Pythium species are “water molds” that produce swimming spores called zoospores. There are many species of Pythium. Some species are saprophytes or weak pathogens that mostly decay dead root tissue. Often, Pythium nibbles on feeder roots of plants. Under the right environmental conditions, however, some Pythium species become destructive pathogens that rot and kill the roots of plants, resulting in stunted growth or death. The pathogen is favored by wet conditions, such as when media does not drain properly or when weather events prevent the soil from drying out completely. Pythium can also be more problematic when the host plants are stressed.
Pythium can be a problem on many annual and perennial hosts and is extremely destructive to greenhouse production of poinsettias and geraniums. Pythium can be introduced into greenhouses on infected plugs or infected plant material. It can also be a year-round “resident” hiding on dirty plant containers, equipment, benches, or even in irrigation water, waiting for the conditions that favor the pathogen and disease development.
|Leaf curling associated with Pythium root rot on poinsettia|
|Stunted growth symptom of Pythium root rot on poinsettia|
Managing Pythium Problems
Sanitation is very important in managing root rot because Pythium produces survival structures, called chlamydospores, that are able to survive for long periods of time on infected plant material or dirty benches and pots. There are some fungicides that are effective against Pythium and other water molds (oomycetes). Some of the most popular oomycete fungicides contain the active ingredient mefenoxam. In recent years, greenhouse growers have seen an increase in oomycete resistance to mefenoxam, resulting in poor disease control. Some good practices that limit the occurrence of fungicide resistance include:
- Time spray applications when the pathogen is weakest or most vulnerable to application
- Use a fungicide registered for your pathogen that is proven to be effective in controlled settings
- Alternate active ingredients in your spray programs
- Use only the labeled rates listed on the container and do not over-apply the fungicide
- Incorporate new fungicides into your spray program as they become available
Unfortunately, even when using good application practices, sometimes the pathogens still develop resistance.
The purpose of my research is to determine which Pythium species are present in North Carolina greenhouses and to evaluate their abilities to be effective pathogens in the greenhouse environment. This will involve morphological and molecular species characterization, pathogenicity assays, growth chamber studies, and checking for mefenoxam sensitivity.
At the plant disease clinic, we routinely run root assays to determine the presence of Pythium. Currently, we are not able to determine which species of Pythium is present and whether it is pathogenic or just saprophytic. My research will allow the diagnosticians at the disease clinic the opportunity to expand their Pythium diagnosis to be more beneficial to the grower. We will be able to tell the grower whether the Pythium we isolated from their plants is resistant to mefenoxam and if so, offer alternative control options.
Fungicides are expensive. Routine mefenoxam screening will help growers develop effective fungicide programs so they are not spraying money down the drain.