Thanks for contacting Ask Extension with your gambel oak question. Could you please send some pictures of the browning leaves-close up as well as a bigger picture of the shrub in entirety. Also, if it is just one plant or several in a group that are affected and browning. 

Thanks,

Pam

Master Gardener

I would like to provide a little more information regarding your question on gamble oak.  We cannot offically diaganose the issue without photos or a sample(best option).  

I am going to attach a fact sheet from USU on Gambel Oak in the Landscape.  Review the fact sheet and the section on fungal diseases.  Again, we would need to have a sample to diagnose. 

Based on research from a resource we have in our office, Anthracnose, is a disease named for the foliar symptoms caused by fungus.   " There are two types of foliar symptoms associated with this term anthracnose.  Could have necrotic irreglular blotchy areas associated with veins that expandinto interveinal areas.  Less common, spot anthracnose, are spots or round necrotic circles scattered throughtout the leaf.  Dark or chlorotic margins (lines of demarcation) between healthy and diseased tissue are easily seen. Twig blighting might occurs in oaks."  

Damage is usually minor.  If there is any leaf debris it should be raked up and disposed of.  If fungus is on the twigs, then you may want to pruning.  Disposing of this material will descrease the overwintering fungus.  

Again, the El Paso Co., Extension office in Colorado Springs would need to have a sample to truely diagnose.  This is a followup for infomational purposes, not diagnostic.  

If you have any additional questions please do not hesitate to contact the: 

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Happy Gardening,

Chella D., Colorado Master Gardener, Volunteer