyne incognita) and cotton seedling disease fungi (primarily Thielaviopsis basicola) in one naturally infested field during 1999 and 2000 and in three additional fields in 2000. Treatments included: seed-applied fungicides (triadimenol + mefenoxam + thiram and carboxin + PCNB + mefenoxam), cultivars (Paymaster [PM] 2326 RR and PM 2200 RR), and a nematicide (aldicarb at 0.83 kg a.i/ha). Plant stands were higher (P = 0.02) in the presence of aldicarb (77% emergence) than in its absence (74% emergence). Hypocotyl disease symptom ratings were lower (P = 0.0001) following triadimenol + mefenoxam + thiram seed treatment (0.53) as compared with carboxin + PCNB + mefenoxam (0.93). Root necrosis was lower (P = 0.002) following triadimenol + mefenoxam + thiram seed treatment (27%) as compared with carboxin + PCNB + mefenoxam (34%). In one field, in both years, aldicarb was associated with more root necrosis (58%) than in its absence (46%) (P = 0.004). At three other sites aldicarb did not affect root necrosis. Population densities of Meloidogyne incognita eggs and juveniles at midseason were greater (P = 0.005, P = 0.003, respectively) on PM 2200 RR (less resistant) than on PM 2326 RR (more resistant). Yield was affected by the plant genotype by aldicarb interaction (P = 0.02) but not by seed treatments. Aldicarb effect on yield was dependent on cultivar, whereas affect of seed treatment on root health was consistent and independent of cultivar and aldicarb. No conditions were identified when use of triadimenol + mefenoxam was detrimental.

First Report of Cylindrocladium Black Rot Caused by Cylindrocladium parasiticum on Peanut in Texas.

During August 2004, Cylindrocladium parasiticum Crous, M.J. Wingf., & Alfenas (teleomorph Calonectria ilicicola Boedijin & Reitsma) was isolated from peanut (Arachis hypogaea L.) cv. NC 7 taken from an irrigated field in Terry County, TX. On 24 September, the mean length of patches with symptoms of dead plants and confirmed for the presence of C. parasiticum was 40 cm (averaged over 52 patches). Pods, pegs, roots, and lower stems of affected plants had decayed black lesions. No perithecia were observed. Roots were plated on water agar, and mycelia growth was transferred to potato dextrose agar (PDA). Conidia and microsclerotia typical of C. parasiticum had formed at 14 days. After 21 days on PDA, the following mean measurements were recorded: macroconidia 57 × 7 µm (usually two septations); stipe 115 × 4 µm; and sphaeropedunculate vesicle 15 × 10 µm. These are within reported ranges for C. parasiticum (1). Four-week-old cultures were comminuted in tap water and added (mycelium and microsclerotia) to potting medium (noninfested nonsterile peanut field soil/peat moss potting mix, 1:1). Peanut-specific Bradyrhizobium sp. was applied before planting. Three 3-day-old peanut cv. Tamrun 88 seedlings were transplanted in infested soil (four replicate pots, five isolates). Root, hypocotyl, pod, peg, and lower stem lesions were evident after 8 (Trial 1, one isolate) and 16 weeks (Trial 2, four isolates). Virulence varied among isolates (3). C. parasiticum was reisolated on acidified PDA after both trials. In Trial 3, six plant species (three replicate pots, five plants per pot) were challenged with one isolate by drenching a tap water suspension of comminuted microsclerotia and mycelium from 4-week-old PDA cultures into potting medium and transplanting peanut seedlings or planting seeds for other hosts. Root rot severity (0 to 5 index) (3) and reisolation percentages were cv. Tamrun 88 peanut 2.0, 100%; cv. Tamrun 96 peanut 2.0, 92%; cv. Hutcheson soybean (Glycine max (L.) Merr.) 3.3, 64%; cv. Blue Lake 274 green bean (Phaseolus vulgaris L.) 2.7, 40%; cv. California Blackeye 8046 southern pea (Vigna unguiculata (L.) Walp. 2.0, 70%; and cv. Jubilee Hybrid sweet corn (Zea mays L.) 0.0, 100%. Noninoculated peanut controls in all trials had no black decayed root lesions and attempts to isolate the fungus were unsuccessful. Peanut seeds planted in the production field were obtained from the southeastern United States. This field had been scheduled for seed production until C. parasiticum was identified. Temperatures and rainfall amounts during the 2004 growing season were less and greater than long-term averages, respectively. No other infested peanut fields have been confirmed or suspected in the region or state. There is continued risk of contamination from shipments of infected seeds from infested areas (2). This pathogen may pose a serious threat to >100,000 ha of peanut production in western Texas and eastern New Mexico. References: (1) P. W. Crous. Taxonomy and Pathology of Cylindrocladium (Calonectria) and Allied Genera. The American Phytopathological Society, St. Paul, MN, 2002. (2) B. L. Randall-Schadel et al. Plant Dis. 85:362, 2001. (3) R. C. Rowe and M. K. Beute. Phytopathology 65:422, 1975.

Relationship of Aerial Broad Band Reflectance to Meloidogyne incognita Density in Cotton.

Aerial images were obtained on 22 July 1999 and 4 August 2000 from five cotton sites infested with Meloidogyne incognita. Images contained three broad bands representing the green (500-600 nm), red (600-700 nm), and near-infrared (700-900 nm) spectrum. Soil samples were collected and assayed for nematodes in the fall at these sites. Sampling locations were identified from images, by locating the coordinates of a wide range of light intensity (measured as a digital number) for each single band, and combinations of bands. There was no single band or band combination in which reflectance consistently predicted M. incognita density. In all 10 site-year combinations, the minimum number of samples necessary to estimate M. incognita density within 25% of the population mean was greater when sampling by reflectance-based classes (3 to 4 per site) than sampling based on the entire site as one unit. Two sites were sampled at multiple times during the growing season. At these sites, there was no single time during the growing season optimal to take images for nematode sampling. Aerial infrared photography conducted during the growing season could not be used to accurately determine fall population densities of M. incognita.

Effects of Quadrat Size and Time of Year for Sampling of Verticillium dahliae and Lesion Nematodes in Potato Fields.

Six potato fields were randomly sampled using quadrat areas ranging from 10 m2 to 4,000 m2. A composite soil sample consisting of 20 soil cores was taken from each of 20 quadrats of each unit area. All samples were assayed for Verticillium dahliae and nematodes. The area in which a composite sample was taken had little effect on estimated mean and variance of V. dahliae until quadrat size was ≥1,000 m2. Then variances dramatically increased with size. With Pratylenchus spp., there was no systematic relationship between quadrat size and variance. Seven additional fields were divided into 2,000 or 4,000 m2 grids, and each grid was sampled in both spring and fall for V. dahliae and plant-parasitic nematodes (Pratylenchus spp. and Meloidogyne hapla). Density of V. dahliae was lower, and in some cases, degree of aggregation was higher, in the spring than in the fall. Sampling for V. dahliae in the fall required 2 to 7 composite samples to estimate mean density within 25% of the true mean, while in the spring, 10 to 19 samples were required. Assays of Pratylenchus spp. required a similar number of samples in spring and fall surveys, while M. hapla required fewer samples with spring than with fall sampling when estimating densities >20 second-stage juveniles per 100 cm3 of soil.

Soybean Cyst Nematode Reproduction in the North Central United States.

An experiment was conducted in Heterodera glycines-infested fields in 40 north central U.S. environments (21 sites in 1994 and 19 sites in 1995) to assess reproduction of this nematode. Two resistant and two susceptible soybean cultivars from each of the maturity groups (MG) I through IV were grown at each site in 6.1 m by 4 row plots. Soil samples were collected from each plot at planting and harvest and processed at Iowa State University to determine H. glycines initial (Pi) and final (Pf) population densities as eggs per 100 cm3 of soil. Overall, reproduction (Pf/Pi) of H. glycines on susceptible cultivars in all MG was similar. Reproduction was higher on MG III and IV susceptible cultivars than on those in MG I and II. Resistant MG I and II cultivars reduced nematode population densities more consistently than those in MG III and IV. Reproduction of the nematode was similar among sites within the same maturity zone (MZ), defined as the areas of best adaptation of the corresponding MG. Nonetheless, careful monitoring of nematode population densities is necessary to assess changes that occur over time in individual fields.

Variability in Time and Space of Meloidogyne incognita Fall Population Density in Cotton Fields.

Three cotton fields infested with Meloidogyne incognita were intensively sampled in the fall for 3 years (1996 to 1998) to determine if intensive sampling for M. incognita, for which spatial location is important, was necessary every year in a continuous cotton system. Two composite soil samples (20 cores each), taken over an area covering one-third of the field length and two rows wide, were averaged to represent that area (row-location combination). Each field (except one) had 24 areas assayed for changes in M. incognita population density (Pf) over a 3-year period. At all three sites, Pf was higher during fall 1998 than fall 1996. There were no differences in Pf between rows within a year or between years (no. row x year interaction) at any of the sites. At all three sites, there was a consistent difference each year in Pf among locations in a field (no. year x location interaction). At each area, M. incognital/500 cm(3) was labeled for one of four Pf classes: <250, 250 to 999, 1,000 to 2,499, and >/= 2,500. Management of root-knot nematode would likely be altered as classification changed. The areas that were reclassified by two classes or more after 1 and 2 years ranged from 0 to 29% and 25 to 54%, respectively. The risk of underestimating Pf of M. incognita was higher in one site 2 years after the initial intensive sampling procedure, whereas in another site there was little change in Pf 2 years after initial sampling. Sampling frequency will need to be decided on a field-by-field basis.

Root Galling and Reproduction of Meloidogyne incognita Isolates from Texas on Resistant Cotton Genotypes.

Several cotton genotypes with resistance to Meloidogyne incognita have been released in recent years. To estimate the durability of this resistance, galling severity on these resistant genotypes by M. incognita was measured. Nematode isolates (115 total) were collected from cotton fields in 14 Texas counties in August and September 1996 and 1997. Four additional isolates from Maryland, Mississippi, and North Carolina were also tested. The isolates were evaluated in 12 greenhouse experiments for their ability to gall roots of the resistant cotton genotypes M315, Acala NemX, and Stoneville LA887 and the susceptible cultivar Deltapine 90. Numbers of galls on each genotype by each isolate were counted 60 days after inoculation with 10,000 eggs/plant. M315 consistently had the fewest galls for each nematode isolate, whereas Deltapine 90 had the greatest number of galls. Numbers of galls on NemX and LA887 were usually intermediate and more variable. For each separate experiment, analysis of variance indicated that the effects of nematode isolates, cotton genotypes, and isolate-genotype interaction were significant (P < 0.05). In two of the experiments, nematode reproduction was also measured and galling was positively correlated (r = 0.68 and 0.86) with egg production by M. incognita. Nematode isolates from one field exhibited higher root galling and reproduction (P < 0.05) on resistant genotypes than other isolates, suggesting a need for gene deployment systems that will enhance the durability of resistance.

Survey of Meloidogyne incognita and Thielaviopsis basicola: Their Impact on Cotton Fruiting and Producers' Management Choices in Infested Fields.

A survey of 100 cotton fields selected randomly in 1995 and 1996 was conducted in the High Plains of Texas to determine the incidence and potential severity of Meloidogyne incognita and Thielaviopsis basicola. Information was obtained from producers for each field on their nematicide application rates and fungicide seed treatments. The percent of squares and bolls set was evaluated for 20 plants in each field during late July 1995 and early August 1996. Thielaviopsis basicola was identified in 55% of the fields in 1995 and 73% of the irrigated fields in 1996. Meloidogyne incognita was found in 39% and 43% of the fields in 1995 and 1996, respectively. Both M. incognita and T. basicola were found together in approximately 30% of the fields. The average rate of aldicarb used in 1995 and 1996 was higher when fields were infested with both T. basicola and M. incognita than for fields infested with none or only one of the pathogens. However, there was no relationship between the use of fungicide treatments active against T. basicola and the potential for root necrosis, or presence of either or both pathogens (T. basicola and M. incognita). Aldicarb rates and usage of fungicide seed treatments were chosen by producers before fields were surveyed (i.e., the survey did not influence grower practices). In 1995, but not in 1996, the association of M. incognita and potential root necrosis (based on a bioassay from soil samples) was negatively correlated with the number of squares, percentage of squares set, and percentage of bolls set. The association between M. incognita and T. basicola, or potential severity of root necrosis, could not be correlated with fruit attributes in 1996 under warm spring conditions but was negatively correlated with fruit attributes in 1995 under cool spring conditions.

Comparison of Variable and Single-Rate Applications of Aldicarb on Cotton Yield in Fields Infested with Meloidogyne incognita.

Variable-rate applications of the nematicide aldicarb were compared to producer standard rates in eight field tests over 3 years. Test areas (308 to 1,015 m long) were divided into eight or five blocks. Each block contained two plots with a variable-rate treatment (VRT) of aldicarb and a producer standard treatment (PST) of aldicarb. Each VRT plot was divided into three subunits and intensively sampled for Meloidogyne incognita in either the fall or spring before planting. Rates of aldicarb were assigned to each subunit for VRT based on M. incognita population density. In three of the eight tests, VRT resulted in either higher yield or similar yields, but less nematicide applied. In two tests there were no differences between PST and VRT in yields or average rates of aldicarb applied. In three tests, VRT used more aldicarb (>0.17 kg a.i./ha difference) than PST and yields were not significantly different between treatments. In two of the cases where VRT was superior to PST, the producer's rate of aldicarb was judged to be either too low or too high for the average M. incognita density present in the field. In all three cases where PST was superior to VRT, perennial weeds were an important factor also limiting yield. Variable-rate application of aldicarb did not consistently provide for higher yields or lower nematicide usage than standard application rates.

Identification of Resistance to Thielaviopsis basicola in Diploid Cotton.

A screening program to find sources of resistance to Thielaviopsis basicola in cotton identified a diploid cotton (Gossypium arboreum, PI 1415) with apparent high resistance to this pathogen. Seedlings were obtained from self-pollinated seed taken from the resistant plant (PI 1415) and grown in growth chamber tests with the cultivars (tetraploid G. hirsutum) Paymaster HS-26 and Paymaster Tejas as controls. Seedlings were grown for 20 days in naturally infested field soil and evaluated for root growth and root necrosis. In all tests, PI 1415 had less (P = 0.001) root necrosis than the cultivars HS-26 and Tejas. A triadimenol seed treatment also resulted in less (P = 0.001) root necrosis for all three cotton genotypes, and the response was additive with level of disease resistance. Incorporation of the resistance factor from PI 1415 into tetraploid upland cotton may greatly reduce damage by black root rot, especially in combination with fungicide seed treatments.

Root Rot by Rhizoctonia solani on Ambrosia grayi in Texas.

Ambrosia grayi (A. Nelson) Shinners is a pernicious perennial weed that infests over 200,000 ha of cotton in Texas and New Mexico. It reproduces primarily by rhizomes, which are found up to 3 m deep in the soil. With the exception of Roundup Ultra on Roundup-ready cotton, no herbicides can be applied in-season to control this weed without injuring cotton. A search was conducted for soilborne pathogens as potential biocontrol agents by surveying cotton fields and small lakes for A. grayi plants with root lesions. Roots were washed with running water for 2 min and segments of root lesions were placed on water agar (1.5%) with hyphae of emerging fungi transferred to potato dextrose agar (PDA). Eighty isolates of fungi were collected; most were Fusarium spp. or Rhizoctonia solani. Replicated experiments with isolates were conducted. Each isolate was grown on PDA and then 1/4 of the media in a petri plate (12.6 cm2) was mixed with 450 cm3 autoclaved soil and placed in a styrofoam cup. A piece of rhizome (3 to 5 cm in length) was placed 5 cm below the soil surface in the infested cup. Roots with necrosis were plated on PDA to recover fungi, which were grown in pure culture and used to reinfest soil. Significant disease (in repeated tests) was caused by two isolates of R. solani, compared with the noninfested check, as indicated by a reduction in number of emerging plants per pot, increase in root necrosis, and decrease in fresh and dried leaf weight. Roots growing from the infected rhizomes were discolored and rotted without distinct lesions. Both of these isolates were identified as AG4 by anastomosis with tester ATCC #46134 by the cellophane technique (1). Reference: (1) D. E. Carling et al. Phytopathology 77:1609, 1987.