We often see Pythium problems on mums in Delaware. The following is an article on this disease that is a problem this year in many areas of the Northeast due to the wet weather.
The 2009 growing season for fall mums has presented real challenges for the growers. The cool and wet weather of June and early July were less than ideal conditions for growing fall mums. As the summer progressed many growers pushed the mums with fertilizer trying to keep the mums in a vegetative state. Now in late August the varied weather has brought on diseases common to fall mums. A Pythium species, most likely Pythium aphanidermatum, was observed in roots and stems of the specimens from several growers. Symptoms of Pythium attack vary with temperature, moisture, and the species of Pythium involved. The first symptom is often a wet, black basal rot which extends several inches from soil surface. Affected plants wilt and collapse rapidly. Often the fungus invades leaves touching the soil and progresses into the stem at the node, causing a black lesion. In such cases, plants may not exhibit root and basal stem rot. Root rots are also common in Delaware.
Pythium is favored by high fertility and high moisture; avoid overwatering and overfertilizing. Pythium is a natural inhabitant of the soil and can survive there indefinitely as well as in dirt and debris in the greenhouse. Use soilless growing media media. Keep hose ends off the floor and avoid contaminating growth medium with soiled hands, tools, or flats. Fungicides for controlling Pythium include Aliette, Alude, Banol, Banrot 40 WP, Subdue Maxx, Stature DM (drench) Terrazole 35 WP, and Truban 30 WP. Many greenhouse isolates of Pythium are resistant to Subdue. Recommend a drench with a systemic chemical, followed by a sprench to target lesions above soil line. Avoid consecutive applications of fungicides within the same chemical class: rotate among different active ingredients' mode of action for best results.
Stem rot on mum caused by Pythium. We also see root rots caused by Pythium in Delaware. Photo by P. Lopes, University of Massachusetts.
Information from M.Bess Dicklow, Extension Plant Diagnostic Clinic and Paul Lopes, Floriculture and Greenhouse Crops Program in the New England Greenhouse Update http://www.negreenhouseupdate.info/index.php/august/508-chrysanthemum-garden-pythium-stalk-rot