Bacterial Disease

• Several bacteria attack the leaves and stems of Philodendron plants, causing yellow discoloration and decay. The bacterial subspecies Xanthomonas campestris pv. dieffenbachiae infects the tips of leaves, causing progressive yellow discoloration that expands along the leaf's surface towards the stem end. Blight disease, caused by Erwinia chrysanthemi, also attacks the leaves of Philodendrons and is often accompanied by an unpleasant odor. The fiddleleaf philodendron, or Philodendron panduraeforme, is vulnerable to another type of leaf spot disease that is caused by the Pseudomonas cichorii bacteria.
  
  

Dasheen's Mosaic Virus

• The dasheen mosaic potyvirus, or DsMV, is responsible for a set of serious symptoms on Philodendrons and related plant genera. This disease affects the leaves, stems and buds of its host. Infected foliage displays large yellow patches that can completely cover the surface of individual leaves. According to the University of Hawaii, this mosaic virus is to blame for economically significant damage to commercial Philodendron crops. Various parasitic insects, including aphids, or Aphis spp., transmit this virus from diseased plants to healthy ones as they migrate.
  
  

Fungal Disease

• Several fungal diseases turn the leaves of Philodendron plants yellow by harming the leaves directly or by damaging leaf-bearing stems and twigs. Pythium rot, caused by fungi in the Pythium genus, decays the roots and stems of its host. According to the University of Florida, an early symptom of this disease on Philodendron plants is yellow discoloration of foliage, which eventually turns to a darker shade of brown as the condition worsens. Other fungal diseases of Philodendrons include Dactylaria leaf spot, caused by Dactylaria humicola, and Phytophthora leaf spot, caused by Phytophthora parasitica.
  
  

Environmental Damage

• Environmental damage is more difficult to detect and diagnose than fungal infestations or bacterial and viral infections. Mysterious wilting and discolorations may occur suddenly as because of adverse conditions. An excessive supply of water, due to overwatering or flood, causes Philodendron leaves to yellow. Sun exposure is another common cause of foliage discoloration --- philodendrons cannot tolerate full sunlight so plant them in partial shade. A deficiency of a vital nutrient or mineral, like Magnesium, may also be to blame for yellowing leaves.