"Did we need another phytophthora? Did mums need another disease?"

Phytophthora chrysanthemi causes stunting, purpling, and death of foliage on one major branch or the whole plant; root rot is apparent as well.

If you asked any mum grower, they would tell you that chrysanthemums already had plenty of diseases and didn’t need another one. But it seems that the flower industry didn’t get to make the call on this one. Growers have been aware of a new threat to mums for five years since an Ohio report described a Phytophthora disease from several cultivars in a large landscape planting (Lin et al., 2017). The pathogen was not one of the Phytophthoras that we are more familiar with, e.g., 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 United States, named Phytophthora chrysanthemi.

This pathogen appears to be favored by high-temperature conditions (30°C/86°F and above) based on its laboratory behavior and the summer timing of the first documented outbreak in the United States. 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 (Agdia, Inc., Elkhart, IL) 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 the culture at 28-30°C,
which is warmer than standard practice.

Read the complete article at www.e-gro.org.


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