Reaction of selected soybean germplasm to fungal seed disease and senescence regulation
Abstract
Environmental and genetic factors influencing fungal pod and seed infection were studied during a four year period on sets of isogenic soybean lines, derived from six different genetic backgrounds, which differed for genes controlling the time of flowering/maturity and growth habit. Selected cultivars/lines with different seed disease reactions were compared to isolines with similar maturity. Under natural field inoculum conditions, pod infection levels of the two dominant pathogens, Diaporthe/Phomopsis spp. and Cercospora kikuchii, were positively correlated with subsequent seed infection, for the isolines but not cvs. with Ransom parentage or PI80837. Conidial inoculation with C. kikuchii resulted in uniformly high ($>$75%) levels of pod infection for all genotypes. Among isolines, C. kikuchii seed infection was negatively correlated with maturity date, and early-maturing Clark, Harosoy, Chippewa, Williams, and Wayne isolines, and cv. Amsoy 71 were highly susceptible ($>$60%). The cvs. with Ransom parentage and PI80837 were resistant, and the Will isolines were moderately resistant to seedborne fungi. Determinate, semideterminate, and indeterminate isolines with Clark or Harosoy genetic backgrounds were equally susceptible to seedborne fungi. The potential confounding effect of late-maturity in fungal seed disease evaluations, with and without inoculation, is indicated by the negative correlations observed between maturity date and C. kikuchii and Diaporthe/Phomopsis spp. seed infection among the susceptible isolines. All genotypes were susceptible to the foliar blight phase of Cercospora disease development, and premature senescence (4 days) and yield reductions (31%) occurred. Cercospora kikuchii was more pathogenic on seed of early-maturity entries (Groups I-II), compared to late maturity entries (Groups III-IV), as indicated by germination of infected seed. Growth regulators and manual defoliation/depodding treatments altered maturation rates and fungal seed disease reactions. CF 125 (chlorflurenol) and manual depodding (50%) delayed maturity and increased Diaporthe/Phomopsis spp. and C. kikuchii seed infection in susceptible isolines and cv. Amsoy 71. Ethrel (ethephon) and manual defoliation (100%) hastened maturity and reduced Diaporthe/Phomopsis spp. and C. kikuchii seed infection in nearly all but the most resistant genotypes. Comparison of a resistant (cv. Sprite) and susceptible (cv. Amsoy 71) soybean genotype, for pod and seed maturation and moisture loss rates, established the relationship between rapid pod and seed moisture loss after physiological maturity, and resistance to fungal seed disease.
Degree
Ph.D.
Advisors
Abney, Purdue University.
Subject Area
Plant pathology
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