Taxonomy and biocontrol of Diaporthe sojae and screening for resistance to Phomopsis seed decay caused by an atypical Diaporthe sojae isolate using various assays
Phomopsis longicolla, a seedborne fungal pathogen of soybean, has been known to be the main cause of Phomopsis seed decay (PSD), which is a major cause of poor seed quality in the soybean growing regions of the United States but especially in the Mid-South. This species has been morphologically and pathologically distinguished from Diaporthe phaseolorum var. sojae; however, most of the characteristics thought to distinguish the two species have been reported and found presently to overlap. Because these two fungi cannot be distinguished morphologically, pathogenically, or phylogenetically, it is likely that they are the same species and the name Diaporthe sojae should be assigned to them. Breeding for resistance to PSD is often a time-consuming endeavor as soybeans need to be grown to maturity before plating seed on agar plates can be done to assay for PSD resistance. Thus, a mature seed inoculation assay, an immature stem assay, and a detached leaf assay were conducted to see if soybean genotypes resistant to PSD would prove to be similarly resistant in any of these assays. Results showed that the mature seed inoculation assay and the detached leaf assay cannot be used to screen for PSD resistance while conditions more favorable to colonization by D. sojae would help verify the utility of the immature stem assay. In addition to the investigation of alternative PSD resistance screening assays, the standard screening assay for PSD resistance that is usually conducted in the field was attempted in the greenhouse. The results from the greenhouse assays reinforced the importance of controlling relative humidity and temperature in obtaining informative PSD incidences, incidences that are statistically different between the susceptible and resistant controls. Planting PSD-resistant cultivars is likely the most effective method of controlling PSD, but such cultivars have yet to be developed. Other management strategies exist, such as later planting to avoid hot and humidity weather during the seed development and maturity stages (R5-R8) and fungicide use, but an effective biological control of PSD has yet to be developed. Therefore, Acremonium strictum, a fungus that was found to parasitize an isolate of D. sojae, was tested in vivo, ex vivo, and in vitro for its biocontrol capabilities. The findings from the series of experiments showed that A. strictum acts as a protectant rather than a curative biocontrol and that it does inhibit mycelial growth in vitro, albeit with a delay. Further testing would be required to quantify the protectant effects of A. strictum on PSD incidence in the greenhouse or in the field.
Phomopsis seed decay, Diaporthe sojae, Acremonium strictum
University of Illinois - Urbana-Champaign
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University of Illinois at Urbana-Champaign