Update:
Sudden death syndrome (Fusarium virguiliforme) is an important disease that affects soybean yields across the United States every year. The pathogen survives in the soil where it starts infection on soybean roots as root rot. After root colonization, F.v. produces a toxin that spreads throughout the plant that kills the vascular system, which in turn, cuts off the plants water supply. Because SDS impacts soybean yields it is important to understand how the disease is spread to understand how to control it. Natural resistance to both the toxin and root phases of the pathogen differs among varieties of soybeans.

OBJ 1. Screen adapted Kansas germplasm and KSVT entries for SDS resistance using three high-throughput methods.

This objective is continued from the 2017-2018 project and overlaps with the final report for last year’s project. Germplasm is being screened using three methods: (i) culture extract/toxin assay method, (ii) rolled-towel method, and (iii) layer-cake method. The culture extract/toxin assay method has been fine-tuned and completed. An important soybean discovery through this assay is that genotypes can be divided into “senescent” and “non-senescent” types based upon their reaction to wounding and placement into water or toxin suspension. See attached PDF for designations of toxin resistance and senescence among Kansas Soybean Variety Trial entries. We have received an additional 250 elite breeding materials from Dr. Bill Schapaugh (KSU Soybean Breeder) and these are being screened using the culture extract/toxin method (see attachment). As a result of the toxin (culture extract) screening approach, the following designations have been made:

RESULTS:

KSVT
Non-senescent & Toxin tolerant = 11 entries
Non-senescent & Toxin sensitive = 50
Senesent & Toxin tolerant = 1
Senescent & Toxin sensitive = 69

KAES Elite Germplasm*
Non-senescent & Toxin tolerant = 9 entries
Non-senescent & Toxin sensitive = 40
Senesent & Toxin tolerant = 6
Senescent & Toxin sensitive = 95
*From Dr. Bill Schapaugh’s breeding program.

It is hypothesized that the non-senescent & toxin tolerant varieties from the culture extract test will prove to be the most resistant to SDS in the field. To test this, examples entries from each of the categories will be planted in the field with a history of SDS disease this summer (2019) and evaluated/compared for disease severity and yield.

As many KSVT entries as possible, given seed availability, that were screened for the culture extract/toxin assay were screened using the rolled towel approach. It is thought that the same entries that showed toxin resistance in the first assay may or may not show seedling disease resistance in the rolled-towel assay, since these assay test different aspects/stages of the disease/types of resistance. Data collected for the rolled towel assay includes (1) disease severity, (2) seedling root & hypocotyl germination, (3) root & hypocotyl length, and (4) seedling fresh weight. There was a positive and significant relationship (r = +0.45; P = 0.012) between toxin rating and seedling disease severity scores in the subset of KSVT entries tested.


OBJ 2. Examine the interaction between ILeVO seed treatment and planting date for SDS.

The first, second, and third planting dates for the ILeVO/planting date study were completed at the Rossville unit of the Kansas Research and Extension fields near Topeka. The first, second, and third planting dates for this study were May 1, May 18, and June 8, respectively. 2018 was the 4th and final year of this study and will contribute to a complete data set for analysis, publication, and extension & communication purposes concerning planting and seed treatment strategies for management of SDS. Data is currently in preparation for this year's plots. Information for planting date/ILeVO and seedling-associated Fusarium spp. will be combined for final publication.

RESULTS:
1. Later planting reduces SDS.
2. However, earlier planting improves yield, which overcomes average SDS effects (if a SDS-resistant variety is used).
3. Yield improves slightly, but not significantly, when ILeVO is seed treatment is used in the presence of SDS.


OBJ 3. Determine modes of residue and soilborne survival of the SDS pathogen in Kansas production fields.

This objective was addressed in two ways: (i) corn residue and (ii) soil. Corn residue will be re-sampled to determine if this substrate is a source of F. virguiliforme survival in the Kansas River Valley. The first experiment, after using molecular confirmation techniques, showed that corn residue did NOT harbor the pathogen. Therefore, a second experiment was conducted. Working with the Co-PI, Eric Adee, we will identify three sites that are in corn-soybean rotation and have a history of SDS disease.

RESULTS:
1. Corn residue processed for the presence of F. virguiliforme. F. solani is related to F. virguiliforme and is present in corn residue, which can be confused with F. virguiliforme in microbiological plating assays.
2. However, PCR showed that F. virguiliforme does not appear to be present in corn residue.


OBJ 4. Determine pathogenic variability of Fusarium virguiliforme isolates from multiple Kansas fields.

Isolates were obtained from symptomatic plants as they began to express symptoms in the field. Several pathogen isolates have been characterized. Additionally, plant disease diagnostic lab samples were also obtained as a source of isolates. A seedling disease assay that integrates germination and hypocotyl length has proven useful in estimating pathogenicity between F.v. isolates.

RESULTS:

1. As predicted there is some variability in pathogenicity between F.v. isolates.
2. However, the majority of isolates tested appear to show at least moderate pathogenicity. About 20% are highly pathogenic. And, about 5-10% of the isolates are non-pathogenic.

This will remain ongoing to increase the number of isolates available for pathogenicity testing.

View uploaded report

View uploaded report 2

View uploaded report 3

1. Resistance to SDS is a combination of resistance to the toxin and root rot phases of the fungal infection. These two parts are related, but both are required to cause disease. Varieties that stay green and healthy when exposed to the fungus toxin have a good chance of resisting SDS.
2. Planting late reduces SDS, but earlier planting gives a yield advantage that overcomes SDS yield impacts. ILeVO can improve yield when plants are under disease pressure, but only slightly. Use a SDS and SCN resistant variety even if seed treatments such as ILeVO are to be used.
3. The SDS fungus can live in the soil as spores and mycelium, but does not appear to survive in corn residue.
4. Isolates of the SDS fungus from different fields vary in their ability to cause disease. Some isolates are non-pathogenic, some are highly pathogenic, but most exhibit moderate pathogenicity.