Bob Mulrooney, Extension Plant Pathologist; bobmul@udel.edu
Soybeans
Phyllosticta leaf spot was diagnosed on soybean this week. We have observed it in years past during periods of hot, dry weather on full season beans with a good canopy. It is a weak, pathogenic fungus and is probably infecting stressed plants. There should be no cause for concern since the disease is considered of minor importance. It will produce irregular spots often with small brown to black fruiting bodies of the fungus in the dead spots.
Phyllosticta leaf spot
Close up of Phyllosticta leaf spot
Alternaria leafspot on mite damaged soybean leaves.
With the dry weather look for the other drought related diseases in soybeans such as charcoal rot, and Alternaria leafspot. Alternaria is often observed when the leaves have been damaged from spider mites and this weak pathogenic fungus colonizes the leaves following the mite damage.