Andreas Westphal

Interaction of Fusarium solani f. sp. glycines and Heterodera glycines in sudden death syndrome of soybean.

ABSTRACT Sudden death syndrome (SDS) of soybean is caused by the soilborne Fusarium solani f. sp. glycines (synonym F. virguliforme). In a sequential approach, two multifactor factorial-design microplot experiments were conducted to investigate the effects of fungal infestation levels and soil moisture on both root necrosis and foliar SDS severity, and the interaction between F. solani f. sp. glycines and Heterodera glycines in fumigated versus nonfumigated soil. In 2003, soybean cv. Spencer was grown in nonfumigated or methyl bromide-fumigated soil and infested with increasing levels of F. solani f. sp. glycines, either under rainfall or irrigated after growth stage V6/R1. In 2004, interactions between F. solani f. sp. glycines and H. glycines were explored in a factorial inoculation design in fumigated or nonfumigated soil, planted to Williams 82 or Cyst-X20-18. In both years, higher levels of foliar SDS severity and root necrosis were found in F. solani f. sp. glycines-infested soils with H. glycines than in soils without the nematode on the soybean cultivars susceptible to both pathogens. Both natural infestations of H. glycines in 2003 and artificially amended populations of H. glycines in 2004 contributed to higher foliar SDS severity. More severe foliar SDS symptoms always were associated with more root necrosis, but elevated levels of root necrosis did not predict severe leaf symptoms. In contrast to the critical role of H. glycines, increasing fungal infestation levels had no significant effects on increasing either foliar SDS symptoms or root necrosis. Effects of moisture regime and fungal infestation levels also were examined in factorial greenhouse and growth chamber experiments. High soil moisture resulted in higher levels of SDS root necrosis. In the greenhouse, root necrosis increased at a higher rate in low soil moisture than the rate in high soil moisture. The two pathogens acted as a complex and the disease development was strongly dependent on high soil moisture.

Identification of root-knot nematode suppressive soils

In three of 12 soils obtained from agricultural fields in California, population density development of Meloidogyne incognita under susceptible tomato was significantly suppressed when compared to identical but methyl iodide (MI)-fumigated, M. incognita re-infested soils. When the 12 soils were infested with second-stage juveniles (J2) of M. incognita and the juveniles were extracted after 3 days, significantly fewer J2 were recovered from 9 of the 12 non-treated soils than from the MI-fumigated equivalents. In one of the 12 soils, infestation 3 weeks before planting resulted in lower nematode population densities than infestation at planting in both MI-fumigated and non-treated soil. The combination of infestation 3 weeks before planting with infestation at planting did not alter the occurrence or degree of root-knot nematode suppressiveness.

First Report of Soybean Cyst Nematode Reproduction on Purple Deadnettle under Field Conditions

Soybean cyst nematode commonly infests soybean fields in the United States and is a threat to profitable soybean production. Purple deadnettle is common in Indiana production fields and frequently occurs at very high densities. Purple deadnettle plants were removed from a research site infested with SCN. Peadnettle roots were found to possess cysts containing eggs. The morphology of cysts and juveniles was consistent with SCN. Based on the distribution of other cyst nematodes in Indiana, a bioassay on soybean from the field, and morphometric observations, we determined the identity of the nematode to be SCN.

Specific Microbial Attachment to Root Knot Nematodes in Suppressive Soil

ABSTRACT Understanding the interactions of plant-parasitic nematodes with antagonistic soil microbes could provide opportunities for novel crop protection strategies. Three arable soils were investigated for their suppressiveness against the root knot nematode Meloidogyne hapla. For all three soils, M. hapla developed significantly fewer galls, egg masses, and eggs on tomato plants in unsterilized than in sterilized infested soil. Egg numbers were reduced by up to 93%. This suggested suppression by soil microbial communities. The soils significantly differed in the composition of microbial communities and in the suppressiveness to M. hapla. To identify microorganisms interacting with M. hapla in soil, second-stage juveniles (J2) baited in the test soil were cultivation independently analyzed for attached microbes. PCR-denaturing gradient gel electrophoresis of fungal ITS or 16S rRNA genes of bacteria and bacterial groups from nematode and soil samples was performed, and DNA sequences from J2-associated bands were determined. The fingerprints showed many species that were abundant on J2 but not in the surrounding soil, especially in fungal profiles. Fungi associated with J2 from all three soils were related to the genera Davidiella and Rhizophydium, while the genera Eurotium, Ganoderma, and Cylindrocarpon were specific for the most suppressive soil. Among the 20 highly abundant operational taxonomic units of bacteria specific for J2 in suppressive soil, six were closely related to infectious species such as Shigella spp., whereas the most abundant were Malikia spinosa and Rothia amarae, as determined by 16S rRNA amplicon pyrosequencing. In conclusion, a diverse microflora specifically adhered to J2 of M. hapla in soil and presumably affected female fecundity.

Evidence for biological nature of the grape replant problem in California

A bioassay was developed to investigate causes of grape replant problems under controlled conditions. Soils were collected from methyl bromide-fumigated and non-fumigated plots at a site cleared from a 65-year-old grape vineyard (Vitis vinifera cv. Thompson seedless) at Parlier, CA. Subsamples of the non-fumigated soil were either left non-treated, subjected to autoclaving (twice 45 min), or heating at 40, 50, 60, 70, 80 or 90 °C for 30 min. Subsequently, the samples were placed in 120-mL pots, planted with rooted hardwood grape cuttings (V. vinifera, cv. Carignane) and placed in a greenhouse or growth chamber. Three months after transplanting, vines from non-treated or 40 °C-treated soil had lower shoot weights and densities of healthy lateral roots than vines from the other treatments. Pythium spp. were isolated from 45 to 55% of the plated root segments from vines grown in non-treated, or soil that had been heated at 40 or 50 °C but were not detected in roots from soil given other treatments. Egg masses of root-knot nematode, Meloidogyne spp., were produced on roots from non-treated or heated at 40 °C soil, but no egg masses were detected on roots of the other treatments. In another test with the same soils, remnant roots from non-fumigated or pre-plant methyl bromide-fumigated soil were extracted and amended to non-fumigated soil, soil from fumigated field plots, soil fumigated in a small container, or autoclaved potting mix. The transfer of old vine roots from non-fumigated field soil resulted in incidence of Pythium spp. on grape assay roots, but there was no measurable effect of the transfer on growth and health of the bioassay plant roots. The results of the bioassays indicate that grape replant problem at the California site had biological causes. The bioassay approach may aid in future determinations of the etiology of grape replant problems.

Depth distribution of Rotylenchulus reniformis under crops of different host status and after fumigation

Population densities of Rotylenchulus reniformis were investigated in 15 cm horizons from the surface to 120 cm deep in field plots under fallow, grain sorghum (cv. Asgrow 571), and susceptible (cv. DP6880RR) or resistant (cv. HY798) soybean. In 2000, population densities were monitored in non-fumigated plots and in plots fumigated pre-season with 1,3-dichloropropene (1,3-D) at 38 cm depth. Fallow, grain sorghum and resistant soybean reduced the incidence of R. reniformis down to 120 cm in comparison to susceptible soybean. In 2001, population densities were monitored under cotton in these plots and in additional plots that had grown susceptible soybean cv. Vernal or resistant cv. Padre in 2000 and that were fumigated before the 2001 crop with 1,3-D at depths of 0-60, 60-120, or 0-120 cm. In 2000 non-fumigated plots, cotton fibre yields were increased by an average 35% after fallow, grain sorghum or resistant soybean compared to those after susceptible soybean. In the 2000 fumigated plots, cotton fibre yields were increased 38% after grain sorghum and resistant soybean compared to those after susceptible soybean or fallow. In plots cropped previously to susceptible soybean, fumigation at 60-120 cm deep increased cotton fibre yields by 68% compared to the non-fumigated control. Population densities in the 0-120 cm horizon were a more accurate predictor of plant damage than those at 0-30 cm. The value of R. reniformis resistant soybean cultivar-cotton crop sequences was confirmed and the importance of their effects on deep-occurring populations of R. reniformis demonstrated.

Soil suppressiveness to Heterodera schachtii under different cropping sequences

Heterodera schachtii population densities were monitored in a H. schachtii -suppressive soil cropped in screenhouse experiments for two consecutive seasons with wheat, susceptible or resistant cultivars of either sugar beet or oilseed radish, or left fallow. Heterodera schachtii population densities under wheat and the resistant cultivars of sugar beet and oilseed radish did not differ significantly from the fallow treatment. Populations declined under all crops, with a reproductive factor between 0.08 and 0.57. In glasshouse experiments, introduced H. schachtii populations increased greatly on susceptible Swiss chard grown in previously wheat-monocultured soils, suggesting that significant loss of H. schachtii suppressiveness occurred during the monoculture. Following fallow, two H. schachtii -resistant or two H. schachtii -susceptible cultivars, introduced sugar beet cyst nematode populations remained small, suggesting that suppressiveness had been maintained. In a field trial with H. schachtii suppressive soil, cyst nematode population densities remained lower under wheat, resistant sugar beet, resistant radish and susceptible radish than under susceptible sugar beet.

Influence of Winter Annual Weed Management and Crop Rotation on Soybean Cyst Nematode (Heterodera Glycines) and Winter Annual Weeds

Abstract Certain winter annual weeds have been documented as alternative hosts to soybean cyst nematode (SCN), and infestations of such species have become common in no-till production fields in the Midwest. This research was conducted to determine the influence of herbicide- and cover-crop-based winter annual weed management systems and crop rotation on winter annual weed growth and seed production, SCN population density, and crop yield. Two crop rotations (continuous soybean and soybean-corn) and six winter annual weed management systems (a nontreated control, fall and spring herbicide applications, spring-applied herbicide, fall-applied herbicide, fall-seeded annual ryegrass, and fall-seeded winter wheat) were evaluated in no-tillage systems from fall 2003 to 2006 at West Lafayette, IN and Vincennes, IN. Fall or spring herbicide treatments generally resulted in lower winter annual weed densities than cover crops. Densities of henbit and purple deadnettle increased over years in the cover crop systems but remained constant in the herbicide systems. Averaged over sites and years, winter annual weed densities were nearly 45% lower in the spring than the fall due to winter mortality. Corn yield was reduced by the cover crops at West Lafayette but not Vincennes. Winter annual weed management system had no influence on soybean yield. SCN population density was reduced by including corn in the crop sequence but was not influenced by winter annual weed management. The density of weedy host species of SCN in the experimental area was relatively low (less than 75 plants m−2) compared to densities that can be observed in production fields. The results of these experiments suggest that inclusion of corn into a cropping sequence is a much more valuable SCN management tool than winter annual weed management. In addition, control of winter annual weeds, specifically for SCN management, may not be warranted in fields with low weed density. Nomenclature: Soybean cyst nematode, Heterodera glycines Ichinohe; corn, Zea mays L.; soybean, Glycine max (L.) Merr; wheat, Triticum aestivum L.

Development of Soybean Cyst Nematode on Henbit (Lamium amplexicaule) and Purple Deadnettle (Lamium Purpureum)

A survey of seven production fields in Indiana, Illinois, and Ohio was conducted to assess henbit and purple deadnettle growth and soybean cyst nematode (SCN) development and reproduction on these weeds. Autumn and spring growth of purple deadnettle and henbit was influenced by location within each state. In general, winter annual weeds were larger in size and reached maturity earlier in the spring at the southern sample sites than those in the north. All growth stages of SCN were found to be associated with henbit and purple deadnettle at both autumn and spring sample timings. SCN juveniles were generally found infecting roots at highest abundance in the spring. SCN cyst and egg production also were widespread and occurred to a much higher degree during the autumn than the spring developmental period. The results of this survey indicate that management tactics designed to minimize the potential for SCN reproduction on winter annual weeds would probably be most effective if conducted in the autumn, when the majority of SCN reproduction occurred. However, spring populations of winter annual weeds that harbor SCN juveniles might facilitate additional SCN reproduction and population increase if the weeds are not controlled in a timely manner prior to planting. Nomenclature: Henbit, Lamium amplexicaule L. LAMAM; purple deadnettle, Lamium purpureum L. LAMPU; soybean, Glycine max (L.) Merr; soybean cyst nematode, Heterodera glycines Ichinohe.

Contributions of Fusarium virguliforme and Heterodera glycines to the Disease Complex of Sudden Death Syndrome of Soybean

Background Sudden death syndrome (SDS) of soybean caused by Fusarium virguliforme spreads and reduces soybean yields through the North Central region of the U.S. The fungal pathogen and Heterodera glycines are difficult to manage. Methodology/Principal Findings The objective was to determine the contributions of H. glycines and F. virguliforme to SDS severity and effects on soybean yield. To quantify DNA of F. virguliforme in soybean roots and soil, a specific real time qPCR assay was developed. The assay was used on materials from soybean field microplots that contained in a four-factor factorial-design: (i) untreated or methyl bromide-fumigated; (ii) non-infested or infested with F. virguliforme; (iii) non-infested or infested with H. glycines; (iv) natural precipitation or additional weekly watering. In years 2 and 3 of the trial, soil and watering treatments were maintained. Roots of soybean ‘Williams 82’ were collected for necrosis ratings at the full seed growth stage R6. Foliar symptoms of SDS (area under the disease progress curve, AUDPC), root necrosis, and seed yield parameters were related to population densities of H. glycines and the relative DNA concentrations of F. virguliforme in the roots and soil. The specific and sensitive real time qPCR was used. Data from microplots were introduced into models of AUDPC, root necrosis, and seed yield parameters with the frequency of H. glycines and F. virguliforme, and among each other. The models confirmed the close interrelationship of H. glycines with the development of SDS, and allowed for predictions of disease risk based on populations of these two pathogens in soil. Conclusions/Significance The results modeled the synergistic interaction between H. glycines and F. virguliforme quantitatively in previously infested field plots and explained previous findings of their interaction. Under these conditions, F. virguliforme was mildly aggressive and depended on infection of H. glycines to cause highly severe SDS.