Watermelon (Citrullus sp.)-Bacterial Fruit Blotch/Seedling Blight

Cause Acidovorax avenae subsp. citrulli (syn. = Pseudomonas pseudoalcaligenes subsp. citrulli), a seedborne bacterium that affects watermelon, cantaloupe, and different types of melons. Damage in the greenhouse can be high. Field losses can be severe particularly in humid environments. Field tests indicate that fruit blotch is an unlikely threat in north-central Oregon where watermelons grow in a hot, dry environment using drip tape and plastic mulch.

Symptoms Greenhouse symptoms begin as small water-soaked lesions on the underside of the cotyledon followed by necrotic lesions. A single infected plant in a tray can infect many nearby plants resulting in a circular pattern of infection. Lesions can develop on the first true leaves before transplanting in the field.

Field symptoms can develop throughout the growing season. Lesions can be anywhere on the leaf but most often are near the midrib. Fruit symptoms begin as small greasy-looking water-soaked spots that often enlarge quickly; infected areas on the melon are a darker green than normal. Large areas of the melon can be involved. Infected areas on fruit become necrotic, either in distinct localized irregular spots or over wide areas. Cracks in the rind occur with time. Fruit blotch will not, until advanced, produce symptoms in the melon's flesh.

Cultural control For the greenhouse:

  • Thoroughly clean the greenhouse and treat with a disinfectant before each planting.
  • Plant only tested bacteria-free seeds where no fruit blotch has been found before.
  • Separate trays by lots to help prevent bacterial movement from one lot to another.
  • If possible, water plants from below trays.
  • Rogue diseased plants, but do not touch healthy plants in the process.

For the field:

  • Rotate out of cucurbits 2 out of every 3 years, or at least every other year.
  • Plant only tested bacteria-free seeds.
  • Do not irrigate from overhead. Use drip tube and plastic mulch.
  • Do not transplant infected plants.

Chemical control Copper treatments (Group M1) may help reduce disease spread.

  • 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 Formula 2 Flowable at 0.66 to 1.33 pints/A. 48-hr reentry.
  • C-O-C-S WDG at 1 to 2 lb/A. 48-hr reentry.
  • 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
  • Previsto at 0.75 to 2 quarts/A on 5- to 7-day intervals. 48-hr reentry. O

Reference Hamm, P.B., Spink, D.S., Clough, G.H., and Mohan, K.S. 1997. First report of bacterial fruit blotch of watermelon in Oregon. Plant Disease 81:113.