Verticillium Wilt

• Verticillium wilt is a soil-borne fungal disease that attacks Japanese maple trees through small wounds in their roots or directly through the root system. The Verticillium albo-atrum or Verticillium dahliae fungi release chemicals that infect the water-transport system and send spores upwards into the plant. Infected trees often have discolored sapwood, dead or dying branches, and yellow leaves. The symptoms, which usually appear in late summer, are more likely to be worse when temperatures are cool.
  
  

Anthracnose

• Several fungi including Gnomonia, Colletotrichum and Discula cause anthracnose. This disease is also called twig, shoot or leaf blight. Dark spots appear on new leaves or shoots, and the leaves may become distorted or curl and fall from the tree. Serious infections may cause severe defoliation and dieback, but does not usually cause permanent or long-term damage. You can reduce outbreaks by pruning trees during their dormant period to improve air circulation in the crown.
  
  

Powdery Mildew

• Powdery mildew infections are usually worse in dry, hot weather, especially when humidity levels are high. Several plant-specific fungi create powdery grayish-white spots on the upper surface of Japanese maple leaves. Younger leaves are at higher risk of infection than mature foliage. Diseased leaves turn yellow, curl or distort, and fall from the tree. The fungus does not enter the plant tissues, but stays on the surface of the leaf. Apply a protective fungicide every one to two weeks to prevent powdery mildew infections from occurring.
  
  

Root Rot

• Several types of Phytophthora fungi cause root rot infections. The pathogen attacks the root hairs first, then moves into the larger roots and crown. Diseased plants turn yellow and wilt. Trees that grow in excessively moist soil or with poor soil drainage are susceptible to root rot. Prevent infections by planting trees in raised beds or well-drained soil, and apply a preventive soil-based fungicide when weather or climate conditions favor phytophthora outbreaks.
  
  

Leaf Spot

• The Phyllosticta minima fungus causes phyllosticta leaf spot in Japanese maples, but this is not a serious infection and rarely requires control. The disease is characterized by rounded, tan leaf spots with red or purple margins and small, round fruiting bodies. Minimize outbreaks by pruning plants to improve air circulation and water plants from above rather than below to keep their leaves dry, since the fungus germinates on wet leaves. Remove diseased leaf litter from the ground to keep fungal spores from infecting trees the following season.