Leaves and needles will turn brown due to environmental causes, insects and fungi.


Here are some resources that may assist in explaining why your leaves are browning: 
Aspen & Poplar Leaf Spots

Fire Blight is a common and fast-spreading bacterial disease that affects trees such as apple, mountain ash, pear, hawthorn etc.
Fire Blight Info
Fire Blight of Apple & Pear
Fire Blight info                                                                       
Integrated Management of Fire Blight

Dutch Elm Disease



For coniferous trees, here are some articles for why your needles may be browning:  

What is Going on With my Blue Spruce?
Lophodermium Needlecast (spruce and pine)    
Pine Needle Scale
Needle Cast of Pine & Spruce
Brown Spot Needle Blight of Pine

Managing Dothistroma and Brown Needle Blight on Pines
Dothistroma Needle Blight
Frost and Winter Damage
Spring Conifer Troubles
Browning of Evergreens
Spruce Problem Diagnosis sheets
Normal Needle Drop: Even Healthy Evergreens are NOT Evergreen

Browning of Evergreens
A Troubleshooting Guide for Evergreens
Evergreen Autumn Needle Shed

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