Update:
(Obj 1) Screen adapted Kansas germplasm and Kansas Soybean Variety Trial (KSVT) entries for SDS resistance using three high-throughput methods.
Germplasm is being screened using three methods: (i) culture extract/toxin assay method, (ii) rolled-towel method, and (iii) layer-cake method. Currently, KSVT entry seed has been obtained. Also, a new Fusarium virguiliforme (Mont-1) isolates was revived from the culture collection in Dr. John Leslie's (KSU) laboratory. Seedlings were grown in vermiculite and F. virguiliforme cultures were grown to test KSVT entries using the culture extract method for high throughput phenotyping (Table 1; Figure 1; see attachment). For the toxin assay, seedlings were bisected at the crown and incubated in F. virguiliforme culture extract and foliar symptoms were evaluated. In this test, germplasm was rated for both senescence and toxin reaction. Seedlings that wilted and showed foliar chlorosis/necrosis (3-6 on the SDS severity scale used) after stems were cut and placed in distilled water (control), were rated as senescent. However, those seedlings that remained relatively green after the same treatment (1-2 on the severity scale) were rated as non-senescent types. Likewise, germplasm was rated for tolerance or susceptibility to F. virguiliforme toxin. As a result of the toxin assay, 26 KSVT entries were rated as “non-senescent” and “toxin resistant” (Table 1). We are also using non-fungicide-treated seed to use for the rolled-towel (Figure 2) and layer cake (Figure 3) screens.
(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 Silver Lake ("Paramore") unit of the Kansas Research and Extension fields near Topeka. 2017 was the third year of this study and has contributed to a complete data set for analysis, publication, and extension & communication purposes. The goal is to confirm the effectiveness claimed by Bayer Crop Science for the ILeVO seed treatment product on SDS disease and to see whether seed treatment and planting date can have a positive interaction for SDS control. See Figure 4 for results. This season, as before, later planting resulted in reduced SDS severity. And, ILeVO did not have a significant effect on yield.
(Obj 3) Determine modes of residue and soilborne survival of the SDS pathogen in Kansas production fields.
This objective will be addressed in two ways: (i) corn residue and (ii) soil. Corn residue has been sampled to determine if this substrate is a source of F. virguiliforme survival in the Kansas River Valley. Working with the Co-PI, Eric Adee, we identified three sites that are in corn-soybean rotation and have a history of SDS disease. This year's soybean plots that were in corn last year, were sampled for corn residue. Corn residue samples were dried and ground to a powder. Using a culture-based method, colonies resembling F. virguiliforme were isolated from corn residue, which initially suggested a positive isolation of the pathogen (Figure 5). However, these isolates are likely F. solani and not F. virguiliforme based on Fusarium-specific PCR. Also, whole corn debris DNA, when amplified using F.v.-specific primers, did not produce a PCR product. This is a positive result since it suggest that F.v. is not surviving in corn residue. To check if F. virguiliforme survives in soil, fungal structures were incorporated into the soil at multiple sites and then checked for recovery after an overwintering period. This part of the project remains in progress. This experiment will be repeated during the 2018-2019 project year.
(Obj 4) Determine pathogenic variability of F. virguiliforme isolates from multiple Kansas fields.
As plants began to show symptoms of SDS, we worked with cooperators and extension personnel to identify and collect symptomatic plants for the isolation of F. virguiliforme from different parts of the state. Isolation of F. virguiliforme remains in progress from these samples. However, see Table 2 for a list of isolates obtained during this study so far. See Figure 6 for an example of pathogenic and less pathogenic F. virguiliforme isolates collected from Topeka and Belleville, KS, respectively.
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