Cause The fungus Phyllosticta sphaeropsoidea (sexual Guignardia aesculi) overwinters in fallen leaves. Ascospores are produced in the spring and infect new leaves from budbreak through shoot elongation. The disease is favored by wet weather in spring. Although not formally reported from the Pacific Northwest, it has been detected on a few plant samples at the OSU Plant Clinic. It is an economic problem for nursery tree production. Established landscape trees may be infected each year without serious injury or decline.
Most species and subspecies are susceptible; however, the following were resistant: A. arguta, A. glabra var. monticola and sargentii, A. parviflora and A. parviflora var. serotina.
Symptoms The first symptoms are discolored, water-soaked spots on the leaf. Later, the irregularly shaped spots turn reddish brown with very bright-yellow margins. Spots may remain small or may enlarge to cover the entire leaf. When the whole leaf is infected, it may cup up and become dry and brittle and fall from the tree or seedling. Small black fruiting bodies (pycnidia) of the asexual form can be seen in the center of the spots. Petioles also may be attacked. Complete defoliation is uncommon for large established trees but common in the nursery if not controlled.
- Rake and destroy fallen leaves during the growing season and at the end of the year.
- Space plants for good air circulation.
- Avoid overhead irrigation or apply such that plants are not wet for extended periods of time.
Chemical control Apply fungicides during wet spring weather from bud break through shoot elongation.
Products available in home packaging:
- Bonide Fung-onil Multi-purpose Fungicide at 2.25 teaspoons/gal water. H
Products available for commercial use: A dormant spray of lime sulfur, suggested by some authors in the past, is of unknown value. Other fungicides should be applied during wet spring weather from bud break through shoot elongation.
- Daconil Weather Stik at 1.4 pints/100 gal water. Group M5 fungicide. 12-hr reentry.
- Mancozeb-based products. Group M3 fungicides. 24-hr reentry.
- Fore 80 WP at 1.5 lb/100 gal water plus a spreader-sticker.
- Protect DF at 1 to 2 lb/100 gal water plus 2 to 4 oz spreader-sticker.
- Spectro 90 WDG at 1 to 2 lb/100 gal water. Group 1 + M5 fungicide. 12-hr reentry.
Reference Neely, D. 1971. Additional Aesculus species and subspecies susceptible to leaf blotch. Plant Disease Reporter 55:37-38.