Cause The only fungus reported on sweet gum in the PNW is a leaf spot caused by Cladosporium sp. from Oregon. The OSU Plant Clinic has gotten many leaf spot samples on sweet gum in June which are likely seen earlier in May. Many of these leaf spots are abiotic while others yield a wide variety of fungi and bacteria including Alternaria, Cercospora, Leptothyvella (red leaf spot), Septoria, and even Pseudomonas.
Symptoms Septoria leaf spots are dark brown, angular to irregular, up to 4 mm across and with a diffuse yellow halo. Spots may coalesce to form larger spots. Pycnidia (fruiting structures) develop on the lower leaf surface, are embedded in leaf tissue, and may exude conidia in tendrils through a wide ostiole.
- Avoid overhead irrigation that keeps leaves wet for extended periods of time.
- Remove all infected plant parts during the growing season.
- Remove and destroy fallen leaves at the end of the season.
Chemical control Leaf spots are rarely serious enough to warrant control.
- Heritage at 1 to 8 oz/100 gal water. Group 11 fungicide. 4-hr reentry.
Reference Bartlett, A. B., Halliwell, R. S. and MacSwan, I. C. 1962. Undescribed plant diseases and diseases not previously reported for Oregon during 1961-1963. Plant Disease Reporter, 47:762.
El-Gholl, N. E. 1989. Septoria leaf spot of sweet gum. Florida Dept of Ag and Consumer Service. Pl. Path. circular No 316.