This fall we received several inquiries concerning problems with lilacs. Lilacs suffer from vascular pathogens and bacterial diseases, but also a blight caused by a water mold. Water molds belong to the Oomycetes (also known as Stramenopila), microorganisms that despite their similarities with fungi, are closely related to algae.
Phytophthora shoot blight in lilacs is caused by the water mold Phytophthora spp., commonly by the species P. cactorum. This pathogen causes extensive shoot blight and root sprouts at the base can be killed over time. But even though the initial symptoms are on the leaves, the pathogen resides in the roots.
The symptoms can be confused with environmental injury, including drift or herbicide misuse, bacterial blight and in some varieties symptoms may be similar to Verticillium wilt.
Lilacs perform well in clay soils, but slow water drainage leads to excess moisture in the root zone, the perfect condition for the pathogen Phytophthora cactorum to thrive.
What can you do after your plant was diagnosed with Phytophthora shoot blight?
Unfortunately, there are no fungicide treatments that can cure lilacs of this malady, and the pathogen can persist in the soil for many years.
To manage this disease remove infected plants and if replanting in the area, select plants that are not susceptible to P. cactorum. When replacing plants in the same location, some plant that should not become affected by this pathogen and that can thrive in Iowa include chokeberry, witch hazel, ninebark, fragrant sumac, spirea, snowberry/coralberry, and arrowwood viburnum. Avoid overwatering if irrigating the area. Avoid planting dogwood's and forsythia as those are susceptible to the P. cactorum.
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