Watermelon (Citrullus sp.)-Alternaria Leaf Spot

Cause Alternaria sp., a fungus. The disease has caused severe losses in Hermiston, OR area fields with overhead irrigation.

Symptoms First are irregular brown spots on leaves. Spots may coalesce, covering large leaf areas; when those are blown or drop out, leaves look severely tattered. Infections may girdle and kill the vines. If the disease is severe in a field, most leaves and vines die (they turn gray). Without vine or plant growth around them, melons do not mature.

Cultural control

  • Use plastic mulch in plant rows.
  • Use drip irrigation instead of overhead irrigation.

Chemical control

  • Bonide Fung-onil is (Group M5) available for home gardens. H
  • Bravo Ultrex (Group M5) at 1.8 to 2.7 lb/A on 7-day intervals. See label for information on when not to spray. 12-hr reentry.
  • Copper (Group M1) products are not recommended as stand-alone materials.
    • Badge SC at 0.5 to 2.5 pints/A on 5- to 7-day intervals. Preharvest interval is 0 days. 24-hr reentry for greenhouse use; 48-hr reentry for all other applications.
    • Champ WG at 1.5 to 2 lb/A on 5- to 7-day intervals. 48-hr reentry. O
    • Cuprofix Ultra 40 Disperss at 1.25 to 2 lb/A on 5- to 7-day intervals. 48-hr reentry.
    • Kocide 2000 at 1 to 2.25 lb/A or Kocide 3000 at 0.5 to 1.25 lb/A on 5- to 7-day intervals. 48-hr reentry. O
    • Liqui-Cop at 2 to 3 teaspoons/gal water. H
    • Previsto at 0.75 to 2 quarts/A on 5- to 7-day intervals. 48-hr reentry. O
  • Echo 720 (Group M5) at 2 to 3 pints/A on 7-day intervals. Preharvest interval is 0 days. 12-hr reentry.
  • Endura (Group 7) at 6.5 oz/A on 7- to 14-day intervals; alternating with another appropriate fungicide. To limit the potential for development of resistance, do not make more than four (4) applications of Endura per season. The preharvest interval is 0 day. 12-hr reentry.
  • JMS Stylet Oil at 3 to 6 quarts/100 gal water. Do not spray when plants are wet or if temperature is below 50°F, above 90°F or when plants are under heat or moisture stress. 4-hr reentry. O
  • Manzate 75 DF (Group M3) at 2 to 3 lb/A. Do not apply within 5 days of harvest. 24-hr reentry.
  • Mettle 125 ME (Group 3) at 8 fl oz/A on 7- to 10-day intervals. Application can be made the day of harvest. 12-hr reentry.
  • Omega 500F at 0.75 to 1.5 pints/A on a 7- to 10-day intervals. Preharvest interval is 30 days. 12-hr reentry.
  • Regalia (Group P5) at 1 to 4 quarts/A plus another fungicide on 7- to 10-day intervals. Preharvest interval is 0 days. 4-hr reentry. O
  • Strobilurin fungicides (Group 11) are labeled for use. Do not make more than one (1) application of a Group 11 fungicide before alternating to a labeled fungicide with a different mode of action.
    • Quadris Flowable at 11 to 15.5 fl oz/A. Do not apply within 1 day of harvest. 4-hr reentry.
    • Reason 500 SC at 5.5 fl oz/A. Do not apply within 14 days of harvest. 12-hr reentry.
  • Trilogy at 0.5% to 1%. Not labeled for use in Oregon. Do not use above 90°F or when plants are under heat or moisture stress. Do not use when foliage is wet as good coverage is essential. Poor control as a stand-alone product. 4-hr reentry. O
  • Premixes of fungicides are available and offer ease for fungicide resistance management. Do not make more than one (1) application of a Group 11 fungicide before alternating to a labeled fungicide with a different mode of action; for other fungicide groups apply no more than two (2) sequential applications alternating with another fungicide with a different mode of action.
    • Aprovia Top (Group 7 + 3) at 10.5 to 13.5 fl oz/A on 7- to 14-day intervals. May be applied the day of harvest. 12-hr reentry.
    • Dexter Max (Group M3 + 11) at 2.1 to 3.2 lb/A on 7- to 10-day intervals. Some cantaloupe varieties are sensitive to Dexter Max. Preharvest interval is 5 days. 24-hr reentry.
    • Dexter Xcel (Group M3 + 11 + 3) at 72 fl oz/A on 7- to 10-day intervals. Some cantaloupe varieties are sensitive to Dexter Xcel. Preharvest interval is 7 days. 24-hr reentry.
    • Gavel 75 DF (Group 22 + M3) at 1.5 to 2.0 lb/A on 7- to 10-day intervals. Preharvest interval is 5 days. 48-hr reentry.
    • Inspire Super (Group 3 + 9) at 16 to 20 fl oz/A on 7- to 10-day intervals. Preharvest interval is 7 days. 12-hr reentry.
    • ManKocide (Group M1 + M3) at 2 to 3 lb/A on 7- to 10- day intervals. Under moderate to severe disease pressure use the higher rate on 5- to 7-day intervals. Do not apply within 5 days of harvest. 48-hr reentry.
    • Merivon Xemium (Group 7 + 11) at 4 to 5.5 fl oz/A on 7- to 14-day intervals. Preharvest interval is 0 days. 12-hr reentry.
    • Miravis Prime (Group 7 + 12) at 9.2 to 11.4 fl oz/A on 7- to 14- day intervals. Preharvest interval is 1 day. 12-hr reentry.
    • Orondis Opti (Group M5 + 49) at 1.75 to 2.5 pints/A on 7- to 14-day intervals. May be applied the day of harvest. 12-hr reentry.
    • Pristine (Group 7 + 11) at 12.5 to 18.5 oz/A on 7- to 14-day intervals. Preharvest interval is 0 days. 12-hr reentry.
    • Quadris Opti (Group 11 + M5) at 3.2 pints/A or Quadris Top (Group 11 + 3) at 10 to 14 fl oz/A on 7- to 14-day intervals. Preharvest interval is 7 days. 12-hr reentry.
    • Switch 62.5WG (Group 12 + 9) at 11 to 14 oz/A on 7- to 10-day intervals. Do not apply within 1 day of harvest. 12-hr reentry.

Biological control

  • Actinovate AG at 3 to 12 oz/A as a foliar spray on 7- to 14-day intervals. 1-hr reentry. O
  • Actinovate Lawn & Garden at 1/2 to 1 teaspoon/gal water. H O
  • Ecoswing at 1.5 to 2 pints/A. Preharvest interval is 0 days. 4-hr reentry. O