In California in 2017 and 2018, parsley fields in coastal counties showed signs of a root disease. Affected plants were stunted. The root systems were reduced and few small feeder roots remained, since most of the small roots had rotted away. The main taproots were dark and discolored both externally and internally. Leaves turned yellow and in advanced stages of the disease the foliage became brown and collapsed.
Our research investigation discovered that affected parsley plants were infected with Pythium mastophorum. This pathogen is soilborne and persists in fields by producing a thick-walled survival structure called an oospore. Like all water molds in this group of organisms, Pythium mastophorum will be spread in contaminated dirt moved by equipment and by movement of surface and sub-surface water. Wet soil conditions will favor growth of the pathogen and will enhance infection and subsequent disease of parsley. Pythium mastophorum appears to have a limited host range; along with parsley, this pathogen has been found to infect only celery in California. At this time we do not expect Pythium mastophorum to infect other crops planted in rotation with parsley, including the following: lettuce, endive, radicchio, spinach, beet, Swiss chard, broccoli, cauliflower, arugula, rappini, pepper, tomato. However, because Pythium mastophorum causes disease on celery and parsley, we would like to hear from growers if new root rot problems are found on cilantro and other crops in the Apiaceae.
Pythium root rot of parsley is a new disease in California and also is apparently new for the USA. Because affected parsley plants were completely unharvestable, this disease could be a significant concern for growers if inoculum spreads throughout other parsley growing regions in the state.
Reference: Plant Disease 102: 1671.
Pythium root rot causes parsley root systems to be severely reduced and decayed (healthy plant on left).
Inoculated parsley plants (on right) show typical symptoms of yellow leaves and eventual collapse of the foliage.
The Pythium mastophorum pathogen persists in soil by producing a resilient survival structure called an oospore.