Petunia (Petunia spp.)-Damping-off

Cause Damping-off refers to a disease that develops on the seed, radicle, hypocotyl, epicotyl, or stem of an emerged seedling. Damping-off is caused by the organisms Pythium spp., Rhizoctonia solani, and Fusarium spp. These organisms also can cause stem lesions and root rot of older plants. None have been reported in the Pacific Northwest, but damping-off caused by one or more of these organisms has been found by the OSU Plant Clinic.

Fusarium survives in the soil as thick-walled, dormant chlamydospores, which germinate in response to exudates from nearby plant roots. Hyphae then penetrate the roots, colonize the cortex, and move into the xylem tissue. Small spores, microconidia, are produced and carried up into the plant.

Rhizoctonia survives in the soil as sclerotia or as mycelium colonizing bits of organic matter. Roots become infected as they grow past these structures. The fungus then grows along root surface, forms an aggregation called an infection cushion, pushes into the root where it colonies, and spreads and kills the tissue. The fungus can form knots of mycelium that develop into sclerotia. These are released into the soil as the plant debris decomposes.

All Pythium species are strongly favored by abundant moisture and can be especially problematic in poorly drained potting media. Pythium usually attacks seeds or seedlings. Pythium spp. can survive in dust, planting media, or soil particles on greenhouse floors, flats, and pots. Wood benches can also be a reservoir. Spread can also occur via movement of infected plants. Damaged plants are more easily infected. It enters roots damaged by high soluble salts, either from overfertilization or from letting the rooting medium dry out, even for short periods, during propagation.

Symptoms Poor stand development or seedling collapse are indicative of damping-off. Seeds or emerging radicles may be rotted. After emergence, stem, root, and cotyledon may rot at or below the soil line. It is difficult to determine which organism is involved by causal observation so send into a Plant Clinic for an accurate diagnosis. Plants may be stunted if the pH is below 5.0. Poor germination can also occur with use of old seed.

Pythium spp. causes a soft, mushy rot of ungerminated seeds. Once the radicle is extended or the plant emerges, tissues can be attacked at any point. The infection starts as a brown, water-soaked spot that will enlarge, become soft, and collapse.

Rhizoctonia solani causes reddish brown to dark brown lesions that become slightly sunken. Stem and root lesions often are shallow but may extend deeper in young stem or root tissue. Sometimes brown mycelia may be visible with a hand lens on the surface of the lesions.

Fusarium spp. causes darker red lesions that become brown with reddish borders, or brown lesions with diffuse margins, or similarly discolored longitudinal streaks. Stem and root lesions extend deep into plant tissue.

Cultural control

  • Plant seeds at the proper depth in well-drained soil.
  • Seed germination usually is higher if soils are moistened regularly but not saturated. It is important for the soil to dry down without drying out.
  • Plant seeds when temperatures favor rapid germination and growth, so plants rapidly emerge and reach a more mature or hardened state.
  • Use a soilless medium or pasteurized soil to start seedlings. Also disinfect any tools and equipment that might be used and contaminate the media.
  • Keep soilless media pH on the acidic side, ideally from 5.5 to 6.0. Note that petunias tend to naturally increase the growing media pH.
  • 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 Chemical treatment should be preceded by an accurate diagnosis since most of the chemicals are not effective against all pathogens that might cause damping-off.

For Rhizoctonia:

  • Banrot 40WP at 4 to 8 oz/100 gal water. Group 1 + 14 fungicide. 12-hr reentry.
  • Emblem at 1 fl oz/100 gal water. Use with oils or adjuvants may cause plant damage. Group 12 fungicide. 12-hr reentry.
  • Empress at 1 to 3 fl oz/100 gal water can be used for seedlings. May discolor blooms that are open during application. Group 11 fungicide. 12-hr reentry.
  • Medallion WDG at 2 oz/100 gal water if not phytotoxic. Using with oils or adjuvants may damage plant. Group 12 fungicide. 12-hr reentry.
  • ProStar 70 WG at 3 to 6 oz/100 gal water. Group 7 fungicide. 12-hr reentry.
  • Thiophanate-methyl-based products. Group 1 fungicides. 12-hr reentry.
    • Cleary's 3336 EG at 8 to 16 oz/100 gal water as a drench.
    • OHP 6672 4.5 F at 7.5 to 20 fl oz/100 gal water.

For Pythium: Rotate fungicides that have a different mode of action for resistant management.

  • Adorn at 1 to 4 fl oz/100 gal water plus another fungicide. Group 43 fungicide. 12-hr reentry.
  • Aliette WDG at 1.25 to 4 lb/100 gal water. Group 33 fungicide. 12-hr reentry.
  • Alude at 5 to 10 fl 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 26 to 54 fl oz /100 gal water at 14- to 21-day intervals. Group 33 fungicide. 4-hr reentry.
  • Areca at 1.25 to 4 lb/100 gal water as a foliar application. Group 33 fungicide. 12-hr reentry.
  • Banol at 2 to 3 fl oz/10 gal water. Group 28 fungicide. 24-hr reentry.
  • Banrot 40 WP at 4 to 8 oz/100 gal water. Group 1 + 14 fungicide. 12-hr reentry.
  • Emblem at 1 fl oz/100 gal water. Use with oils or adjuvants may cause plant damage. Group 12 fungicide. 12-hr reentry.
  • Fenstop at 7 to 14 fl oz/50 to 100 gal water/400 sq ft. For greenhouse use only. 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 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. Good control. 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. Group 4 fungicide. 48-hr reentry.
  • 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. 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. Apply 1 pint of solution per 1 sq ft. Group 4 fungicide. 48-hr reentry.
  • Terrazole 35 WP at 3.5 to 10 oz/100 gal water. Group 14 fungicide. 12-hr reentry.

For Rhizoctonia and Pythium

  • Hurricane WDG at 0.75 oz/100 gal water as a drench at seeding. Group 4 + 12 fungicide. 48-hr reentry.

For Fusarium:

  • Use thiophanate-methyl products. See above.

Note: Do not use Compass on petunia, violet or New Guinea impatiens as it might cause injury.

Biological control Use in conjunction with other control tactics such as thorough sanitation.

  • RootShield Plus Granules (Trichoderma harzianum Rifai strain T-22 and T. virens strain G-41) at 1 to 3 lb/ cubic yard soil mix. Zero-hr reentry. O
  • Bio-Tam 2.0 or Tenet WP (Trichoderma asperellum and T. gamsii) at 0.5 to 1.5 lb/cubic yard of substrate. See label for details and other application methods. 4-hr reentry unless soil incorporated. O

Reference Harman. G.E. 2000. Myths and dogmas of biocontrol. Plant Disease 84:377-393.