Watermelon (Citrullus sp.)-Damping-off

Cause The fungus-like microorganism, Pythium spp., and the fungus, Rhizoctonia solani, are reported on watermelon. Fusarium spp. also are common damping-off fungal organisms. These pathogens are likely to be in any soil. Damping-off frequently attacks almost all kinds of vegetable seedlings just before or after they emerge from the soil. Plants that never emerge or seedlings that die or fall over are said to have "damped-off." Destructiveness depends on environmental conditions and the amount of pathogen in the soil. Wet soil, high humidity, and cloudy days are especially favorable for damping-off.

Symptoms Just after seedlings emerge, they develop a sunken, necrotic lesion near the soil line. The lesion girdles the stem, and the seedling collapses and dies quickly.

Cultural control

  • In the greenhouse or seedbed, use only new, pasteurized soil mix. Heat treat any other soil; for example, most pathogens die if soil is heated to 158°F (70°C) for 30 min. with aerated steam.
  • Use new flats or pots for planting, or thoroughly wash and disinfect old containers before using.
  • If damping-off starts, stop watering for a while. Allow soil around the plants to dry somewhat. If the seedlings are in flats or in cold frames, give them as much air and light as possible.
  • Use transplants beyond the first-true-leaf stage for field or garden plantings.

Chemical control

  • Seed treatment.
    • Captan 4F at 2 fl oz/100 lb seed plus a dye.
    • Dynasty at 0.1 to 0.38 fl oz/100 lb seed plus a dye. 4-hr reentry.
    • Maxim 4FS at 0.08 to 0.16 fl oz/100 lb seed plus a dye. To control pathogenic Fusarium and Rhizoctonia spp. See label for reentry restrictions.
    • Mertect at 0.1 mg a.i./seed plus a dye. To control pathogenic Fusarium spp. See label for reentry restrictions.
    • Thiram 50WP Dyed at 4.5 oz/100 lb seed (not labeled for Oregon) or 42-S Thiram at 4.5 fl oz/100 lb seed plus a dye. See label for reentry restrictions.
  • Regalia (Group P5) at 1 to 2 quarts/A as an in-furrow treatment. 4-hr reentry. O
  • Ridomil Gold SL at 0.25 to 0.4 pint/A in a directed spray to the base of the plant. Apply at planting; two (2) additional applications at 20- to 30-day intervals may be required during the season. 48-hr reentry.

Biological control

  • Actinovate AG at 3 to 12 oz/A as a soil drench at planting. 1-hr reentry. O
  • Actinovate Lawn & Garden at 1/2 to 1 teaspoon/gal water. H O
  • Cease at 4 to 8 quarts in 100 gal water applied to soil at 1 pint/sq ft for each 3-inches of soil depth on 21- to 28-day intervals. For greenhouse plants only. 4-hr reentry. O
  • Double Nickel LC at 0.5 to 4.5 pints/A for soil application on 14- to 28-day intervals. Can be applied the day of harvest. 4-hr reentry. O
  • Prestop at 1.4 to 14 oz/10 gal water (0.1% to 1.0% suspension) as a soil drench or incorporation into growing medium. 0-hr reentry. O
  • Rootshield Home and Garden at 1 to 3 Tbsp/gal water as a soil drench at planting. H O
  • Rootshield WP at 3 to 5 oz/100 gal water for greenhouse soil drench and field chemigation or at 16 to 32 oz/A as an in-furrow spray. 0-hr reentry. O
  • SoilGard 12G at 0.5 to 10 lb/A at 4-week intervals will suppress Pythium and Rhizoctonia. See label for specific application types and timings. Can be applied on the day of harvest. O
  • Stargus at 3 to 4 quarts/A as a soil drench on 10- to 21-day intervals. Preharvest interval is 0 days. 4-hr reentry. O