Glomerella leaf blight seems to be rearing its ugly head. Symptoms include lesions with concentric rings in them that can grow together and give the leaf a blighted appearance (Fig. 1). Leaves may yellow and fall off (Fig. 2) and may appear like Golden Delicious necrotic leaf blotch on a variety of apple cultivars (Fig. 3).
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- Figure 1. Radiating bands within the lesions are a characteristic of Glomerella. Photo by Janna Beckerman
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- Figure 2. Significant leaf drop can affect some varieties. Shown here is ‘Princess’, which is almost defoliated by the most recent outbreak. Note fallen mummies and thick, weedy undergrowth. Photo by Janna Beckerman
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- Figure 3. This pathogen can be mistaken for Golden Delicious necrotic blotch. Photo by Janna Beckerman
Before anyone panics, Glomerella is not a new pathogen, but the sexual state of a fungus that was first identified it its asexual state and was named Colletotrichum. Fungi known to infect apples and belonging to this group (genus) cause bitter rot. Previously, the bitter rot pathogens were divided into two species complexes: C. acutatum and C. gloeosporioides. Recent work in Kentucky found five different species of Colletotrichum associated with bitter rot: C. fioriniae, C. nymphaeae, C. siamense, C. theobromicola, and C. fructicola. The relationship between bitter rot and Glomerella leaf spot, if any, is not all that clear here in Indiana. Or anywhere.
Colletotrichum species are cosmopolitan fungi (by this, I mean they grow everywhere, not that they are found in vodka-based drinks) and infect almost everything (fruit, vegetables, flowers, etc.). It isn’t clear if the fungi that infect leaves also infect fruit on the tree, but we do know that they are all capable of causing post-harvest rot on wounded fruit in storage.
For now, keep your eyes peeled and below is a list of fungicides that are effective in controlling this disease, provided applications are made to protect growing fruit and that fungicides are reapplied after rain events that provide more than 1-2 inches of precipitation. At Meigs, trees that had better pruning and increased air flow had less Glomerella than overgrown trees.
| Trade Name | Active ingredient | Pre-harvest interval -PHI (days) | FRAC Code |
| Roper, Koverall | Mancozeb | 77 (3 lb/A rate only) | M3 |
| Flint 50WG | Trifloxystrobin | 14 | 11 |
| Luna Sensation | Trifloxystrobin + Fluopyram | 14 | 11+7 |
| Ziram DF | Ziram | 14 | M3 |
| Merivon | Pyraclostrobin + Fluxapyroxad | 0 | 11+7 |
| Pristine WG | Pyraclostrobin + Boscalid | 0 | 11+7 |
| Captan 80WDG | Captan | 0 | M4 |