See:
Greenhouse Plants, Ornamental - Pythium Seed Rot, Damping-off, and Root Rot
Cause Pythium spp., a fungus-like microorganism. Pythium ultimum and P. irregulare have been associated with geraniums in the Northeastern United States as a problem on both cuttings and mature plants. The organism is soilborne and favored by overwatering and poor soil drainage. It survives unfavorable periods in soil and infected plant debris. It can be found in field soil, sand from streams or rivers, and pond or lake sediment. Under favorable conditions, spores germinate and infect roots. After infection, the fungus-like microorganism spreads mainly in the inner bark tissues of the root and stems. A thick-walled survival structure, oospores, are produced that can persist several years. Movement of infected plants and/or soil can spread the organism. Plants grown at high fertility can be most susceptible.
Symptoms Poor stand development or seedling collapse are indicative of damping-off. Seeds or emerging radicles may be rotted. After emergence, stems, roots, and cotyledons may rot at or below the soil line. Cutting rot starts at the cut end as a brown water-soaked rot and progresses up, turning coal black until leaves wilt and the stem dies. Mature plants may be stunted, drop leaves, and die. Stems turn black, also progressing up. Root tips appear translucent and water-soaked. The outer layer of root tissue easily strips off leaving the central core of tissue bare. Fusarium sp. infection also can cause a black stem disease.
Cultural control Difficult to manage once rot has begun so focus on preventing disease development.
- Use pathogen-free propagation material.
- Steam the rooting media or use a sterile, soilless potting mix. Also disinfect any tools and equipment that might be used and contaminate the media.
- Clean and disinfect cutting knives often.
- Avoid overwatering plants.
- Keep hose ends off the ground.
- Maintain optimum (not excessive) plant fertility.
- Media pH should be around 6 to 6.6 (5.5 for ivy geraniums) and soil temperature at 65°F; check the electrolyte concentration at regular intervals. Note that geraniums tend to naturally lower the growing media pH.
- Remove and destroy infected plants and their containers.
- Control shoreflies that can spread the fungus. Fungus gnats are not a vector of Pythium. See the PNW Insect Management Handbook for details.
- Avoid reusing pots or trays from a previous crop for propagation. If pots must be reused then wash off all debris and soak in a sanitizing solution or treat with aerated steam for 30 min.
Chemical control Use only with cultural controls. Rotate or tank-mix materials from different groups with different modes of action. Isolates resistant to Banol (group 28 fungicide) and/or Mefenoxam or Subdue (group 4 fungicides) have been reported from North America. Isolates resistant to Mefenoxam are able to cause more damping-off in the presence of the chemical.
- Adorn at 1 to 4 fl oz/100 gal water plus another fungicide. Group 43 fungicide. 12-hr reentry.
- Aliette at 1.25 to 4 lb/100 gal water. Do not use with adjuvants. Group P7 fungicide. 24-hr reentry.
- Areca at 1.25 to 4 lb/100 gal water as a foliar application. Group P7 fungicide. 48-hr reentry.
- Banol at 2 to 3 fl oz/10 gal water. Group 28 fungicide. 12-hr reentry.
- Banrot 40 WP at 6 to 12 oz/100 gal water. Group 1 + 14 fungicide. 12-hr reentry.
- Fenstop at 14 fl oz/100 gal water. For greenhouse use only. Plant stunting may occur. Group 11 fungicide. 12-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 P7 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; see label for media incorporation. Group 4 fungicide. No restrictions on reentry when used as a soil drench or media incorporation.
- MetaStar 2E at 0.25 to 0.5 fl oz/100 gal water as a soil drench at seeding, at 0.5 to 2 fl oz/100 gal water at transplanting, and see label for media incorporation. Group 4 fungicide. No restrictions on reentry when used as a soil drench or media incorporation.
- Monterey Garden Phos at 1 teaspoons/8 gal water as a soil drench. Group P7 fungicide. H
- Proplant at 2 to 3 fl oz/10 gal water. Group 28 fungicide. 12-hr reentry.
- Segway SC at 1.5 to 3 fl oz/100 gal water. Group 21 fungicide. 12-hr reentry.
- Subdue MAXX at 0.5 to 1 fl oz/100 gal water as a soil drench. Group 4 fungicide. No restrictions on reentry when used as a soil drench.
- Terrazole 35 WP at 3.5 to 10 oz/100 gal water. Group 14 fungicide. 12-hr reentry.
- Truban 30 WP at 3 to 10 oz/100 gal water. Group 14 fungicide. 12-hr reentry.
Some registered products offer only suppression of this disease and thus are not recommended for use. These products include Heritage.
Biological control Use in conjunction with other control tactics. May have to use in potting mix several days before transplanting. Do not apply fungicides to the potting mix 10 days before or after application of the biological.
- Actinovate SP at 6 oz/100 gal water as a soil drench was effective in a trial in Michigan. Do not mix with Regalia or certain copper or sulfur-based products. 4-hr reentry. O
- Subtilex NG (Bacillus subtilis stain MBI 600) at 0.2 to 0.4 oz/100 gal is registered for soaking the soil; however, it was not effective in two trials out of Connecticut. Group BM02 fungicide. 4-hr reentry. O
References Moorman, G.W., Kang, S., Geiser, D.M., and Kim, S.H. 2002. Identification and characterization of Pythium species associated with greenhouse floral crops in Pennsylvania. Plant Disease 86:1227-1231.
Garzón, C.D., Molineros, J.E., Yánez, J.M., Flores, F.J., Jiménez-Gasco, M.M., and Moorman, G.W. 2011. Sublethal doses of mefenoxam enhance Pythium damping-off of geranium. Plant Disease 95:1233-1238.