Pythium species are soilborne pathogens that can promote root and basal-stem rots in garden mums. The aggressive species of Pythium aphanidermatum is often associated with garden mums. Disease outbreaks are encouraged by high media temperatures often related to summer production on black groundcover fabrics. Growing mums in areas of poor drainage, with high soluble salts or the presence of other plant stressors, creates optimum conditions for this pathogen to develop and spread quickly.
Know The Symptoms
Symptomatic plants may first exhibit a wet, black-brown basal-stem rot that can extend several inches above the media surface. Examination of the root system will reveal brown, soft roots that, when pulled slightly, will slough off the decayed epidermis and cortex, leaving the inner vascular strand. Once the root system is compromised, affected plants wilt and look dull in color or turn yellow. In the early stages of the disease, plants may wilt during the day but recover somewhat overnight.
Preventive Measures Reduce The Risk Of Root Rot
Manage your crop to minimize the risk of Pythium by using a soilless media that has drains well. High EC (soluble salts) levels in the media can damage the root system, particularly if the media is allowed to dry down. The resulting damage can become an entry point for Pythium. Be conscious of this risk if you feed your crop heavily early in production (300 ppm or more on a constant basis). Monitor the EC and avoid large swings in soil moisture.
Ponding of water in fields provides zoospores, the mobile swimming structures, with ready access to adjacent healthy mum plants. This is an important method of disease spread. Drip- or trickle-irrigation systems greatly reduce the risk of spreading this water-borne pathogen. If overhead watering, look for low-lying areas that collect water. Provide additional drainage, elevate plants out of the standing water, or avoid using these areas.
Utilizing integrated pest management (IPM) practices can reduce the risk of Pythium, but sometimes fungicides are needed. Several microbial fungicides can be used at planting. Examples include Actinovate SP (Streptomyces lydicus WYEC 108), Companion (Bacillus subtilis GB03), and RootShield Plus (Trichoderma harzianum Rifai strain T-22). These products are preventative only. If signs of root disease are evident in the crop at planting, treat with a fungicide that has eradicant properties before applying the microbial fungicide.