2013 Annual Report
1a.Objectives (from AD-416):
Screening soybean varieties for resistance to Phomopsis seed decay (PSD).
1b.Approach (from AD-416):
Field screening and laboratory seed quality assays will be conducted to determine:.
1)the disease resistance of commercial available soybean varieties to Phomopsis seed decay (PSD); and.
2)the virulence/aggressiveness of PSD isolates collected in Mississippi.
This project started on April 1, 2012, and finished on March 31, 2013. During this reporting period, 16 selected soybean lines including six maturity group (MG) and 10 MG V varieties were planted on April 25, 2012, to evaluate them for resistance to Phomopsis seed decay (PSD). Those lines were selected based on data from seed assays for high germination rate and good visual scores/appearance in 2011. Two experiments were set up in a split-plot design for each maturity group IV and V, where the inoculation treatments (inoculated and non-inoculated) were the main units in a randomized complete design with four replications. Soybean varieties in each maturity group were the sub-unit, which was a repeated measure of two harvest times at R8 and R8+2weeks stages (normal vs. delayed). Seeds were planted in 10-foot (ft) long row plots with 8 seeds/ft of row on 40-inch row spacing. A total of 256 plots were planted. The inoculated plots were sprayed with Phomopsis spores at the R5 to R6 growth stage. A home-made sprinkle watering system was built and used routinely to promote disease development and increase seed infection by Phomopsis during the hot and dry season. At the end of October, 2012, we finished hand-harvesting of all maturity group IV and V plants at R8 (normal time) and R8 stage plus 2 weeks (delayed harvest) from inoculated and non-inoculated plots. Seed plating assays were performed to determine the PSD incidence. Germination test and visual seed quality scoring of seeds harvested from each field plots also were conducted. Differences of the Phomopsis seed infection were observed among soybean varieties with some varieties having 5% seed infection while others had levels as high as 76%. In general, delayed harvested seed had higher percentage of Phomopsis seed infection than the seed harvested on time. The differences among soybean varieties also were reflected in visual seed quality and seed germination. The range for visual seed quality was from 1.5 to 3.8, and the seed germination rate was from 30.0% to 97.8%.
Based on our seed assay data, Morsoy R2 491 had the lowest percentage of Phomopsis seed infection and highest germination rate in both normal and delayed harvest trials among the MG IV varieties evaluated. Although Morsoy R2S 480 also had the lowest Phomopsis seed infection (the same as Morsoy R2 491) in the normal harvest trial, it had the highest Phomopsis seed infection in the delayed harvest trial. This result indicates that a variety that is resistant to PSD at normal harvest time may be susceptible to PSD at delayed harvest time if the environmental condition is favorable to PSD development. Therefore, testing of varieties at delayed harvest time or under the conditions which favor PSD disease development is important for the identification of PSD-resistant varieties.
In the testing of MG V varieties, Progeny 5650 and plant introduction (PI) 42324B had the lowest percentage of Phomopsis seed infection in both normal and delayed harvest trials. This was true for both inoculated and non-inoculated treatments. Asgrow 5831 also had the lowest percentage of Phomopsis seed infection (the same as Progeny 5650 and PI 42324B) in the non-inoculated trial, but had high percentage of Phomopsis seed infection in the inoculated and normal harvest trial. Varieties may escape pathogen attack under natural screening. Inoculated treatment provided even disease pressure and therefore provided a useful approach for identification of durable PSD-resistant varieties.