Lilac Pseudocercospora Leaf Spot

Need to know:

• Disease occurs during high humidity and moderate temperatures.
• Symptoms don’t develop until seven or more days after infection.
• Preventative measures include removal and destroying of leaf debris and pruning out dead branches.
• Fungicides may be beneficial for recurring severe infections.

Overview of Pseudocercospora leaf spot

Leaf spot on Japanese tree lilac has been identified as a fungus belonging to the genus Pseudocercospora. Other leaf spots caused by members of this group of fungi include leaf spot on olives, guava, and mulberry.

Symptoms of Pseudocercospora leaf spot 

Leaf spots start small, enlarge and are restricted by veins, eventually cause blight (rapid tissue death)

Signs of Pseudocercospora leaf spot 

Long conidia (spores) can be seen under magnification in the spots, under humid conditions.

Disease cycle of Pseudocercospora leaf spot 

Pseudocercospora pathogens seem to enjoy high humidity and moderate temperatures (~76 °F). It is during these periods that high infection often occurs, however, symptoms don’t often develop for 7 or more days after infection

Type of Sample Needed for Diagnosis and Confirmation

The Iowa State University Plant & Insect Diagnostic Clinic can help you to investigate and confirm if you plant has this disease. Please see our website for current forms, fees, and instructions on collecting and packing samples. Contact information for each states diagnostic laboratory for U.S. residents.  If your sample is from outside of Iowa please do not submit it to the Plant & Insect Diagnostic Clinic without contacting us

Management of  Pseudocercospora leaf spot

Pseudocercospora is often able to survive 2 years in the plant debris that is scattered on the ground. Removing and destroying leaf debris and pruning out dead branches may be the best and easiest strategy in reducing disease. Pseudocercospora leaf spots rarely become severe enough to cause the decline of the plant. However, if repeated severe infections occur, preventative spring fungicide applications may help prevent disease. However, no fungicides have been specifically tested for leaf spot on Japanese tree lilac. Pseudocercospora fungal leaf diseases on ornamental plants are controlled with fungicide applications in the spring- starting when the leaves first emerge from the buds and repeated every 14 days (or however the label instructs) through the rainy period of spring.

Fungicide applications may be avoided by following good Integrated Pest Management practices like those listed in this encyclopedia article. Often, the only preventative application is effective to manage plant diseases. If the problem requires a fungicide, state law requires the user to read and follow all labels accordingly. For more information, read Proper fungicide use.