Phytophthora is a destructive pathogen in the genus of oomycetes or water molds that can cause major damage to crops, plants, shrubs, and trees. It was previously believed to be a fungus but unlike the cell wall of fungi, which are primarily made of glucans, chitin, and glycoproteins, Phytophthora cell walls are made of cellulose. There are approximately 170 species of Phytophthora known and up to 500 species are estimated to yet be discovered. Phytophthora is a soil-borne pathogen that can survive in slightly moist soil for long periods of time. A good rain shower can trigger reproduction and allow the Phytophthora to attach to the roots of plants and trees. Phytophthora ramorum is a species best known for sudden oak death (SOD) in Texas. Phytophthora causes the roots to rot and dries out the leaves of trees almost mimicking Oak wilt. A few visual observations can help differentiate between the two. This is important when treating sick trees so that the proper chemical is used. Oak wilt typically loses leaf from the top of the crown and then progresses down and outward along the canopy. Phytophthora usually shows defoliation from the bottom of the canopy moving upward as the roots become compromised. The leaves that are affected by both pathogens are very similar with just a few differences to note. Oak wilt leaves can have veinal necrosis which will show through on both sides of a leaf. Phytophthora leaves can have veinal necrosis but it will not show through on the back of the leaf. Leaves can show margin burn or tip burn from both pathogens but the Phytophthora leaf will look “damp” and not “crispy” like an Oak wilt leaf. Phytophthora is not host-specific like Oak wilt and can be treated with the proper fungicide. If you have a tree that shows symptoms like this, give Rockoff Tree Solutions a call.
Example of Phytophthora leaves. 
Example of Oak wilt leaves.