The poor quaking aspen has so many disease problems it's a wonder there are any around to grow in cultivation. With thin bark, the slightest encounter with a lawn mower or weed-eater or even a speck of a hole from an insect can lead to life-threatening fungal diseases. One of the most serious foliar diseases of the quaking aspen is black leaf spot, caused by a Marssonina, a fungus. Symptoms include dark spots on the foliage in spring that eventually grow together to form large dead areas.
Clean the garden bed in fall to remove all fallen leaves and twigs. The fungus overwinters in garden debris.
Spray the quaking aspen tree with a fungicide containing mancozeb chlorothalonil. Apply the first treatment when the buds begin to open in spring and repeat every two weeks, unless the fungicide package states otherwise. Fungicide won't cure the disease, but it will protect new foliage from becoming infected.
Water the quaking aspen tree at the soil to keep water off the foliage. If you must water overhead, do so in the morning so that the leaves have time to dry before sundown.