New Phytophthora on Mums Has Growers on the Lookout
October 13, 2022 
Photo: Francesca Peduto Hand (The Ohio State University)
If you ask any mum grower, they will likely tell you that chrysanthemums already have plenty of diseases and don’t need another one. However, ornamental researchers have discovered a new threat to mums for 5 years, following an Ohio report from a grower in 2017 describing a Phytophthora disease in several cultivars in a large landscape planting.
The pathogen was not one of the Phytophthoras that the industry is familiar with, such as Phytophthora nicotianae or P. cactorum. Instead, researchers in Ohio worked from root and stem rot symptoms to isolate a Phytophthora species not previously known to occur in the U.S., named Phytophthora chrysanthemi.
This pathogen appears to be favored by high temperature conditions (86°F and above) based on its laboratory behavior and the summer timing of the first documented outbreak in the U.S. Infection is associated with stunting, purpling, and death of foliage, on one major branch or the whole plant. Root rot is also apparent.
Immunostrip tests and likely other serological tests for Phytophthora can be used on discolored stem tissue to get an initial indication that this pathogen may be responsible for symptoms in production or the landscape. In order to confirm the pathogen, diagnostic labs will find it helpful to culture at warmer conditions than standard practice.
To learn more, and to check out photos highlighting visual symptoms of the disease, check out this recent e-GRO alert from Margery Daughtrey (Cornell University), Francesca Peduto Hand (The Ohio State University), and Nora Catlin (Cornell University).
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