Leaf Scorch is common when hot dry winds remove moisture from plants faster than roots can keep up with the loss. Leaves usually appear brown around leaf edges and/or between veins. Make sure plants are receiving adequate water. Be aware that the same leaf symptoms appear when roots are not functioning properly from compacted soils, overwatering, drought and mechanical injury.
Marssonina leaf spot begins appearing on aspens in the middle to late July. Look for symptoms that include brown spots with yellow halos. Small spots may merge together, creating large blotches. Leaves may drop prematurely. Rake up fallen leaves in the autumn and dispose or compost.
See CSU Extension fact sheet 2.920, Aspen and poplar leaf spots, at http://extension.colostate.edu/topic-areas/yard-garden/aspen-and-poplar-leaf-spots-2-920/ for more information or request a copy from a county Extension office.