J. A. Wrather

Effects of Tillage, Cultivar, and Planting Date on Percentage of Soybean Leaves with Symptoms of Sudden Death Syndrome

A 4-year experiment, 1991 to 1994, was conducted in a field where sudden death syndrome (SDS) had previously been observed. The objective was to determine the effects of tillage, planting date, and soybean cultivar on the percentage of leaves with symptoms of SDS at R6 growth stage. The soybean cultivars Essex, Forrest, Hartwig, and Rhodes were each planted in 75-cm-wide rows in disk-till, ridge-till, and no-till plots. The planting dates were mid-May, mid-June, and late June to early July each year. Symptoms of SDS developed in 1991, 1992, and 1994, but in not 1993. There were significant year × cultivar (P = 0.0001) and tillage × planting date × cultivar (P = 0.05) interactions for the percentage of leaves with symptoms of SDS. Essex, Forrest, and Rhodes had a greater percentage of leaves with symptoms of SDS than did Hartwig in 1991 and 1994; differences among cultivars did not occur in 1992. The percentage of Essex, Forrest, and Rhodes leaves with symptoms of SDS was greater for no-till than for either disk-till or ridge-till in mid-May plantings. There were no significant correlations between the percentage of leaves with SDS and yield

Somaclonal variation in soybean plants regenerated from tissue culture.

Callus cultures of soybean (Glycine max (L.) Merr.) genotypes PI 88788, PI 438489B, and cultivar Bedford were initiated in vitro from seedling explants consisting of the cotyledonary node plus epicotyl from germinated mature seed. Plants were regenerated from these callus cultures and subsequently evaluated for qualitative variation in three to four subsequent generations. Variant phenotypes observed that have not been previously reported from tissue culture include lanceolate leaves, leaf variegation (chimeral variegated plants), pod variegation on otherwise normal plants, and change in growth habit from indeterminate to determinate. The lanceolate leaf, chimeral variegated plant, and change from indeterminate to determinate growth habit characters were inherited through at least three generations (R0-R2), and segregation occurred in each generation. Pod variegation was inherited through the two generations tested thus far and segregation occurred in each generation. No variation was observed in control plants derived from normal seed. Variants appeared more frequently in regenerants from PI 88788 and PI 438489B than from Bedford. These results confirm and extend the finding that certain tissue culture techniques may be used to induce novel plant formation from somatic tissue of soybean.

Genetic diversity among isolates of Heterodera glycines and sources of resistance in soybeans

Populations of the soybean cyst nematode (Heterodera glycines) (SCN) are extremely variable. A recent nematode classification system includes 11 new races, for a total of 16. Little is known about newly described races 4, 6-13, 15, and 16. We have attempted to examine the genetic diversity among isolates of H. glycines, isolate sources of resistance in soybeans, and identify soybean differentials for describing intrarace variation of SCN (.)

Salt tolerant sugarbeet progeny from tissue cultures challenged with multiple salts.

Lines of sugarbeet (Beta vulgaris L.) tolerant of multiple salts was accomplished by an in vitro multiple salt challenge. Petioles were placed on RV medium amended with 5 different salts along with Murashige and Skoog base salts for one month. Surviving shoots were cultured on RV medium to obtain petioles for subsequent challenges. During the first, second and third challenges, organogenically regenerated shoots developed from 5%, 46%, and 80% of the petioles, respectively. After the third multiple salt challenge, tolerant shoots were rooted and transplanted in soil. Salt was added to this soil at 1.0% by weight and plants were observed for 2 months. The ten most salt tolerant plants were vernalized to obtain seed. The R1 seed and controls were planted in soil containing 0%, 0.61% or 0.77% multiple salts per dry soil weight. Emergence of R1 seedlings was significantly greater than the controls under salt stress. Multiple salt tolerant R1 plants were maintained in salt amended soil to the 8–10 leaf stage and appeared as healthy and vigorous as the control growing in salt free soil.

Planting Date and Cultivar Effects on Soybean Yield, Seed Quality, and Phomopsis sp. Seed Infection

Incidence of Phomopsis seed decay is frequently high and quality low in seed from early-maturing maturity group III and IV soybean cultivars planted in early to mid-April in the southern United States. Cultivars resistant to this disease have not been available until the recent release of germ plasm lines SS 93-6012 and SS 93-6181. Our objective was to determine the effects of planting dates with these lines and one Phomopsis seed decay-susceptible soybean cultivar, Asgrow 3834, on seed infection by Phomopsis spp. and on yield and the correlation between percentage of Asgrow 3834 infected with Phomopsis spp. and seed quality. Generally, yields averaged over years were significantly greater for mid-April than mid-June plantings, and yields of cultivars were similar within a planting date. Soybean lines SS 93-6012 and SS 93-6181 were highly resistant to Phomopsis seed decay compared with the susceptible cultivar, Asgrow 3834. There was a significant, negative correlation between germination of seed from mid-April plantings of Asgrow 3834 and percentage of these seed infected with Phomopsis spp. Moreover, there were significant correlations between fatty acid composition of Asgrow 3834 seed and the percentage of these seed infected with Phomopsis spp. This altered composition of fatty acids may be responsible for reduced quality of oil derived from seed infected with this fungus. Phomopsis seed decay-resistant soybean lines SS 93-6012 and SS 93-6181 should be useful in breeding programs focused on developing high-yielding cultivars resistant to this disease.

Cultivar and planting date effects on soybean stand, yield and Phomopsis sp. seed infection

Experiments were conducted in a field near Portageville, MO, from 1992 to 1994 to determine cultivar and planting date effects on soybean stand, yield, and seed infection with fungi. Soybean cultivars Asgrow 3733 (maturity group III), Ring Around 452 (maturity group IV), Hutcheson (maturity group V), and Asgrow 6785 (maturity group VI) were each planted in 75-cm-wide rows in tilled soil. The planting dates were mid-April, mid-May, and mid-June. In 1992, stands of Hutcheson and Asgrow 6785 were greater than stands of Asgrow 3733 and Ring Around 452 for mid-April plantings. Stands were similar among all cultivars and planting dates in 1993 and 1994. Yields of Hutcheson and Asgrow 6785 were similar for all planting dates and were greater than yields of Asgrow 3733 for all planting dates. Yields of Hutcheson and Asgrow 6785 were greater than those of Ring Around 452 for mid-April and mid-May plantings. The percentage of seed infected with Phomopsis sp. was greater in Asgrow 3733 than for the other cultivars in mid-April and mid-May plantings. The percentage of seed infected with Phomopsis sp. was low in Asgrow 6785 each year and was similar for all planting dates. Low yields and problems of seed infection with Phomopsis must be resolved for early planting of Asgrow 3733 to be a practical production system in the upper mid-south. Mid-April planting of Hutcheson and Asgrow 6785 may be a practical cropping system for the upper mid-south because yields, stands, and seed infection with Phomopsis sp. were similar to those of mid-May plantings and harvest was earlier.

Soybean Cultivar and Foliar Fungicide Effects on Phomopsis sp. Seed Infection

Phomopsis seed decay (PSD) caused by Phomopsis spp. can be severe when soybean seed producers in the southern United States use the early soybean production system (ESPS) to avoid late-July through early-September drought damage to soybean. The usefulness of this production system would be greater if developing seed could be protected from PSD by foliar application of fungicides or by planting Phomopsis spp.-resistant soybean lines. The objective of this research was to determine the affects of the fungicides benomyl and azoxystrobin applied to soybean, at various times, on percent Phomopsis spp. infection of seed in Asgrow 3834, a PSD-susceptible cultivar, and SS93-6012, a PSD-resistant soybean line, planted in mid-April. The percent Phomopsis spp. infection of Asgrow 3834 seed averaged over years was significantly less for the benomyl (0.28 kg a.i. ha-1) applied at R3 + R5 treatment (48.6% seed infection) than the control (52.8% seed infection) and significantly greater for the azoxystrobin (0.17 kg a.i. ha-1) applied at R3 + R5 treatment (61.6% seed infection) than the control (52.8% seed infection). This method of managing PSD will not be acceptable to soybean growers. The percent of Phomopsis spp. infection of Asgrow 3834 seed averaged over years (52.8% seed infection) was significantly greater than for line SS93-6012 (2.8% seed infection). There were no differences in percent Phomopsis spp. infection of SS93-6012 seed between the control (2.8% seed infection) and benomyl treatment (4.0% seed infection). The most effective method for PSD management was to plant a resistant soybean line. Line SS93-6012 will be useful in breeding programs focused on developing high yielding PSD-resistant cultivars.

Lack of predictable race shift in Heterodera glycines-infested field plots

Soybean cultivars with different sources of resistance to Heterodera glycines were grown at three locations initially infested with races 2, 3, and 6 in order to investigate H. glycines race shift in field populations. Each spring and fall, soil samples were taken from each plot and race tests were conducted to evaluate effects of cultivar and time of sampling. Field experiments were paired field plots rotated annually with corn since 1991. Cultivars included at the northern and central Missouri sites were Williams 82 (susceptible to H. glycines), Linford (PI 88788 source of resistance), MFA 9043 (Peking) replaced by Morsoy 9345 (Peking and PI 88788) from 1995 to 1997, and Jackson II (Peking + PI 88788) replaced by Asgrow 3431 (Peking and PI 88788) in 1996-97. Cultivars at the southern Missouri site were Essex or Hutcheson (susceptible to H. glycines), Forrest (Peking), Hartwig (PI 437654), and Rhodes (PI 88788 + Peking). In 1995, race tests were performed at four temperature regimes to determine temperature effects on race designations. Race shifts were not predictable based on the source of resistance of the soybean cultivar planted. Variability in female numbers on Lee 74 among tests caused changes in female indices (FI). Furthermore, race designations were influenced by the time of sampling and temperature at which the race tests were conducted. The variability of H. glycines populations in both field and greenhouse situations diminishes the value of race test results when making cultivar recommendations.

Distribution and virulence phenotypes of Heterodera glycines in Missouri

The soybean cyst nematode, Heterodera glycines, is the most economically important pathogen of soybean in Missouri. Knowledge of the nematodes distribution and ability to adapt to resistant varieties is important for determining crop losses and establishing research priorities. No previous surveys of Missouri have provided reliable population density and phenotypic diversity data; therefore, we conducted a random survey to obtain both. Two samples from each of 200 fields were collected; 392 samples were processed for extractions of cysts and eggs. Two hundred and forty seven (63%) of the samples had detectable cyst nematode populations, which ranged from 15 to 149,700 eggs per 250 cm3 of soil. The lowest average population densities were observed in the east-central region of Missouri (2,260 eggs per 250 cm3 of soil), and the highest were observed in the northeast (9,238 eggs per 250 cm3 of soil), but among the eight regions sampled, mean population densities did not differ significantly. These population densities were potentially responsible for losses worth over

Selection of atrazine tolerant soybean calli and expression of that tolerance in regenerated plants.

58 million in 1999 in Missouri. Race tests were conducted on populations from 183 samples. In order of frequency, races 3, 1, and 2 accounted for 86% of H. glycines populations. Nearly 60% of the populations were virulent (able to produce females) on plant introduction (PI) 88788, which is the source of resistance for most H. glycines-resistant cultivars. More than a third of the populations were virulent on cv. Peking, another common resistance source. Very few populations were virulent on PI 90763 or PI 437654, suggesting that these sources of resistance should be exploited more frequently.