Common fungal victims are crabapple trees, which are prone to apple scab. Some of the most susceptible varieties are the ones with bright magenta flowers in spring, like Profusion and Prairifire. They put on a gorgeous display of blooms, leaf out nicely with purplish-green leaves, then by mid-August, the leaves start to yellow and drop until they are nearly bare when September rolls around. Then the cycle repeats itself next year. Generally, white-flowering crabapples like Firebird Crabapple and Adirondak, tend to have more resistance (though not always- fruitless Spring Snow is one of the worst options). If you want something a little showier, Royal Raindrops® Crabapple is a superior alternative with purple leaves and magenta flowers.
Rust is another fungus to watch out for, which can affect anything in the Malus genus as well as Hawthorns (Crataegus). See the picture below, left side. Trees will first show the effects of rust with yellow spots on the leaves in spring. As the disease matures, the spots turn orange and create visible elongated lesions on the leaves, fruit, and stems. Cedar Apple Rust is a very interesting disease. It needs a date at the party, Eastern Red Cedar (Juniperus virgiana or J. scopulorum), to complete the life cycle.
You will notice chocolate-colored “galls” that sit dormant on the evergreen host until conditions are just right in spring. Then the galls burst with what looks like fingers of orange Jell-O (actually called telial horns) which propel billions of fungal spores into the air, looking for a tree to infect. By September, you will have lost most of the leaves and, if you were trying to grow edible apples, your crop will most likely be deformed.
I suggest staying away from native juniper trees if you want to grow apples, crabapples, or hawthorns without doing fungicide applications. We say “trees” because shrub-form junipers don’t harbor the disease – Juniperus communis and J. horizontalis are OK. You may choose the Asian Juniper species, Juniperus chinensis, which doesn’t have the same co-dependent, dysfunctional relationship with those ornamental trees.