Independent Evolution with the Gene Flux Originating from Multiple Xanthomonas Species Explains Genomic Heterogeneity in Xanthomonas perforans.

Xanthomonas perforans is the predominant pathogen responsible for bacterial leaf spot of tomato and X. euvesicatoria for that of pepper in the southeast United States. Previous studies have indicated significant changes in the X. perforans population collected from Florida tomato fields over the span of 2 decades, including a shift in race and diversification into three phylogenetic groups driven by genome-wide homologous-recombination events derived from X. euvesicatoria In our sampling of Xanthomonas strains associated with bacterial spot disease in Alabama, we were readily able to isolate X. perforans from symptomatic pepper plants grown in several Alabama counties, indicating a recent shift in the host range of the pathogen. To investigate the diversity of these pepper-pathogenic strains and their relation to populations associated with tomatoes grown in the southeast United States, we sequenced the genomes of eight X. perforans strains isolated from tomatoes and peppers grown in Alabama and compared them with previously published genome data available from GenBank. Surprisingly, reconstruction of the X. perforans core genome revealed the presence of two novel genetic groups in Alabama that each harbored a different transcription activation-like effector (TALE). While one TALE, AvrHah1, was associated with an emergent lineage pathogenic to both tomato and pepper, the other was identified as a new class within the AvrBs3 family, here designated PthXp1, and was associated with enhanced symptom development on tomato. Examination of patterns of homologous recombination across the larger X. euvesicatoria species complex revealed a dynamic pattern of gene flow, with multiple donors of Xanthomonas spp. associated with diverse hosts of isolation.IMPORTANCE Bacterial leaf spot of tomato and pepper is an endemic plant disease with a global distribution. In this study, we investigated the evolutionary processes leading to the emergence of novel X. perforans lineages identified in Alabama. While one lineage was isolated from symptomatic tomato and pepper plants, confirming the host range expansion of X. perforans, the other lineage was isolated from tomato and acquired a novel transcription activation-like effector, here designated PthXp1. Functional analysis of PthXp1 indicated that it does not induce Bs4-mediated resistance in tomato and contributes to virulence, providing an adaptive advantage to strains on tomato. Our findings also show that different phylogenetic groups of the pathogen have experienced independent recombination events originating from multiple Xanthomonas species. This suggests a continuous gene flux between related xanthomonads associated with diverse plant hosts that results in the emergence of novel pathogen lineages and associated phenotypes, including host range.

Predicting the risk of cucurbit downy mildew in the eastern United States using an integrated aerobiological model.

Cucurbit downy mildew caused by the obligate oomycete, Pseudoperonospora cubensis, is considered one of the most economically important diseases of cucurbits worldwide. In the continental United States, the pathogen overwinters in southern Florida and along the coast of the Gulf of Mexico. Outbreaks of the disease in northern states occur annually via long-distance aerial transport of sporangia from infected source fields. An integrated aerobiological modeling system has been developed to predict the risk of disease occurrence and to facilitate timely use of fungicides for disease management. The forecasting system, which combines information on known inoculum sources, long-distance atmospheric spore transport and spore deposition modules, was tested to determine its accuracy in predicting risk of disease outbreak. Rainwater samples at disease monitoring sites in Alabama, Georgia, Louisiana, New York, North Carolina, Ohio, Pennsylvania and South Carolina were collected weekly from planting to the first appearance of symptoms at the field sites during the 2013, 2014, and 2015 growing seasons. A conventional PCR assay with primers specific to P. cubensis was used to detect the presence of sporangia in rain water samples. Disease forecasts were monitored and recorded for each site after each rain event until initial disease symptoms appeared. The pathogen was detected in 38 of the 187 rainwater samples collected during the study period. The forecasting system correctly predicted the risk of disease outbreak based on the presence of sporangia or appearance of initial disease symptoms with an overall accuracy rate of 66 and 75%, respectively. In addition, the probability that the forecasting system correctly classified the presence or absence of disease was ≥ 73%. The true skill statistic calculated based on the appearance of disease symptoms in cucurbit field plantings ranged from 0.42 to 0.58, indicating that the disease forecasting system had an acceptable to good performance in predicting the risk of cucurbit downy mildew outbreak in the eastern United States.

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 Soybean Rust (Phakopsora pachyrhizi) on Florida Beggarweed (Desmodium tortuosum) in Alabama.

Soybean rust (SBR), caused by the fungus Phakopsora pachyrhizi, was detected on Florida Beggarweed (Desmodium tortuosum) for the first time in Alabama in November, 2009. The pathogen was not observed in 2010 or 2011, probably because of the exceptionally dry, hot weather in the region. The pathogen was observed on multiple mature leaves of plants, evenly distributed through a field at the Wiregrass Research and Extension Center in Headland, Alabama, located in the southeast region of the state. Florida Beggarweed can serve as an overwintering host for SBR. Symptoms on leaves were consistent with SBR symptoms previously described on soybeans (1). Sori in multiple pustules were observed on the undersurface of the leaves. Urediniospores and paraphyses were observed microscopically and identified as P. pachyrhizi. Symptomatic leaves from 20 plants were analyzed using an Envirologix monoclonal antibody strip test kit at the Auburn University Plant Diagnostic Laboratory. A subsample of 20 plants were positive for the pathogen. Representative symptomatic leaves were sent to the USDA Molecular Diagnostic Laboratory in Beltsville, Maryland, for confirmation. DNA was extracted from sori aseptically removed from leaves using a Qiagen DNeasy Plant Mini Kit, and amplified with primers Ppa1 and NL4. The resulting partial ITS2 and 28S ribosomal RNA sequences were 100% identical to GenBank entry DQ354537. Voucher specimens were deposited in the USDA Agricultural Research Service, National Fungus Collection (BPI). To our knowledge, this is the first report of the disease on Florida Beggarweed in Alabama. References: (1) A. Carcamo Rodriguez et al. Plant Dis. 90:1260, 2006. (2) R. D. Frederick et al. Phytopathology 92:217, 2002.

First Report of the Cucurbit Yellow Vine Disease Caused by Serratia marcescens in Watermelon and Yellow Squash in Alabama.

More than 3,000 acres of watermelon were planted in Alabama in 2010 with a production value more than $4 million (J. Kemble, personal communication). Symptoms typical of cucurbit yellow vine disease (CYVD) were observed in a 2-ha watermelon field in Crawford, AL on 8 June 2010. Watermelon, cv. Jubilee, exhibited a yellow appearance and some plants were completely wilted. Incidence of affected plants was estimated at 25%. On 24 June, plant samples were collected from a 1-ha watermelon (cv. Jubilee) field near Dadeville, AL. Approximately 30% of the plants exhibited yellowing and wilting, which is symptomatic of CYVD. Samples were also collected from a small planting of yellow crooked-neck squash at the same location. Approximately 20% of the squash plants had symptoms typical of CYVD. Cross-sections of belowground stem and primary root revealed a honey-brown phloem discoloration and a healthy appearing xylem, symptoms consistent with CYVD caused by the phloem-colonizing bacterium, Serratia marcescens Bizio (1). Isolations were made from the crown on four symptomatic watermelon and two squash plants. Approximately 2.5-mm3 tissue pieces from the phloem were excised, surface sterilized in 10% sodium hypochlorite, and ground in 1-ml PBS (phosphate buffer with saline). A 10-µl aliquot of slurry was plated onto nutrient agar (NA) (Difco, Detroit, MI) and the plates were stored at room temperature for 4 days. Individual colonies were selected and purified by serial dilution plating. Isolates from watermelon and squash were consistent with S. marcescens in colony morphology, color, and texture. Three isolates obtained from watermelon were grown on NA and suspended in sterile water at 108 cells per ml for mechanical transmission experiments on 'Lemondrop' squash. Sterile water served as a negative control. After 28 days, plants were cross-sectioned at the juncture of the root and stem and observed for phloem discoloration. Of the 56, 58, and 62 plants inoculated in three replicate studies, 78.6, 56.9, and 62.9% developed CYVD symptoms, respectively, while none of the controls were positive. Cultured bacteria from six of the symptomatic, greenhouse-inoculated plants representing the three watermelon isolates were subjected to multiplex end-point PCR using primer sets YV1/YV4, specific for the species S. marcescens, and a79F/R, which amplifies only the CYVD strains of S. marcescens (3). All six bacteria cultures along with the positive control (reference isolate W01 obtained from watermelon in Texas) were positive, while the negative PBS control was negative. Although rhizosphere-inhabiting and plant growth promoting endophytic strains of S. marcescens have been reported from Alabama (2), to our knowledge, this is the first known report of CYVD and phytopathogenic S. marcescens in Alabama cucurbits. References: (1) B. D. Bruton et al. Plant Dis. 87:937, 2004. (2) J. A. McInroy and J. W. Kloepper. Plant Soil 173:333, 1995. (3) Q. Zhang et al. Appl. Environ. Microbiol. 71:7716, 2005.

First Report of Soybean Rust Caused by Phakopsora pachyrhizi on Pachyrhizus erosus in the United States.

Soybean rust, caused by the fungus Phakopsora pachyrhizi, was detected on jicama (Pachyrhizus erosus L. Urban) for the first time in the United States in November 2009. The pathogen was observed on leaves of a single, potted jicama plant grown outdoors in a residential area and on leaves of all plants in a 12-m2 demonstration plot located at the Auburn University Teaching Garden in Auburn, AL. Symptoms on the upper leaf surfaces were isolated chlorotic areas near the leaf edges in the lower part of the canopy. The abaxial surface was first observed to exhibit brown lesions and subsequently produced volcano-shaped uredinia. These symptoms are consistent with a rust previously described on jicama in Mexico (1). Representative symptomatic plant tissue was sent to the USDA National Identification Services (Mycology) Laboratory in Beltsville, MD for diagnostic confirmation at both the Urbana, IL lab and the USDA National Plant Germplasm and Biotechnology Laboratory for DNA testing. From an infected leaf, samples of approximately 5 mm2 were excised from a microscopically observed rust lesion and an apparently noninfected area. Total DNA was purified with the FastDNA Spin Kit (MP Biomedicals, Solon, OH) followed by the E.Z.N.A. MicroElute DNA Clean-Up Kit (Omega Bio-tek, Inc, Doraville, GA) per manufacturer's instructions. Detection of P. pachyrhizi and P. meibomiae DNA was achieved by quantitative PCR using the method of Frederick et al. (2) and a DNA standard of previously prepared P. pachyrhizi spores. The observed rust pustule was found to contain P. pachyrhizi DNA in excess of 28,000 genomes, while no P. pachyrhizi DNA was observed from the asymptomatic sample. Both samples were negative for P. meibomiae. The fungal structures present were confirmed to be Phakopsora spp. DNA was extracted from sori aseptically removed from leaves with a Qiagen (Valencia, CA) DNeasy Plant Mini Kit and amplified with primers Ppa1 and NL4. The resulting partial ITS2 and 28S ribosomal RNA sequences were 100% identical to GenBank entry DQ354537 P. pachyrhizi internal transcribed spacer 2 and 28S ribosomal RNA gene, partial sequence. Sequences from jicama from Alabama were deposited in GenBank. Voucher specimens were deposited in the USDA Agricultural Research Service, National Fungus Collection (BPI). To our knowledge, this is the first report of the disease on jicama in the United States. References: (1) A. Cárcamo Rodriguez et al. Plant Dis. 90:1260, 2006. (2) R. D. Frederick et al. Phytopathology 92:217, 2002.

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.

First Report of Asian Soybean Rust Caused by Phakopsora pachyrhizi on Soybean in Alabama.

On November 4, 2004, soybean leaves (Glycine max (L.) Merr) were submitted to the Auburn University Plant Diagnostic Lab by a State Department of Agriculture and Industries Inspector. Samples were collected from an 80-ha field of soybean plants in a late-reproductive-growth stage in Mobile County, Alabama. Under microscopic examination, leaves showed rust pustules in advanced stages of development with urediniospores and sori characteristic of Phakopsora spp. Uredinia were ostiolate in small, brown, angular leaf spots (2 to 3 mm) on lower leaf surfaces. Urediniospores were pale yellow-to-white, globose or ovate, 20 to 40 × 15 to 25 µm. In a subsequent visit to the field, symptoms and signs of the rust disease were observed on plants bordering the edge of the field since the majority of plants were senescent. Tan lesions on lower leaf surfaces contained small pustules surrounded by a small zone of slightly discolored necrotic tissue. Masses of tan spores covered the lower leaf surface pustules. Leaves were mailed overnight to the USDA National Identification Services (Mycology) Laboratory in Beltsville, MD. The fungal structures were confirmed to be a Phakopsora sp., and the sample was forwarded to the USDA National Plant Germplasm and Biotechnology Laboratory in Beltsville, MD. DNA was extracted from leaf pieces containing sori using the Qiagen DNeasy Plant Mini kit (Qiagen, Valencia, CA). Phakopsora pachyrhizi was detected using a real-time polymerase chain reaction (PCR) protocol (1) performed in a Cepheid SmartCycler (Sunnyvale, CA). The PCR master mix was modified to include OmniMix beads (Cepheid). The field and microscopic suspect diagnosis of P. pachyrhizi was confirmed officially by APHIS on November 18, 2004. This was the fourth USDA official confirmation of Asian soybean rust in the continental United States during 2004, and to our knowledge, this is the first report of the disease in Alabama. This report helps confirm that early occurrences of Asian soybean rust in the United States were present in other areas in addition to the first reported finding in Louisiana (2). References: (1) R. D. Frederick et al. Phytopathology 92:217, 2002. (2) R. W. Schneider et al. Plant Dis. 89:774, 2005.

First Report of Bean pod mottle virus in Soybean in Alabama.

During October 2003, soybean (Glycine max (L.) Merr.) plants showing symptoms of delayed maturity of stems, or green stem, were observed in a soybean cultivar trial on Dee River Ranch in Pickens County, Alabama. Symptoms were characteristic of those caused by Bean pod mottle virus (BPMV). BPMV infections have been identified in other southern states, including Arkansas, Louisiana, and neighboring Mississippi, but had not yet been identified in Alabama (1,2,3). In this study, a cultivar trial was established as a nonreplicated strip test to evaluate the performance of nongenetically modified soybean cultivars in high-pH soils. The trial consisted of 12 maturity group V cultivars planted side by side in 24-row plots approximately 1 km long. The cultivars consisted of Anand, Asgrow 5547, Asgrow 5944, Delta King 5995, Deltapine 4748, Deltapine 5110, Deltapine 5989, Essex, Hutcheson, Pioneer 9594, Pioneer 9597, and USG5601T. During the season, a known vector of BPMV, the bean leaf beetle (Cerotoma trifurcate Forster), was identified in the plots (4). On 10 October, the majority of plants in the trial had senesced; however, it was observed that plants of 6 of the 12 cultivars were showing symptoms of green stem typical of BPMV infection. A visual assessment was taken to determine incidence of green stem for Asgrow 5547, Delta King 5995, Deltapine 5110, Deltapine 5989, Pioneer 9594, and USG5601T. Incidence between 1 and 5% was observed for Delta King 5995, Deltapine 5989, and Pioneer 9594. Incidence of less than 1% was observed for Asgrow 5547, Deltapine 5110, and USG5601T. Twenty soybean plants showing symptoms of green stem and retaining green leaves were sampled from each of the six cultivars by collecting one trifoliate leaf near the top of the plant. All samples were tested for BPMV using double-antibody sandwich enzyme-linked immunosorbent assay (DAS-ELISA) according to the manufacturer's instructions (Agdia, Inc., Elkart, IN). BPMV was detected in 30% of Deltapine 5989, 10% of Delta King 5995, and 45% of Pioneer 9594 plants. BPMV was not detected in Asgrow 5547, Deltapine 5110, and USG5601T. Ten of the samples shown to be infected with BPMV using DAS-ELISA were mechanically transferred to soybean seedlings in the greenhouse. These plants developed systemic mottle symptoms typical of those caused by BPMV and tested positive for the virus BPMV using DAS-ELISA. To our knowledge, this is the first report of BPMV in Alabama. References: (1) N. S. Horn et al. LA. Agric. 13:12, 1970. (2) H. N. Pitre et al. Plant Dis. Rep. 63:419, 1979. (3) J. P. Ross. Plant Dis. Rep. 47:1049, 1963. (4) H. J. Walters. (Abstr.) Phytopathology 48:346, 1958.

First Report of Plectosporium Blight on Pumpkin Caused by Plectosporium tabacinum in Alabama.

In October of 2001, Plectosporium tabacinum (van Beyma) M.E. Palm, W. Gams, & H.I. Nirenberg (formerly known as Microdochium tabacinum (von Arx, 1984) and Fusarium tabacinum (Gams & Gerlagh, 1968)) was observed in field plantings of pumpkin (Cucurbita pepo L.) in Cullman and Jackson counties in north Alabama. Symptoms were white or tan, spindle-shaped lesions on the stems and leaf petioles and slightly raised, corky, white or light brown lesions on pumpkin fruit and fruit stems. Pumpkin symptoms were identical to a previous description of P. tabacinum (published as M. tabacinum) on pumpkin, zucchini, and yellow summer squash (1). Disease severity ranged from less than 10% stem tissue damage on pumpkins in Cullman County to 40 to 45% stem tissue damage on pumpkins in Jackson County. A field section of pumpkins in Jackson County sprayed with azoxystrobin (Quadris 2.08F, 0.20 kg a.i./ha) alternated weekly with chlorothalonil (Bravo Ultrex, 2.44 kg a.i./ha) beginning at vine-run had stem damage of approximately 5% compared to approximately 45% stem damage on pumpkins in an unsprayed field section. A 50% reduction in marketable fruit due to P. tabacinum was observed in the unsprayed field section compared to the section sprayed weekly with fungicides. When thin slices of lesions were taken from stem and fruit surfaces using a scalpel and examined microscopically, one- or two-celled, hyaline, bilaterally symmetric spores, 7.0 to 8.5 × 2.8 to 3.0 µm were observed. The ends of the spores were slightly narrowed and rounded. Spore characteristics were identical to previous descriptions of P. tabacinum produced in culture and on diseased pumpkins and squash (2,3). Surface-sterilized tissue from fruit lesion margins was plated on potato dextrose agar and incubated under light (Sylvania Gro-Lux, 40w) with a light/dark cycle of 12 h at 23°C. After 10 days, spores were observed that were similar to those from fruit except they were multiguttulate and had a phialide arrangement. The fungal mycelium was pale pink to pale orange and closely appressed to the agar. Fungal characteristics in culture agree with a previous description of P. tabacinum in culture (2). To our knowledge, this is the first report of P. tabacinum in Alabama. References: (1) S. C. Bost and C. A. Mullins. Plant Dis. 76:861, 1992. (2) M. E. Palm et al. Mycologia 87(3):397, 1995; (3) T. A. Zitter. Microdochium blight. Page 28 in: Compendium of Cucurbit Diseases. T. A. Zitter, D. L. Hopkins, and C. E. Thomas, eds. The American Phytopathological Society, St. Paul, MN, 1996.

Hatch and Emergence of Heterodera glycines in Root Leachate from Resistant and Susceptible Soybean Cultivars.

Egg hatch and emergence of second-stage juveniles (J2) of Heterodera glycines races 3 and 4 from cysts exposed to soybean root leachate of cv. Fayette (resistant to H. glycines) and H. glycines-susceptible cultivars A2575, A3127, and Williams 82 were determined in three sets of experiments. In the first experiment, cysts of both race 3 and race 4 were exposed to leachate of 8-week-old plants for a 2-week period. In the second experiment, cysts from populations of races 3 and 4 were raised on cultivars A2575, A3127, and Williams 82. Cysts then were exposed to leachate from 8-week-old plants for a 2-week period in all possible race-per-cultivar combinations. In the third experiment, cysts of races 3 and 4 were exposed at 4-day intervals to leachate from plants as the plants developed 7 to 59 days after planting. In experiments 1 and 2, leachate from 8-week-old Williams 82 and A3127 stimulated more hatch and emergence of H. glycines than leachate from A2575, Fayette, or the control. In the first experiment, cumulative hatch and emergence were greater for race 3 than for race 4. In experiment 2, no apparent relationship developed between leachate from a cultivar and the population developed on that cultivar in terms of stimulation of hatch and emergence. In the third experiment, A2575 stimulated more hatch and emergence of both race 3 and race 4 than A3127, Fayette, and Williams 82. Leachate from Fayette stimulated less hatch and emergence of both race 3 and race 4. Hatch and emergence were greatest during the initial 12 days of the experiment.

Influence of temperature on development of pine wilt in scots pine.

The effect of temperature on pine wilt development in Scots pine (Pinus sylvestris) was examined in three experiments. Container-grown pines (4-6 years old) inoculated with 1,500 Bursaphelenchus xylophilus were incubated at constant temperatures in growth chamber for 8 weeks, then at a temperature range of 15-30 C in a greenhouse for 10-12 weeks. Nematode infection was greater, tree mortality was higher, and disease incubation was shorter at 32 and 30 C than at 25, 23, 18, and 11 C. Foliar symptoms developed more rapidly and uniformly at higher temperatures. Ninety-five percent of tree deaths at 32 and 30 C and 88% at 25 and 23 C occurred within the 8-week exposure to constant temperatures. Mortality at 18, 16, and 11 C occurred only after transfer to the greenhouse. Results indicate that pine wilt incidence is directly related and disease incubation period is inversely related to temperature and that high-temperature stress predisposes Scots pine to lethal infection by B. xylophilus.

Distribution of Heterodera glycines Races in Illinois.

Because the race scheme for Heterodera glycines was expanded recently from 5 to 16 races, the occurrence and distribution of races in Illinois need clarification. Forty-four populations of H. glycines were collected from sites in 23 of the 88 infested counties. Populations were tested using the differential soybean lines Pickett 71, Peking, PI 88788, and PI 90763. Lee 68 and Williams 82 were used as standards. Seedlings were grown in 7.5-cm-d clay pots and inoculated with 1,000 eggs and second-stage juveniles obtained from H. glycines-infested field soil. Plants were maintained in a greenhouse at 22-28 C. After 1 month, the number of first-generation white females that developed on each differential was determined and the race of the population was designated. Twenty-eight populations were race 3, twelve were race 1, two were race 5, one was race 2, and one was race 4. Populations of races 3 and 1 were widely distributed in the state. In 26 of the 28 race determinations, race designations using Williams 82 and Lee 68 were the same, indicating that if Lee 68 is not available, Williams 82 may be a suitable alternative for race tests done in the north central United States.

Evaluation of Soybeans in Maturity Groups I-IV for Resistance to Heterodera glycines.

Forty-seven private and public soybean (Glycine max) cultivars in maturity groups I, II, III, and IV were evaluated in the field for resistance to Heterodera glycines race 3 and race 4 at two sites, Kilbourne and Urbana, Illinois. The soil at Kilbonrne was an irrigated sand infested with race 3. The soil at Urbana was a nonirrigated silty clay loam infested with race 4. Yield of nematode-susceptible controls was reduced at both locations, and yield differences were observed among the other cultivars. Soybeans grown in sand yielded less than the soybean grown in silty clay loam. Although populations of H. glycines recovered at planting were lower at Kilbourne than at Urbana, a greater percentage of loss in yield was associated with the sand at Kilbourne. Several soybean cultivars had a high level of resistance to both populations of H. glycines.