Irrigation and Time of Harvest Effects on Evaluation of Selected Soybean Accessions Against Phomopsis longicolla

Abstract

Phomopsis seed decay (PSD) of soybean (Glycine max L. Merr) is primarily caused by Phomopsis longicolla. Currently, percent seed infection is used as a method for evaluating resistance to PSD. However, researchers need a common system for classifying soybean genotypes for their reaction to P.longicolla that is consistent across environments. The purpose of this research was: (i) to evaluate the reaction of a select set of soybean accessions to P. longicolla, and (ii) determine the effects of irrigation and two harvest regimes (normal and delayed) on seed infection. We propose a classification system for comparing reactions to P. longicolla among accessions based on a Phomopsis seed infection index (PSII), derived by dividing the percent seed infection of each accession by percent seed infection values of susceptible standards selected within early and late maturity groups. Percent seed infection by P. longicolla infection was greater when accessions were irrigated during the growing season and harvested at normal maturity. Some lines such as MO/PSD‐0259 were rated as resistant to P. longicolla (7% seed infection) when not irrigated and harvested at maturity, but were rated as susceptible (39% seed infection) with irrigation and delayed harvest. The resistance ratings to P. longicolla using percent seed infection and PSII varied among accessions. SS93–6012, for example, had similar ratings for both (9% seed infection and 16% PSII). PI 416942 exhibited some resistance to P. longicolla based on percent seed infection (8%), but no resistance based on PSII (32%). Accessions PI 594478, Delmar, SS93–6012, SS93–6181, PI 594603A, and PI 594712 were classified by PSII as moderately resistant under irrigation and with delayed harvest. These results indicate that simple comparisons of percent seed infection among accessions without irrigation, without delayed harvest, and without a standard control of a similar maturity group might result in false conclusions about resistance to P. longicolla.

Soybean Seed Composition Constituents as Affected by Drought andPhomopsisin Phomopsis Susceptible and Resistant Genotypes

“…The irrigated and non-irrigated plots were separated by a buffer of 15 rows (9.9 m) to prevent irrigation drift. The detailed description of the irrigation, design, and equipment used in this experiment were previously reported elsewhere (Mengistu et al 2010). Within each irrigation environment, genotypes were grouped by MG.…”

“…Soybean genotypes of MG III and MG V were planted in single-row plots 2.7 m long, 0.66 m wide, and at a seeding rate of 75 seed per row. Susceptible (S), moderately resistant (MR), and resistant (R) genotypes were used and classified as indicated by Mengistu et al (2010). The MG III genotypes were Cumberland and Fremont (susceptible), PI 594401A and Williams 82 (moderately resistant), and PI 594618B and PI 594778 (resistant).…”

“…Innoculation and disease ratings were described in Mengistu et al (2010). A conidial suspension (106 conidia mL-1) was sprayed onto the plants.…”

“…However, declines in seed quality under the early soybean production system conditions due to phomopsis seed decay, which is primarily caused by Phomopsis longicolla Hobbs (Mengistu et al 2010;Wrather et al 1996) and heat (Smith et al 2008), have been a major concern. Smith et al (2008) reported that early maturing soybean cultivars mature in the early soybean production system between mid-August and mid-September under high temperatures and high relative humidity.…”

The Diaporthe sojae species complex: Phylogenetic re-assessment of pathogens associated with soybean, cucurbits and other field crops

“…For example, until recently, both D. phaseolorum and D. phaseolorum var. sojae has been used for the fungus causing pod and stem blight of soybean (Kmetz et al 1978;Wrather et al 1997;Zhang et al 1997;Mengistu et al 2010;Sun et al 2013). Diaporthe sojae (h D. phaseolorum var.…”