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.

First Report of Meloidogyne enterolobii on Cotton and Soybean in North Carolina, United States.

Stunted cotton plants (Gossypium hirsutum L. cvs. PHY 375 WR and PHY 565 WR) from two separate fields near Goldsboro in Wayne County, North Carolina were collected by the NCDA&CS Agronomic Division nematode lab for nematode assay and identification in December 2011. The galls on cotton plants were very large in comparison with those commonly associated with Meloidogyne incognita Kofoid and White (Chitwood) infected cotton. In August 2012, the lab also received heavily galled roots of soybean (Glycine max (L.) Merr. cv. 7732) from Wayne and Johnston counties. Population densities of the 2nd-stage juveniles ranged from 150 to 3,800 per 500 cc soil. Female perineal patterns were similar to M. incognita, but PCR and DNA sequencing matched that of M. enterolobii Yang and Eisenback (4). DNA sequences of ribosomal DNA small subunit, internal transcribed spacer, large subunit domain 2 and 3, intergeneric spacer, RNA polymerase II large subunit, and histone gene H3, were found to be 100% homologous when comparing populations of M. enterolobii from North Carolina and China. Species identification was also confirmed using PCR by a species-specific SCAR primer set MK7-F/MK7-R (2). M. enterolobii Yang & Eisenback was described in 1983 from a population causing severe damage to pacara earpod tree (Enterolobium contortisiliquum (Vell.) Morong) in China (4). In 2004, M. mayaguensis Rammah & Hirschmann, a species described from Puerto Rico, was synonymized with M. enterolobii based on esterase phenotype and mitochondrial DNA sequence (3). M. enterolobii is considered to be a highly pathogenic species and has been reported from vegetables, ornamental plants, guava, and weeds in China, Africa, Central and South America, the Caribbean, and Florida in the United States (1,3,4). Of particular concern is its ability to develop on crop genotypes carrying root-knot-nematode resistance genes (Mi-1, Mh, Mir1, N, Tabasco, and Rk) in tobacco, tomato, soybean, potato, cowpea, sweet potato, and cotton. Consequently, this species was added to the European and Mediterranean Plant Protection Organization A2 Alert list in 2010. Two populations of M. enterolobii one from soybean and one from cotton were reared on tomato (Solanum lycopersicum L. var. lycopersicum) in a greenhouse setting. Eggs were extracted using NaOCl and inoculated, at a rate of 7,000 per 15-cm-diameter clay pot, into a sandy soil mixture (1:1 washed river sand and loamy sand). Tomato, peanut (Arachis hypogaea L.), cotton, watermelon (Citrullus lanatus (Thunb.) Matsum. & Nakai), pepper (Capsicum annuum L.), and root-knot-susceptible and -resistant tobacco (Nicotiana tabacum L. cvs. K326 and NC 70, respectively) were transplanted immediately into the infested soil with four replications. Root galls on the host differentials were evaluated after 90 days. Reproduction occurred on all hosts except for peanut, which is consistent with reports for M. enterolobii and M. incognita race 4 (4). Adult females from pepper plants used in the host differential test were sequenced on partial 18S and ITS1 region and confirmed to be M. enterlobii. To our knowledge, this is the first report of a natural infection of North Carolina field crops with M. enterolobii. References: (1) J. Brito et al. J. Nematol. 36:324, 2004. (2) M. S. Tigano et al. Plant Pathol. 59:1054, 2010. (3) J. Xu et al. Eur. J. Plant Pathol. 110:309, 2004. (4) B. Yang and J. D. Eisenback. J. Nematol. 15:381, 1983.

The future of nematode management in cotton.

The importance of plant-parasitic nematodes as yield-limiting pathogens of cotton has received increased recognition and attention in the United States in the recent past. This paper summarizes the remarks made during a symposium of the same title that was held in July 2007 at the joint meeting of the Society of Nematologists and the American Phytopathological Society in San Diego, California. Although several cultural practices, including crop rotation, can be effective in suppressing the populations of the important nematode pathogens of cotton, the economic realities of cotton production limit their use. The use of nematicides is also limited by issues of efficacy and economics. There is a need for development of chemistries that will address these limitations. Also needed are systems that would enable precise nematicide application in terms of rate and placement only in areas where nematode population densities warrant application. Substantial progress is being made in the identification, characterization and mapping of loci for resistance to Meloidogyne incognita and Rotylenchulus reniformis. These data will lead to efficient marker-assisted selection systems that will likely result in development and release of nematode-resistant cotton cultivars with superior yield potential and high fiber quality.

First Report of Phoma terrestris Causing Red Root Rot on Sweet Corn (Zea mays) in North Carolina.

Red root rot, caused by Phoma terrestris E. M. Hansen, caused premature senescence and yield reductions to fresh-market sweet corn in Hyde County, North Carolina in July 2006. Foliar symptoms developed over a period of 5 to 8 days approximately 1 to 2 weeks after anthesis and included desiccation of leaves and poor development of ears. By 3 weeks after pollination, when the sweet corn was harvested, crowns and the first aboveground internode of affected plants were rotted and reddish colored, but roots appeared normal. The root mass of affected plants tended to be greater than that of unaffected plants. Incidence of symptomatic plants was greater than 30% in some fields and was lower on crops planted and harvested early. Symptomatic and asymptomatic plants were adjacent in affected fields. Diseased plants were more common in fields of sweet corn that followed soybean (Glycine max) or a double-crop of onions (Allium cepa) than in fields that followed corn. Incidence of symptomatic plants also differed among adjacent plantings of different sweet corn hybrids. Hybrids '173A', '182A', '378a', and 'XTH1178' had a high incidence of symptomatic plants and '372A', '278A', '8101', and '8102' were less affected. Samples of symptomatic plants of the hybrid '182A' were examined at the North Carolina Plant Disease and Insect Clinic during August. Olivaceous black pycnidia with long setae around the ostioles were imbedded in the stalk near the first node aboveground. Numerous conidia (1.8 to 2.3 × 4.5 to 5.5 µm) were released in cirri from pycnidia. When cultured on potato dextrose agar (PDA), the fungus produced a red pigment and intercalary and terminal chlamydospores. Pathogenicity was demonstrated in the greenhouse by transplanting corn seedlings or direct-seeding corn into pots of soil infested with plates of PDA containing chlamydospores and hyphae. A suspension of chlamydospores and hyphae also was injected into the stems of plants 28 days after transplanting. Five replicates of the pathogenicity experiments were repeated twice with noninoculated controls. After 8 weeks, P. terrestris was recovered from the roots of all inoculated plants. Soil inoculation resulted in necrotic root tissue in approximately 25% of inoculated plants. Approximately 90% of inoculated plants had discolored crowns that resembled symptoms from field infected plants. Stem inoculations resulted in necrosis extending 2 to 5 cm from the point of injection and resulted in shoot death of 40% of inoculated plants that resulted in the development of an adventitious shoot. Red root rot was prevalent on field corn in the Delmarva Peninsula throughout the late 1980s and 1990s (1). To our knowledge, this is the first report of this disease causing damage to sweet corn in North Carolina. Foliar symptoms and discoloration of crowns of diseased sweet corn plants were similar to previously described symptoms of red root rot on field corn (2), however, roots of affected sweet corn plants were not substantially rotted and did not have a symptomatic reddish pink or dark carmine color, presumably because sweet corn is harvested prior to the development of root symptoms. References: (1) K. W. Campbell et al. Plant Dis. 75:1186, 1991. (2) D. G. White, ed. Compendium of Corn Diseases. The American Phytopathological Society, St Paul, MN, 1999.

First Report of Phakopsora pachyrhizi on Kudzu (Pueraria montana var. lobata) in North Carolina and Increased Incidence of Soybean Rust on Soybean in 2006.

Asian soybean rust, caused by Phakopsora pachyrhizi H. Sydow & Sydow, was first detected in the continental United States in soybean (Glycine max (L.) Merr.) in Louisiana on 6 November 2004 (3) and in kudzu (Pueraria montana var. lobata) in Florida during February 2005 (1). Soybean rust was first confirmed in North Carolina in commercial soybean fields in Brunswick, Columbus, and Robeson counties on 25 October 2005 (2). Subsequently, the disease was detected in soybean in 18 counties, but not in kudzu, even when it was growing adjacent to infected soybean. During 2006, soybean rust was first detected in North Carolina in soybean on 14 September 2006 from a sample from Columbus County that was submitted to the North Carolina State University Plant Disease and Insect Clinic (NCSU-PDIC). Thus, the first detection of soybean rust in North Carolina occurred almost 6 weeks earlier in 2006 than in 2005. Subsequently, in 2006, soybean rust was found in soybean in 42 counties in North Carolina through survey, sentinel plot monitoring, and samples submitted to the NCSU-PDIC. In addition, what appeared to be soybean rust was observed in two samples of kudzu collected on 3 and 6 November 2006 from Moore (35.28313°N, 79.38020°W) and Johnston (35.42742°N, 78.18154°W) counties of North Carolina. The diagnosis of P. pachyrhizi in kudzu was confirmed visually and by ELISA protocol supplied with the EnviroLogix QualiPlate kit (Portland, ME). ELISA tests for each kudzu sample were run in triplicate. PCR was also conducted on infected kudzu samples with a protocol previously reported (1). The PCR master mix that was used came from a dilution scheme based on previous PCR work completed by G. Z. Abad. A total of 24 reactions were run, including four 1-kb molecular markers, four positive controls, four negative controls, and four infected kudzu leaf tissue samples. The results of all diagnostic techniques confirmed the presence of P. pachyrhizi in diseased kudzu. To our knowledge, this is the first report of P. pachyrhizi in kudzu in North Carolina. References: (1) P. F. Harmon et al. Online publication. doi:10.1094/PHP-2005-0613-01-RS. Plant Health Progress, 2005. (2) S. R. Koenning et al. Plant Dis. 90:973, 2006. (3) R. W. Schneider et al. Plant Dis. 89:774, 2005.

First Report of Soybean Rust Caused by Phakopsora pachyrhizi in North Carolina.

Asian soybean rust, caused by Phakopsora pachyrhizi Sydow, has been known to occur in the eastern hemisphere for nearly a century. More recently, it was reported from South America in 2002 and the continental United States in Louisiana in November 2004 (1,2). Subsequently, P. pachyrhizi was confirmed in Alabama, Arkansas, Georgia, Florida, Missouri, Mississippi, South Carolina, and Tennessee in 2004. Surveys conducted in North Carolina in late November 2004 failed to detect this pathogen. Symptoms of the disease were first observed on soybean (Glycine max (L.) Merr.) in North Carolina on 25 October 2005 in farmers' fields in the counties of Brunswick, Columbus, and Robeson. Typical pustules and urediniospores were readily apparent on infected leaves when viewed with a dissecting microscope. Urediniospores were obovoid to broadly ellipsoidal, hyaline to pale yellowish brown with a minutely echinulate thin wall, and measured 18 to 37 × 15 to 24 µm. This morphology is typical of soybean rust caused by P. pachyrhizi or P. meibomiae, the latter is a less aggressive species causing soybean rust in the western hemisphere (1). DNA was extracted from leaves containing sori using the Qiagen DNeasy Plant Mini kit (Valencia, CA). P. pachyrhizi was detected using a real-time polymerase chain reaction (PCR) protocol that differentiates between P. pachyrhizi and P. meibomiae in a Cepheid thermocycler (Sunnyvale, CA) with appropriate positive and negative controls. The PCR master mix was modified to include OmniMix beads (Cepheid). Field diagnosis of P. pachyrhizi was confirmed by the USDA/APHIS on 28 October 2005. Soybean rust was identified in subsequent surveys of soybean fields and leaf samples submitted by North Carolina Cooperative Extension Agents in an additional 15 counties. These samples also were assayed using a traditional PCR protocol and by the enzyme-linked immunosorbent assay protocol included in the EnviroLogix QualiPlate kit (Portland, ME) for soybean rust. Ten soybean specimens from 10 sites were confirmed positive by these methods. Disease was not found on three kudzu samples, although one kudzu sample was adjacent to a soybean field that was positive for P. pachyrhizi. Although soybean rust was eventually detected in 18 North Carolina counties in 2005, no soybean yield loss occurred since the pathogen was detected when more than 80% of the soybean crop was mature. To our knowledge, this is the first report of P. pachyrhizi in North Carolina and the northern most find on soybean in the continental United States in 2005. References: (1) R. D. Frederick et al. Phytopathology 92:217, 2002. (2) R. W. Schneider et al. Plant Dis. 89:774 2005.

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.

Quantifying Potential Tolerance of Selected Cotton Cultivars to Belonolaimus longicaudatus.

Glyphosate-tolerant cotton cultivars were evaluated for tolerance to Belonolaimus longicaudatus in field experiments conducted from 2004 to 2005. Field trials were arranged in a split-plot design that included treatment with four levels of 1, 3-dichloropropene (0.0, 13.9, 27.8, and 41.7 1 a.i./ha) to establish a range of population densities of B. longicaudatus. Six cotton cultivars (early-to-mid maturity: DP444BG/RR SG501BR, ST5242BR; mid-to late maturity: DP451B/RR, ST5599BR, DP655BRR) were planted as whole plots. Fumigation was effective in suppressing B. longicaudatus population densities at mid-season, but not at cotton harvest, and increased cotton lint yield. The cultivar x fumigation interaction for cotton lint yield was not significant for the six cultivars evaluated, indicating that tolerance did not occur in this nematode-host combination. Early-to-mid maturity cultivars yielded significantly more than mid-to-late maturity cultivars in both years. Small but significant differences in nematode final population density were observed between cultivars that may be related to relative maturity.

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.

Influence of poultry litter applications on nematode communities in cotton agroecosystems.

The effects of the application of poultry litter at 0.0, 6.7, 13.4, and 20.1 tons/ha on population changes during the growing season on nematode communities were evaluated in two cotton production fields in North Carolina. Numbers of bactivorous nematodes increased at midseason in response to the rate at which litter was applied but decreased with increasing litter application rates at cotton harvest. Numbers of fungivores at cotton harvest were related positively to the rate of litter applied, and this affected a positive increase in the fungivore-to-bacterivore ratio at this sampling date. The rate at which poultry litter was applied resulted in an increase in the bacterivore to plant-parasite ratio, and this corresponded with increased cotton lint yield. Trophic diversity was increased by litter application rate at cotton harvest at one location but not at another. The plant-parasite maturity index was greater consistently at one site than at a second site where the Hoplolaimus columbus population density was above the damage threshold for cotton. The population density of H. columbus was suppressed with increasing rates of poultry litter application, but other plant-parasitic nematodes were affected marginally.

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.

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.

Rotylenchulus reniformis Management in Cotton with Crop Rotation.

One-year crop rotations with corn or highly resistant soybean were evaluated at four locations for their effect on Rotylenchulus reniformis population levels and yield of a subsequent cotton crop. Four nematicide (aldicarb) regimes were included at two of the locations, and rotation with reniform-susceptible soybean was included at the other two locations. One-year rotations to corn or resistant soybean resulted in lower R. reniformis population levels (P

Impact of Cotton Production Systems on Management of Hoplolaimus columbus.

The effectiveness of selected cultural practices in managing the Columbia lance nematode, Hoplolaimus columbus, on cotton was evaluated in experiments in growers' infested fields. The effects of planting date, cotton cultivar, treatment with the growth regulator mepiquat chloride, and destruction of cotton-root systems after harvest on cotton-lint yield and population densities of H. columbus were studied. The yield of cotton cultivar Deltapine 50 was negatively related (P = 0.054) to initial population density of H. columbus whereas the yield of Deltapine 90 was not affected by preplant density of this nematode, indicating tolerance in Deltapine 90. Reproduction of this nematode did not differ on the two cultivars. Planting date and treatment with the growth regulator mepiquat chloride did not influence cotton yield in a consistent manner. Application of mepiquat chloride suppressed (P

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.

Tolerance to Hololaimus columbus in Glyphosate-Resistant, Transgenic Soybean Cultivars.

Transgenic soybean cultivars, resistant to glyphosate herbicide in maturity groups V and VI, were evaluated for tolerance to the Columbia lance nematode, Hoplolaimus columbus, in field experiments conducted in 1998 and 1999. Treatment with 43 liter/ha of 1,3-dichloropropene was effective in suppressing H. columbus population densities in a split-plot design. Fumigation increased soybean yield, but a significant cultivar x fumigation interaction indicated variation in cultivar response to H. columbus. A tolerance index (yield of nontreated / yield of treated x 100) was used to compare cultivar differences. Two cultivars in maturity group VI and one cultivar in maturity group V had a tolerance index greater than 90, indicating a high level of tolerance.

Resistance as a Tactic for Management of Meloidogyne incognita on Cotton in North Carolina.

Selected cotton cultivars were evaluated for resistance to the southern root-knot nematode, Meloidogyne incognita, in greenhouse and field experiments. Cotton cultivars LA 887, Auburn 634, and NemX cotton were highly resistant to three North Carolina populations of root-knot nematode in greenhouse experiments compared to susceptible cultivars. The relative susceptibility of cultivars tested in the greenhouse from most to least susceptible were Deltapine 16 > Deltapine 50 > LA 887 or NemX > Auburn 634. The yields of resistant and susceptible cotton cultivars were increased by fumigation in fields infested with root-knot nematode. Reproduction of M. incognita in field plots on NemX, Paymaster H 1560, and Stoneville LA 887 was less than on susceptible cultivars. Diminished reproduction of the nematode on resistant cultivars may reduce the need for nematode control tactics in subsequent years.

Stem Canker on Cotton Caused by Phoma exigua in North Carolina and Virginia.

Numerous reports about a disease of unknown etiology on cotton, Gossypium hirsutum L., in northeastern North Carolina and southeastern Virginia were received on 18 June 1999 following several days of cool weather with persistent mist and fog during the week of 14-19 June. Several fields were visited by consultants and county extension staff the following week. In some instances, the cotton stem was girdled, causing the upper portion of the plant to wilt and die. Cotton plants exhibiting various symptoms, including death, wilting, streaking of the vascular system, black sunken lesions on stems, and terminal necrosis were collected for examination and isolation. Pycnidia and spores of the fungus Phoma exigua were abundant in stem and terminal tissues. The fungus was isolated from infected stem tissue and cultured on PDA. A suspension containing 2.5 × 108 spores of P. exigua was sprayed on cotton leaves or injected into the stems to confirm pathogenicity. Controls were sprayed or injected with distilled water. Plants were placed in 100% humidity for 72 h and maintained in the greenhouse thereafter. The experiment was replicated five times and repeated once. Typically, streaking of the vascular system extended 1 to 5 cm from the point of stem inoculation. Inoculated cotton leaves had lesions resembling those attributed to Ascochyta gossypii. Reisolation of the fungus P. exigua from inoculated tissue on potato-dextrose agar (PDA) was successful in all treatments. Crossan (2) considered many isolates of Ascochyta taken from various hosts in North Carolina, including A. gossypii, to be synonymous with Ascochyta phaseolorum. A. phaselorum was subsequently synonomyzed with P. exigua (1). Ascochyta blight (also called ashen spot, or wet weather blight [4]) is usually a minor leaf spot caused by P. exigua (syn. Ascochyta gossypi) and is common in North Carolina. Stem canker caused by P. exigua has not been reported previously in North Carolina (3) or Virginia. The sunken canker at a node is the best diagnostic symptom for cotton stem canker. Dark streaks in vascular tissue extend below and above the canker but do not usually extend to the root system, as with wilt diseases. The disease was widespread and found in most fields north of I-40 in North Carolina into Virginia and east of I-95. Crop consultants and county extension staff estimated disease incidence in individual fields from less than 1 to over 90% in North Carolina and 6 to 25% in Virginia. Disease incidence did not appear to be affected by cotton cultivar, tillage, or crop rotation. This pathogen was also responsible for brittle cotton stems late in the season, resulting in boll loss. Proper identification of the causal organism is essential in formulating management strategies, since P. exigua has an extensive host range and rotation is unlikely to aid in management of this disease in the future. References: (1) G. H. Boerema. Ascochyta phaseolorum synonymous with Phoma exigua. Neth. J. Plant Pathol. 78:113-115, 1972. (2) D. F. Crossan. The relationships of seven species of Ascochyta occurring in North Carolina. Phytopathology 48:248-255, 1958. (3) L. F. Grand, ed. North Carolina Plant Disease Index. Tech. Bul. 240, 1985. (4) G. M. Watkins. Leaf spots. Pages 28-30 in: Compendium of Cotton Diseases, 1st ed. American Phytopathological Society, St. Paul, MN, 1981.

Density-Dependent Yield of Heterodera glycines-Resistant and -Susceptible Cultivars.

Yield of the soybean (Glycine max) cultivar Hartwig with resistance to all races of Heterodera glycines was compared to that of the susceptible cultivar, Deltapine 105, in a field infested with race 2 of this pathogen. The field had previously been in a cropping sequence experiment that provided a range of H. glycines population densities affording the opportunity to evaluate yield potential of resistant and susceptible cultivars in the presence of different levels of soybean cyst nematode in 1992. Plots were planted again in 1993 with the two cultivars in sequences that included Hartwig following Hartwig or Deltapine 105, and Deltapine 105 following Hartwig or Deltapine 105. The yield of Hartwig was inferior to Deltapine 105 at low population densities of H. glycines, but Hartwig yielded more than Deltapine 105 at high population densities. Hartwig was effective in suppressing H. glycines population density compared to susceptible Deltapine 105. The seed yield of Hartwig following Deltapine 105 or Hartwig, and Deltapine 105 following Hartwig yielded more than Deltapine 105 grown for 2 years.

Tolerance of Selected Cotton Lines to Rotylenchulus reniformis.

The reproductive and damage potential of the reniform nematode, Rotylenchulus reniformis, on five cotton breeding lines reported as tolerant to this nematode in Texas were compared with two standard cotton cultivars, Deltapine 50 and Stoneville LA 887, in a North Carolina field naturally infested with R. reniformis. Numbers of R. reniformis in soil were suppressed at mid-season, and cotton-lint yield was increased by preplant fumigation with 1,3-dichloropropene. Population densities of R. reniformis at cotton harvest were unaffected by fumigation in 1998, but were affected in 1999. Some of the putatively tolerant breeding lines supported lower levels of R. reniformis and had higher tolerance indices to reniform nematode than the standard cultivars, but the yields of the breeding lines were significantly lower than the standard cultivars. Fumigation resulted in a 100- to 200-kg/ha increase in cotton lint yield for cultivars LA 887 and Deltapine 50.