Cause Several organisms. Pythium sp. has been reported from Washington as causing damping-off. The OSU Plant Clinic has found both Pythium sp. and Rhizoctonia sp. Pythium is favored by cool, wet soils, Rhizoctonia by warm soils. Rhizoctonia grows across the soil/media surface to attack the base of stems or leaves in contact with the ground. Both Pythium and Rhizoctonia are soilborne pathogens that can survive in infected plant debris in soil or in used pots and flats. The New Guinea impatiens cultivars 'Aurora', 'Cosmos', and 'Twilight' are susceptible to both fungi.
Symptoms Wilting, rotted roots, and death are common for either disease. Pythium infections are black and moist and may extend into the basal stem. Blackening of the stem in cross section may also occur. Rhizoctonia-infected tissue is drier, brown, and cracked. A web-like growth may be seen on infected plant parts. Leaves touching the ground may be infected first.
- Use sterile, pathogen-free potting mix or soil. Also disinfect any tools and equipment that might be used and contaminate the media.
- Growing media pH should be between 6.2 to 6.5 and temperatures at 73°F. Check regularly to ensure low electrolyte concentration levels.
- Use new or thoroughly cleaned and disinfected pots and flats or treat with aerated steam for 30 min.
- Purchase new seed each year, as it will not last over a year even with good seed storage.
- Do not overwater or let the watering hose touch the ground below greenhouse benches.
- Rotate flower beds.
Chemical control Focus on cultural controls first. Rotate among fungicides from different groups with different modes of action.
To control Pythium only:
- Aliette WDG at 1.25 to 4 lb/100 gal water. Group 33 fungicide. 12-hr reentry.
- Alude at 6.3 to 12.7 oz/100 gal water applied as a soil drench at a rate of 25 gal solution/100 sq ft. Use only once per month. Can also be used as a foliar spray at 1 to 2 quarts/100 gal water at 14- to 21-day intervals. Group 33 fungicide. 4-hr reentry.
- Banol at 2 to 3 fl oz/10 gal water. Group 28 fungicide. 24-hr reentry.
- Fosphite at 1 to 2 quarts/100 gal water. Do not use copper products within 20 days of treatment and do not use spray adjuvants. Group 33 fungicide. 4-hr reentry.
- Heritage at 0.2 to 0.9 oz/100 gal water then apply 1 to 2 pints solution/sq ft. Group 11 fungicide. 4-hr reentry.
- Mefenoxam 2 AQ at 0.12 to 0.24 fl oz/100 gal water as a soil drench at seeding, at 0.49 to 0.98 fl oz/100 gal water at transplanting, and see label for media incorporation. Resistant isolates have been detected in the Northeast United States. Group 4 fungicide. 48-hr reentry.
- Proplant at 2 to 3 fl oz/10 gal water. Group 28 fungicide. 12-hr reentry.
- Subdue MAXX at 0.13 to 0.25 fl oz/100 gal water at seeding, or 0.5 to 1 oz/100 gal water at transplanting. Resistant isolates have been detected in the Northeast United States. Group 4 fungicide. 48-hr reentry.
- Terrazole 35 WP at 3.5 to 10 oz/100 gal water as a soil drench. Group 14 fungicide. 12-hr reentry.
To control Rhizoctonia only:
- Banrot 40 WP at 4 to 8 oz/100 gal water. Group 1 + 14 fungicide. 12-hr reentry.
- Compass O 50 WDG at 0.5 oz/100 gal water. Do not use on New Guinea impatiens or with organosilicate additives. Group 11 fungicide. 12-hr reentry.
- Empress at 1 to 3 fl oz/100 gal water. May discolor open flowers. Do not tank-mix with a growth regulator. Group 11 fungicide. 12-hr reentry.
- Heritage at 1 to 2 oz/100 gal water plus a non-silicone-based wetter sticker. Group 11 fungicide. 4-hr reentry.
- Terraguard SC at 4 to 8 fl oz/100 gal water. Group 3 fungicide. 12-hr reentry.
- Veranda O at 0.5 lb/100 gal water used as a drench. Group 19 fungicide. 4-hr reentry.
Biological control Use in conjunction with other control tactics such as thorough sanitation.
- RootShield Granules (Trichoderma harzianum Rifai strain KRL-AG2) at 1 to 1.5 lb/ cubic yard soil mix or 1000 sq ft. Zero reentry interval.
References Castillo, S. and Peterson, J.L. 1990. Cause and control of crown rot of New Guinea impatiens. Plant Disease 74:77-79.
Harman. G.E. 2000. Myths and dogmas of biocontrol. Plant Disease 84:377-393.