Overview: Pod and stem blight, first observed in the United States in 1920, is now endemic throughout most soybean growing areas of the world.
Symptoms: Stems, petioles, pods, seed and less frequently leaf blades may be infected. The fungus that causes pod and stem blight initially infects the host early in the growing season without causing symptoms. Under field conditions, no definite leaf or stem lesions are produced. Pycnidia may be found in dead tissues. Dead stems may be covered with speck-sized pycnidia, usually arranged linearly or the pycnidia may be limited to small patches, generally near the nodes sometimes being confused with stem canker disease.
Causal Organisms: Colonies of Diaporthe phaseolorum var. sojae on acidified (pH 5.0) potato-dextrose agar are floccose and ropy, turning tan to brown as the culture ages. The undersides of colonies are tan to dark brown with black, pulvinate stromata. Perithecia are produced on old agar cultures incubated in light or on overseasoned soybean stems.
Disease Cycle: The pod and stem blight pathogen overseasons as dormant mycelium in soybean or other host debris and in infected seeds. Pycnidia are produced on overseasoned debris and on petioles of the current year's abscised leaves. Perithecia are produced in early summer on overseasoned stem debris. The fungus colonizes plant tissues within 2 cm of the point of infection until the plant begins to senesce.
Management: The following PIs (maturity group) have been shown to be resistant to the pod and stem blight fungus: 80837 (IV), 204331 (VIII), 205089 (IV), 205907 (VIII), 205908 (VIII), 205912 (VIII), 219635 (?), 259539 (VIII), 279088 (VIII), 341249 (IX), 360835 (II), 360841 (III), 385942 (IV), 417479 (IV), and 423903 (IV). Two complementary dominant genes control resistance in PI 417479.