As most of the newer varieties grown in the UK are micropropagated and bought-in as young plants, there was concern that the disease was being transferred onto nurseries via this route too. Diseases symptoms are often reported two-to-five months after potting and it was suggested that problems arose from latent infection.
In AHDB Horticulture project HNS 191, growers were surveyed so as to identify the source and extent of the disease, and current knowledge about the pathogen was reviewed. A molecular test was developed to look for the presence of the pathogen in plant material at various stages of production and from a range of sites. It was also used to find out whether the disease had a latent period and whether it became systemic in the infected plant.
The conditions conducive to disease development were investigated to establish the importance of leaf wetness and high humidities for disease development and spread. DNA tests confirmed the pathogen causing heuchera rust was Puccinia heucherae. No rust was detected in micropropagated or seed-raised plants, so young plants propagated by these methods are unlikely to be the source of infection.
The molecular test detected rust in four out of 36 overwintered heuchera plants that were showing no symptoms that had been collected from one of two nurseries. One developed a rust pustule on a leaf five weeks after quarantine in a glasshouse. Results from other tests in this project suggest systemic infection is unlikely.
Infection was found to be favoured by periods of leaf wetness of about five hours at temperatures of 17-24 degsC.
For details on all AHDB Horticulture activity, see horticulture.ahdb.org.uk.