Green Stem Disorder of Soybean.

Green stem disorder of soybean (Glycine max) is characterized by delayed senescence of stems with normal pod ripening and seed maturation. Three different field research approaches were designed to determine the relationship of green stem disorder to Bean pod mottle virus (BPMV) and other potential factors that may be involved in causing this disorder. The first research approach surveyed green stem disorder and BPMV in individual plants monitored in several commercial soybean fields during three growing seasons. Leaf samples from maturing plants (growth stage R6) were tested by enzyme-linked immunosorbent assay (ELISA) for BPMV. The percentage of monitored plants infected with BPMV at growth stage R6 in some fields was higher than the incidence of green stem disorder at harvest maturity. Many plants infected with BPMV did not develop green stem disorder, and conversely, many plants that had green stem disorder were not infected with BPMV. According to a chi-square test of independence, the data indicated that green stem disorder was independent of BPMV infection at growth stage R6 (P = 0.98). A second research approach compared green stem disorder incidence in an identical set of soybean entries planted in two locations with different levels of natural virus infection. Despite differences in virus infection, including BPMV incidence, 20 of 24 entries had similar green stem disorder incidence at the two locations. A third research approach completed over two growing seasons in field cages showed that green stem disorder developed without BPMV infection. BPMV infection did not increase green stem disorder incidence in comparison to controls. Bean leaf beetle, leaf hopper, or stinkbug feeding did not have an effect on the incidence of green stem disorder. The cause of the green stem disorder remains unknown.

Mycelial Compatibility Grouping and Aggressiveness of Sclerotinia sclerotiorum.

Population variability of Sclerotinia sclerotiorum, the causal organism of Sclerotinia stem rot of soybean, was determined by mycelial compatibility grouping (MCG) and isolate aggressiveness comparisons. MCG and aggressiveness of S. sclerotiorum isolates from diverse hosts and geographic locations (Diverse Set, 24 isolates), from a soybean field in Argentina (Argentine Set, 21 isolates), and from soybean fields in DeKalb and Watseka, Illinois (DeKalb Set, 124 isolates, and Watseka Set, 130 isolates) were assessed. Among 299 isolates tested, 42 MCGs were identified, and 61% were represented by single isolates observed at single locations. Within the Diverse Set, 17 MCGs were identified; 1 MCG consisted of six isolates, and 16 MCGs consisted of one isolate each. Nine MCGs were identified within the Argentine field with two MCGs composed of either five or six isolates, two MCGs composed of two isolates, and the remaining composed of one isolate each. Each Illinois field was a mosaic of MCGs, but MCG frequencies differed between the two fields. Common MCGs were identified among the Diverse, DeKalb, and Watseka Sets, but no MCGs within the Argentine Set were observed with other sets. MCG 8 was the most frequently sampled and widely dispersed MCG and occurred at a frequency of 29, 36, and 62% in the Diverse, DeKalb, and Watseka Sets, respectively. Variation in isolate aggressiveness was assessed using a limited-term, plug inoculation technique. Isolate aggressiveness varied (P = 0.001) within the Diverse, Argentine, DeKalb, and Watseka Sets. Within widely dispersed MCGs, isolate aggressiveness varied (P ≤ 0.10); however, within locally observed MCGs detected only in single fields, isolate aggressiveness did not vary. Additionally, individual MCGs within the DeKalb and Watseka Sets differed in isolate aggressiveness. Using six soybean cultivars and six S. sclerotiorum isolates, no cultivar-isolate interaction was detected, but resistant and susceptible cultivars performed similarly when inoculated with either less or highly aggressive isolates. Pathogen population structure and variability in isolate aggressiveness may be important considerations in disease management systems.

Impact of Herbicides on Heterodera glycines Susceptible and Resistant Soybean Cultivars.

Several abiotic and biotic stresses can affect soybean in a growing season. Heterodera glycines, soybean cyst nematode, reduces yield of soybean more than any other pathogen in the United States. Field and greenhouse studies were conducted to determine whether preemergence and postemergence herbicides modified the reproduction of H. glycines, and to determine the effects of possible interactive stresses caused by herbicides and H. glycines on soybean growth and yield. Heterodera glycines reproduction factor (Rf) generally was less on resistant than susceptible cultivars, resulting in a yield advantage for resistant cultivars. The yield advantage of resistant cultivars was due to more pods per plant on resistant than susceptible cultivars. Pendimethalin reduced H. glycines Rf on the susceptible cultivars in 1998 at Champaign, Illinois, and in greenhouse studies reduced dry root weight of H. glycines-resistant and susceptible cultivars, therefore reducing Rf on the susceptible cultivars. The interactive stresses from acifluorfen or imazethapyr and H. glycines reduced the dry shoot weight of the resistant cultivar Jack in a greenhouse study. Herbicides did not affect resistant cultivars' ability to suppress H. glycines Rf; therefore, growers planting resistant cultivars should make herbicide decisions based on weeds present and cultivar tolerance to the herbicide.

Occurrence of Seed Coat Mottling in Soybean Plants Inoculated with Bean pod mottle virus and Soybean mosaic virus.

Soybean seed coat mottling often has been a problematic symptom for soybean growers and the soybean industry. The percentages of seed in eight soybean lines with seed coat mottling were evaluated at harvest after inoculating plants during the growing season with Bean pod mottle virus (BPMV), Soybean mosaic virus (SMV), and both viruses inside an insect-proof cage in the field. Results from experiments conducted over 2 years indicated that plants infected with BPMV and SMV, alone or in combination, produced seed coat mottling, whereas noninoculated plants produced little or no mottled seed. BPMV and SMV inoculated on the same plants did not always result in higher percentages of mottled seed compared with BPMV or SMV alone. There was significant virus, line, and virus-line interaction for seed coat mottling. The non-seed-coat-mottling gene (Im) in Williams isoline L77-5632 provided limited, if any, protection against mottling caused by SMV and none against BPMV. The Peanut mottle virus resistance gene Rpv1 in Williams isoline L85-2308 did not give any protection against mottling caused by SMV, whereas the SMV resistance gene Rsv1 in Williams isoline L78-379 and the resistance gene or genes in the small-seeded line L97-946 gave high levels of protection against mottling caused by SMV. The correlations (r = 0.77 for year 2000 and r = 0.89 for year 2001) between virus infection of the parent plant and seed coat mottling were significant (P = 0.01), indicating that virus infection of plants caused seed coat mottling.

Distribution of Leaf-Feeding Beetles and Bean pod mottle virus (BPMV) in Illinois and Transmission of BPMV in Soybean.

Bean leaf beetles (BLB; Cerotoma trifurcata) were collected in soybean (Glycine max) fields in 58 and 99 Illinois counties surveyed during the 2000 and 2001 growing seasons, respectively. In 2000, BLB counts were highest in the central portion of the state. BLB counts were lower the following year, but were more uniformly distributed throughout the state. BLB tested positive for Bean pod mottle virus (BPMV) in 37 of 41 counties assayed in 2000. In 2001, BLB tested positive for BPMV in 86 of 99 counties sampled. In 2000 and 2001, western corn rootworm (WCR; Diabrotica virgifera virgifera) adults were abundant in soybean fields only in east central Illinois. WCR adults tested positive for BPMV in 21 of 21 east central Illinois counties in 2000 and 20 of 24 sampled in 2001. BPMV was detected in soybean plants in 38 of 46 counties sampled in 2000. Field-collected WCR adults transmitted BPMV to potted soybean plants at low rates either directly from BPMV-infected soybean fields or with prior feeding on BPMV-infected plants. This is the first report of the distribution of BLB, WCR adults, and BPMV in Illinois and of BPMV transmission by adult WCR.

Response of Commercially Developed Soybean Cultivars and the Ancestral Soybean Lines to Fusarium solani f. sp. glycines.

Sudden death syndrome, caused by Fusarium solani f. sp. glycines, has caused severe damage to soybean production in recent years. One way to control sudden death syndrome is with resistant cultivars. Over a 3-year period, 2,335 publicly and privately developed soybean entries were inoculated and evaluated for their response to F. solani f. sp. glycines under greenhouse conditions. The entries were compared with the susceptible check, Great Lakes 3302 (GL3302), and the moderately resistant checks, plant introductions (PIs) 520733 and 567374. Thirty-eight entries were identified with moderate levels of resistance. Based on foliar ratings, there were no differences (P < 0.05) between the Roundup Ready and conventional cultivars. In all, 90 ancestral lines that represent 99% of the genes in modern U.S. cultivars and 55 lines found in the pedigrees of public cultivars reported to have some resistance were evaluated for their response to F. solani f. sp. glycines. Nine ancestral lines (Aoda, Kim, Jackson, Sioux, Mammoth Yellow, T117, PI 171450, PI 54615-1, and PI 71506) and 12 cultivars or experimental lines (Ina, D83-3349, LN98-4340, LN83-2356, Hartwig, Harosoy, Bedford, Merit, Cutler, Calland, Hill, and Evans) had disease ratings not significantly different (P < 0.05) from PI 520733 or PI 567374. PI 54610, a putative ancestral line, also was found to be moderately resistant.

Use of Aeroponic Chambers and Grafting to Study Partial Resistance to Fusarium solani f. sp. glycines in Soybean.

Several plant introductions (PIs) and cultivars have been classified as partially resistant (PR) to sudden death syndrome. However, little is known about the nature of resistance to this disease. Seedlings of two PR PIs and two susceptible cultivars were inoculated with Fusarium solani f. sp. glycines in aeroponic chambers. Plants were inoculated by taping two sorghum seeds infested with F. solani f. sp. glycines to the main root. Foliar symptoms of the susceptible cultivars were higher than those on the PR PIs and were associated with lower root and plant dry weight. Root lesion lengths of the four soybean lines differed (P < 0.05), but did not correlate with foliar disease or any other variable. To better understand the resistance mechanism by distinguishing between root and plant resistance, three partially resistant PIs (PI 520.733, PI 567.374, and PI 567.650B) and one susceptible soybean cultivar (GL3302) were compared using different grafting combinations in aeroponic chambers. Results of sudden death syndrome evaluation indicated that resistance is conditioned by both the scion and the rootstock. All three PIs evaluated had resistance associated with the scion; resistance in PI 567.650B also was associated with the rootstock. Although the PR PIs used appear to have little or no root resistance, an aeroponic system and grafting may help identify new sources of resistance to F. solani f. sp. glycines with root- or whole-plant resistance.

Quality of Harvested Seed Associated with Soybean Cultivars and Herbicides Under Weed-Free Conditions.

Different herbicides were applied to soybean plants in field plots planted to different soybean cultivars located at four locations in Illinois between 1997 and 2000. Treatments varied from hand weeded to preemergence herbicides to postemergence herbicides. Soybean seeds were harvested and evaluated for different seed quality parameters. The percentage of seeds infected with Phomopsis spp. ranged from 1 to 40%, and the percentage of seeds infected with Cercospora kikuchii was low, ranging from 0 to 4%. Herbicides had little or no effect on seed quality parameters such as percent germination and incidence of seed pathogens or on protein and oil concentrations. Soybean seed quality was affected by Phomopsis spp. in that there were significant (P ≤ 0.05) inverse correlations between Phomopsis spp. incidence and percentage seed germination. It appears that Phomopsis spp. may be a more prevalent seed pathogen than C. kikuchii for soybean fields in central to northern Illinois.

Selected Soybean Plant Introductions with Partial Resistance to Sclerotinia sclerotiorum.

Sclerotinia stem rot, caused by Sclerotinia sclerotiorum, is a major soybean (Glycine max) disease in north-central regions of the United States and throughout the world. Current sources of resistance to Sclerotinia stem rot express partial resistance, and are limited in number within soybean germ plasm. A total of 6,520 maturity group (MG) 0 to IV plant introductions (PIs) were evaluated for Sclerotinia stem rot resistance in the United States and Canada in small plots or in the greenhouse from 1995 to 1997. Selected PIs with the most resistance were evaluated for resistance in the United States and Canada in replicated large plots from 1998 to 2000. The PIs in the MG I to III tests in Urbana, IL were evaluated for agronomic traits from 1998 to 2000. The selected PIs also were evaluated with an excised leaf inoculation and petiole inoculation technique. After the 1995 to 1997 evaluations, all but 68 PIs were eliminated because of their susceptibility to Sclerotinia stem rot. In field tests in Urbana, higher disease severity in selected MG I to III PIs was significantly (P< 0.05) associated with taller plant heights and greater canopy closure. All other agronomic traits evaluated were not associated or were inconsistently associated with disease severity. MG I to III PIs 153.282, 189.931, 196.157, 398.637, 417.201, 423.818, and 561.331 had high levels of resistance and had canopies similar to the resistant checks. The resistance ratings from the petiole inoculation technique had a high and significant (P< 0.01) correlation with disease severity in the MG I and II field tests. The partially resistant PIs identified in this study can be valuable in incorporating Sclerotinia stem rot resistance into elite germ plasm.

Evaluation of Glycine max Germ Plasm for Resistance to Fusarium solani f. sp. glycines.

Sudden death syndrome, caused by Fusarium solani f. sp. glycines, has caused increased losses in soybean production in recent years. This study was done to identify potential sources of resistance to sudden death syndrome. Using a greenhouse screening procedure, 6,037 soybean plant introductions (PIs) were compared with a susceptible check, Great Lakes 3302, and two moderately resistant checks, PI 520.733 and PI 567.374, for resistance to sudden death syndrome. Only 57 PIs had foliar disease ratings that were not significantly different from PI 567.374 (P ≤ 0.05) 3 weeks after inoculation. Six PIs had lower ratings than PI 567.374 at 4 weeks after inoculation, while none had lower area under the disease progress curve (AUDPC) values. When comparing the PIs to PI 520.733, 209 PIs had foliar disease ratings not significantly different from PI 520.733 (P ≤ 0.05) 3 weeks after inoculation. Eight PIs had significantly lower disease severity ratings 4 weeks after inoculation, and 38 PIs had significantly lower AUDPC values than PI 520.733. Additionally, root lesion lengths were measured 4 weeks after inoculation and ranged from 25.2 to 41.5 mm for all the PIs; none of the entries had smaller lesion lengths than the susceptible check Great Lakes 3302. The correlation between lesion length and disease foliar severity rating was not significant. There also were no plant morphological characteristics (i.e., flower color or seed coat color) associated with higher sudden death syndrome foliar symptoms. Eighteen PIs previously identified as moderately resistant with differing agronomic traits were inoculated with five different isolates of Fusarium solani f. sp. glycines. Results indicated that resistance in these 18 PIs was effective against all five isolates of Fusarium solani f. sp. glycines. Isolate Mont-1 caused the greatest disease severity ratings. These PIs that exhibited low foliar severity ratings may provide new sources of resistance for the development of new sudden death syndrome-resistant lines and cultivars.

Efficacy of Fungicides on Sclerotinia sclerotiorum and Their Potential for Control of Sclerotinia Stem Rot on Soybean.

Sclerotinia stem rot of soybean, caused by Sclerotinia sclerotiorum, is a major disease in the north central region of the United States. One approach to managing Sclerotinia stem rot on soybean is the use of fungicides. S. sclerotiorum was assayed for sensitivity to benomyl, tebuconazole, thiophanate methyl, and vinclozolin in pure cultures on agar medium, inoculated soybean seedlings, detached inoculated leaves, and in experimental field plots. To evaluate the inhibitory effect of four fungicides on growth of S. sclerotiorum in vitro, potato dextrose agar (PDA) was amended with the fungicides at six concentrations. Based on measurements of fungal radial growth, vinclozolin was the most effective in inhibiting S. sclerotiorum mycelial growth at 1.0 µg a.i./ml of PDA. Ranges of reduction of radial growth of 91 isolates of S. sclerotiorum on PDA amended with thiophanate methyl and vinclozolin were 18 to 93% and 93 to 99%, respectively, when compared with the nonamended agar control. Benomyl, thiophanate methyl, and vinclozolin applied to greenhouse-grown seedlings prevented S. sclerotiorum from expressing symptoms or signs on leaf tissue. Detached leaves sprayed with thiophanate methyl and then inoculated with mycelial plugs of S. sclerotiorum did not express symptoms or signs. Of 13 different environments in Illinois, Indiana, Ohio, and Wisconsin from 1995 through 2000, six had low Sclerotinia stem rot incidence (<1%), three environments had low to moderate Sclerotinia stem rot incidence (5 to 25%), and four environments had high Sclerotinia stem rot incidence (>25%). When disease incidence was high, no consistent control of Sclerotinia stem rot was observed with benomyl or thiophanate methyl using different application systems. However, under low disease incidence, spray systems that were able to penetrate the canopy reduced the incidence of Sclerotinia stem rot an average of 50%.

First Report of Phomopsis longicolla from Velvetleaf Causing Stem Lesions on Inoculated Soybean and Velvetleaf Plants.

Reddish brown lesions were observed on the lower stem and upper root area of velvetleaf (Abutilon theophrasti Medik.) plants growing in an Illinois soybean field in June 2000. The lesions were similar in appearance to those caused by Rhizoctonia root rot of soybean. Stems and roots with lesions were cut into ≍5-mm pieces, surface-disinfested, and placed on 2% water agar at pH 4.5. The cultural morphology of the two isolates fit the description of Phomopsis longicolla Hobbs (1). Colonies on potato dextrose agar (PDA) were floccose, dense, and white. The undersides of the cultures were colorless. Stromata were large, black, and spreading. The pattern of stromata in one isolate was effuse, and most of the stromata were immersed or semiimmersed in the medium, whereas the stromata from the other isolate were massive and prominent. Neither isolate turned green on PDA. Alpha conidia were hyaline, ellipsoidal to fusiform, and guttulate. DAPI (4',6-diamidino-2-phenylindole)-stained alpha conidia were uninucleate. Beta conidia and perithecia did not occur on either PDA or oat flakes on water agar from 1 to 10 weeks at 25°C under a 12-h photoperiod. The DNA sequences of the mitochondrial small subunit rRNA genes of the two isolates were identical and shared 100% sequence identity with two P. longicolla soybean isolates that we had identified previously. Pathogenicity tests were conducted in a greenhouse by cutting the stems of 3-week-old soybean and velvetleaf plants at the second internode. Mycelial plugs (4 mm in diameter) from the margin of 1-week-old cultures of the two isolates from velvetleaf and one from soybean were individually placed mycelial side down directly on the top of cut stems of 10 to 15 plants per isolate. Controls included noninoculated plants with and without PDA plugs. Plants were kept in a mist chamber in the dark at 25°C for 4 days and were then transferred to a greenhouse with a 16-h photoperiod at 24 ± 3°C. Stem lesions were measured 7 days after inoculation. The experiment was repeated once. Mean stem lesion lengths caused by the velvetleaf and soybean isolates were 23 and 20 mm, respectively, on soybean stems, while negative controls produced no lesions. Mean stem lesion lengths caused by the velvetleaf and soybean isolates were 23.5 and 12 mm, respectively, on velvetleaf stems. P. longicolla was reisolated from the stem lesions of five randomly collected plants. This is the first report of P. longicolla being isolated from velvetleaf and causing stem lesions on inoculated soybean and velvetleaf plants. Reference: (1) T. W. Hobbs et al. Mycologia 77: 535, 1985.

Races of Phytophthora sojae on Soybean in Illinois.

Phytophthora root rot of soybean (Glycine max (L.) Merr.), caused by Phytophthora sojae M. J. Kauffmann & J. W. Gerdemann, has been isolated throughout the soybean-producing regions of the United States. There are more than 39 identified races of P. sojae pathogenic on soybean, and 13 host resistance alleles have been identified at 7 loci (1). None of these alleles confers resistance to all races of P. sojae. The most commonly used resistance allele, Rps1k, confers resistance to the greatest number of races (2). The objective of this study was to identify races of P. sojae in Illinois soybean fields to determine whether the currently used resistance alleles are effective against the P. sojae races found in Illinois. Soybean breeders must be aware of the existence and distribution of races to incorporate appropriate sources of genetic resistance into cultivars. From 192 soil samples collected throughout Illinois in 1997, 33 isolates were obtained and identified to race by inoculating Rps isolines of soybean cv. Williams. A new race with virulence to the Rps1d and Rps7 alleles, designated as race 54, accounted for 48% of the isolates. Another new race with virulence to Rps1d, Rps3a, Rps3c, Rps4, Rps5, Rps6, and Rps7 alleles, designated race 55, was identified in one sample. One isolate, identified as race 41, was obtained from a diseased plant with the Rps1k allele. Another isolate, identified as race 43, was obtained from a diseased plant with the Rps1c allele. Based on virulence patterns of P. sojae, most of the isolates obtained from Illinois soils were races 1, 3, and 4 or variants of these races, such as race 54, with added virulence to the Rps1d allele. References: (1) A. F. Schmitthenner. 1999. Compendium of Soybean Diseases. 4th ed. G. L. Hartman, J. B. Sinclair, and J. C. Rupe, eds. The American Phytopathological Society, St. Paul, MN. pp. 39-42. (2) A. F. Schmitthenner, M. Hobe, and R. G. Bhat. Plant Dis. 78:269, 1994.

Molecular Detection of Diaporthe phaseolorum and Phomopsis longicolla from Soybean Seeds.

ABSTRACT Species-specific detection of Diaporthe phaseolorum and Phomopsis longicolla from soybean seeds was accomplished using polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) and TaqMan chemistry. To use these detection systems, fungal DNA was released from soybean seed coats using an ultrasonic processor to break the cells. DNA fragment lengths ranged from 200 to 1,200 base pairs (bp), with the majority of fragments <500 bp. Based on DNA sequences of the internal transcribed spacer (ITS) regions of ribosomal DNA, three TaqMan primer/probe sets were designed. Primer/probe set PL-5 amplified a 96-bp fragment within the ITS1 region of P. longicolla, D. phaseolorum var. caulivora, D. phaseolorum var. meridionalis, and D. phaseolorum var. sojae. Set PL-3 amplified a 86-bp DNA fragment within the ITS2 region of P. longicolla. Set DPC-3 amplified a 151-bp DNA fragment within the ITS2 region of D. phaseolorum var. caulivora. TaqMan primer/probe sets were able to detect as little as 0.15 fg (four copies) of plasmid DNA. When using PCR-RFLP for Diaporthe and Phomopsis detection, the sensitivity was as low as 100 pg of pure DNA. Among 13 soybean seed lots from Italy and the United States, the total Diaporthe and Phomopsis detected using a traditional seed-plating technique ranged from 0 to 32%. P. longicolla was most prevalent, followed by D. phaseolorum var. sojae. D. phaseolorum var. caulivora, which only occurred in 0.5% of the Italian seed lots, was not detected in the U.S. seed lots. D. phaseolorum var. meridionalis was not detected in either the U.S. or Italian seed lots. Using TaqMan primer/probe set PL-3, the frequency of P. longicolla was 18% in seed lot I3, similar to the frequency obtained from PCR-RFLP and potato dextrose agar plating detection. The frequencies of D. phaseolorum and P. longicolla in each seed lot obtained by the different detection methods were comparable with respect to total infection and individual species detection. However, TaqMan detection provided the fastest results of all the methods tested.

Development of Sclerotia and Apothecia of Sclerotinia sclerotiorum from Infected Soybean Seed and Its Control by Fungicide Seed Treatment.

Field and laboratory studies were done to evaluate the development of sclerotia and apothecia of Sclerotinia sclerotiorum from soybeans and its control with fungicide seed treatment. Soybean seed infected with S. sclerotiorum produced mycelia on both seed coats and cotyledons after 48 h on potato dextrose agar (PDA). Obviously infected soybean seed also were placed in aluminum pans containing field soil and placed in soybean fields near Urbana, Illinois and Clinton, Wisconsin. In 1997, a total of 553 sclerotia, 20 stipes, and 10 apothecia were produced from 500 infected seeds. In 1998, 201 sclerotia and 22 stipes were produced, but no apothecia were observed from the 500 infected seeds. Fludioxonil was the most effective fungicide for reducing radial growth of S. sclerotiorum on PDA plates and suppressed 99% of the radial growth at 0.1 µg a.i./ml. S. sclerotiorum was recovered from 2% of soybean seed lots containing infected seed. When this seed lot was treated with several fungicides, captan + pentachloronitrobenzene + thiabendazole and fludioxonil completely inhibited mycelial growth from infected seed; thiram and thiabendazole each reduced recovery of S. sclerotiorum by 90%. In the field, 754 sclerotia and 10 apothecia were produced from 1,000 infected seeds over a two-year period. When evaluating fungicide control in the field, thiram, fludioxonil, and captan + pentachloronitrobenzene + thiabendazole reduced sclerotia formation from infected seed by more than 98%.

Symptomatology of Barley Yellow Dwarf Virus-RMV Infection in Sweet Corn.

Infection of sweet corn (Zea mays L.) by barley yellow dwarf viruses (BYDVs) caused different symptoms on hybrids with shrunken-2 (sh-2) when compared with hybrids with sugary-1 (su-1) endosperm mutations. Sweet corn hybrids inoculated with BYDV-RMV in Urbana, IL, developed either yellow or red-purple leaf symptoms similar to those caused by phosphorus and/or potash deficiency (1). In a field trial near Rochelle, IL, 329 of 339 sweet corn hybrids naturally infected by BYDV exhibited one or the other of these symptoms. Eighteen of the hybrids with severe symptoms were tested for two strains of BYDVs in enzyme-linked immunosorbent assays (1). All of the hybrids tested were positive for BYDV-RMV; none were positive for BYDV-PAV. Symptoms on su-1 hybrids were typically characterized by dark red to purple coloration of leaf margins from the tip down to the mid-leaf. Leaves of sh-2 hybrids exhibited chlorosis in a similar pattern. A single sh-2 hybrid exhibited red-purple leaf symptoms, probably as a result of a crossover between the anthocyanin production (a1) and sh-2 genes, which are linked about 0.25 map units apart on chromosome 3 (2). Infection of sh-2 sweet corn hybrids by BYDV-RMV is most likely to induce chlorosis, whereas infection of su-1 sweet corn hybrids can result in either chlorosis or red-purple discoloration of leaves. Sweet corn hybrids with these symptoms should be tested for BYDVs, particularly the RMV strain. References: (1) R. L. C. Itnyre et al. Plant Dis. 83:566, 1999. (2) E. B. Mains. J. Hered. 40:21, 1948.

Reaction of Sweet Corn to Inoculation with Barley Yellow Dwarf Virus RMV-IL.

The reaction of five sweet corn hybrids to barley yellow dwarf virus (BYDV-RMV-IL) was determined in 1992 and 1993. In 1992, symptoms were observed in three of the five hybrids planted 20 May and four of the five hybrids planted 20 June. No symptoms were observed in hybrids planted June or July 1993. The mean virus incidences of RMV-IL determined by enzyme-linked immunosorbent assay (ELISA) in all plots for the May and June 1992 planting dates were 3.5 and 21%, respectively. The mean virus incidence for the inoculated plots for the June 1992 planting date was significantly higher than incidence for the control plots (29 versus 13%). Ear weights were significantly lower for inoculated plots than for the control plots (1.2 kg versus 1.4 k/10 ears). The mean virus incidences of RMV-IL in all plots for the June and July 1993 planting dates were 31 and 23%, respectively. The mean virus incidence for the inoculated plots for the June 1993 planting date was significantly higher than incidence for the control plots (49 versus 14%). Plots inoculated in June 1993 also had a significantly lower ear weight than the control plots (1.8 kg versus 2.0 kg/10 ears). A range of symptoms, infection levels, and yield responses of sweet corn hybrids to BYD-RMV-IL was demonstrated in this study. Hybrid susceptibility to this strain of BYDV should be considered when planting sweet corn from middle to late June in order to limit potential yield losses.

Yield and Seed Quality of Soybean Cultivars Infected with Sclerotinia sclerotiorum.

Sclerotinia stem rot (SSR) is one of the most important diseases of soybean in the United States. Five maturity group III cultivars, Asgrow A3304 STS (A3304), Pioneer Brand 9342 (P9342), Pioneer Brand 9381 (P9381), Probst, and Yale, grown in fields in east-central Illinois, were used to determine the relationship of SSR incidence to yield, 100-seed weight, seed protein and oil content, visual seed quality, and seed germination. In addition, the number of sclerotia in seed samples and the seedborne incidence of Sclerotinia sclerotiorum were determined. For each cultivar, at least 23 two-row plots, 3 m long, that represented a range of SSR incidence from low to high, were used to count the number of plants with and without SSR stem symptoms and were used to estimate yields and evaluate seed quality. Disease incidence ranged from 2 to 45% for Probst, 0 to 65% for P9381, 0 to 68% for P9342, 1 to 93% for Yale, and 0 to 95% for A3304. Regression of yields on SSR incidences for each cultivar was significant (P < 0.05); for every 10% increase in SSR incidence, yields were reduced by 147, 194, 203, 254, and 263 kg/ha for Probst, A3304, P9342, Yale, and P9381, respectively. Disease incidence was negatively correlated (P < 0.05) with seed germination for all cultivars but Probst, and to oil content and seed weight for P9381 and Yale. Disease incidence was positively correlated (P < 0.05) with seed quality for all cultivars and to the number of sclerotia in harvested seeds for P9342, P9381, and Probst. The seedborne incidence of S. sclerotiorum was 0.3, 0.3, 0.3, 0.4 and 0.7% in A3304, P9381, Yale, Probst, and P9342, respectively, and represents a significant potential for further spread of this pathogen and disease.

Response of Soybean Accessions from Provinces in Southern China to Phytophthora sojae.

Phytophthora rot, caused by Phytophthora sojae, is a damaging disease of soybean (Glycine max (L.) Merr.) throughout the soybean-producing regions of the world. The discovery of new sources of resistance in soybean is vital in maintaining control of Phytophthora rot, because races of the pathogen have been discovered that can attack cultivars with commonly used resistance genes. The objectives of this study were to investigate the distribution and diversity of Phytophthora-resistant soybean in southern China and identify sources that confer resistance to multiple races for implementation into breeding programs. Soybean accessions obtained from southern China were evaluated for their response to races 1, 3, 4, 5, 7, 10, 12, 17, 20, and 25 of P. sojae using the hypocotyl inoculation technique in the greenhouse at Urbana, Illinois in 1996 and 1997. Accessions were identified that confer resistant responses to multiple races of the pathogen. These accessions may provide sources of resistance for control of Phytophthora rot of soybean in the future. The majority of the accessions with resistance to eight or more of the ten races tested were from the provinces of Hubei, Jiangsu, and Sichuan in southern China. Based on the evaluated accessions, these provinces appear to be valuable sources of Phytophthora-resistant soybean.

Molecular Identification and Phylogenetic Grouping of Diaporthe phaseolorum and Phomopsis longicolla Isolates from Soybean.

ABSTRACT Diaporthe phaseolorum and Phomopsis longicolla isolates from soybean were examined using traditional mycological characteristics and molecular methods. Cultural characteristics including types of fruiting bodies and conidia were assessed for isolates collected from soybean stems and seeds. Cultures were identified as P. longicolla, D. phaseolorum var. caulivora, D. phaseolorum var. meridionalis, or D. phaseolorum var. sojae. Molecular markers for these groups were developed and analyzed using polymerase chain reaction restriction fragment length polymorphisms (PCR-RFLP) and DNA sequencing in the internal transcribed spacer (ITS) and the 5.8S ribosomal DNA. The ITS(4) and ITS(5) primers amplified PCR products for all isolates studied. Gel electrophoresis of undigested PCR products and DNA sequencing produced various fragment lengths including 604 bp for P. longicolla, 602 and 603 bp for D. phaseolorum var. caulivora, 603 bp for D. phaseolorum var. meridionalis, and from 597 to 609 bp for D. phaseolorum var. sojae. Digestion of these PCR products with enzymes AluI, HhaI, MseI, RsaI, and ScrFI resulted in distinct bands for identification of P. longicolla and the varieties of D. phaseolorum I. All P. longicolla, D. phaseolorum var. caulivora, and D. phaseolorum var. meridionalis isolates were distinguished using AluI and HhaI with RsaI or ScrFI. The banding patterns of D. phaseolorum var. sojae isolates were complex and were separated into 11 subgroups after digestion with AluI, HhaI, MseI, RsaI, and ScrFI. Phylogenetic analysis of 20 isolates of D. phaseolorum and P. longicolla based on the DNA sequence of the ITS region resolved six clades termed A, B, C, D, E, and F. Clade A included all sequenced D. phaseolorum var. caulivora isolates, two from Italy and one from the United States. Isolates in clade B were exclusively associated with D. phaseolorum var. meridionalis. Clades A and B formed a well-supported monophyletic group. Isolates in clades C, D, E, and F were morphologically defined as isolates of P. longicolla, D. phaseolorum var. sojae, and Diaporthe spp. The ITS sequences similarity of seven geographically diverse P. longi-colla isolates illustrated that P. longicolla isolates have a similar genetic background, with some affiliations to some D. phaseolorum var. sojae isolates. Morphological characteristics of the isolates along with the terminal clades of the ITS phylogeny suggest that P. longicolla is an individual species, D. phaseolorum var. caulivora and D. phaseolorum var. meridionalis are varieties of D. phaseolorum, and D. phaseolorum var. sojae is either several varieties of D. phaseolorum or possibly several distinct species.