Red Leaf Blotch

Alternate Name(s): Pyrenochaeta leaf spot, Dactuliophora leaf spot and Pyrenochaeta leaf blotch

Overview: The fungus that causes red leaf blotch was first reported in Africa in 1957 as Pyrenochaeta glycines. It is now known as Phoma glycinicola. The disease currently affects countries in central and southern Africa, with losses of 10-50% reported in Zambia and Zimbabwe. Recently the disease was classified as Phoma glycinicola. The pathogen is not known to be seed or wind disseminated.

Symptoms: Initial symptoms include lesions that appear first on unifoliate leaves associated with primary leaf vein. At this point the disease is easily confused with other diseases or cultural conditions affecting soybeans. As the disease progresses, more characteristic lesions develop on trifoliate leaves, appearing as dark red spots on upper leaf surfaces and with similar spots with reddish brown and dark borders on the lower leaf surfaces.

Causal Organisms: Phoma glycinicola; previously named Dactuliochaeta glycines (1988), Dactuliophora glycines (1964), Pyrenochaeta glycines (1957).

Disease Cycle: The disease cycle has not been fully characterized on either soybean or N. wightii and the initial source of inoculum is not known. Infection most likely occurs when rain splashes soilborne sclerotia onto leaf surfaces, where germination and infection occur. Heavily diseased leaves drop prematurely, and eventually all the foliage from a diseased plant will drop, releasing the sclerotia back into the soil. A similar process may happen in the soil itself. Sclerotia then overseason in the soil, providing initial inoculum for the next season. Wet and humid conditions promote disease development.

Management: Resistant germplasm (if available), fungicides and cultural practices.