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Detection of Asymptomatic Fungal Infections of Soybean Seeds by Ultrasound Analysis

Different levels of asymptomatic, seed-borne infection by storage fungi (Aspergillus and Penicillium spp.) or Phomopsis seed decay (PSD) (Phomopsis longicolla, Diaporthe phaseolorum var. sojae, and D. phaseolorum var. caulivora) were induced in sub-lots of separate soybean seed lots by incubation of seeds or pods, respectively, for different times at 25°C and at a relative humidity >95%. Seeds were then air-dried to a constant moisture content in the laboratory atmosphere, and each sub-lot was tested for incidence of infection, germination, and moisture content. Individual seeds in each sub-lot also were dropped 10 cm onto a transducer in an ultrasound analyzer. The average peak value of the ultrasound signals for each sub-lot, which indicates the weight of seeds, decreased linearly as the incidence of seed infection by storage fungi (r² = 0.85) or PSD (r² =0.82) increased. The slope and width of the signal, which indicates seed softness, increased as seed infection increased for both groups of fungi, although coefficients of determination were lower (r² ranged from 0.42 to 0.59). Germination values, which decreased as seed infection for both pathogens increased, showed similar but inverse relationships to ultrasound parameters. Peak values of ultrasound signals decreased, and slope and width increased, as seed moisture content increased for sub-lots of soybeans at three levels of infection by Phomopsis seed decay. The potential for ultrasound technology to identify soybean seeds with asymptomatic infections of seed-borne pathogens was thus established.