Rhubarb (Rheum rhabarbarum)-Gray Mold

Cause Botrytis cinerea, a fungus that overwinters on decayed rhubarb plant parts. Spores also are abundant in the air and on decayed parts of many other plant species. The disease can be a problem in the field but is more destructive in forced crops.

Symptoms On older leaves and stems, a powdery gray mold develops under moist or humid conditions. During harvest, packing, and shipping, stems are further infected. Reddish brown, water-soaked areas develop on stems as the disease advances, producing additional gray mold spores. Especially under warm storage conditions, a watery decay may follow.

Cultural control

  • Remove all broken root and crown pieces from forcing house.
  • Circulate air through the forcing house to keep leaves dry.
  • Proper plant spacing ensures good air circulation and drying.
  • Remove leaves from stalks before packing and shipping.
  • Refrigerate harvested stalks.

Chemical control

  • Botran 75 WSB (Group 14) at 1.33 lb/100 gal water; not registered in Oregon. Do not use within 3 days of harvest. For greenhouse use only. 12-hr reentry.
  • Cannonball WG (Group 12) at 7 oz/A on 7- to 10-day intervals. Preharvest interval is 0 days. 12-hr reentry.
  • Fontelis (Group 7) at 14 to 24 fl oz/A on 7- to 14-day intervals. Preharvest interval is 3 days. 12-hr reentry.
  • Merivon Xemium (Group 7 + 11) at 8 to 11 fl oz/A on 7- to 14-day intervals. Do not make more than two (2) applications before alternating to a fungicide in different FRAC groups (non-Group 7 and non-Group 11). First test on a small portion of crop for phytotoxicity. Preharvest interval is 1 day. 12-hr reentry.
  • Miravis Prime (Group 7 + 12) at 13.4 fl oz/A on 7- to 10-day intervals. Do not make more than two (2) sequential applications. Preharvest interval is 0 days. 12-hr reentry.
  • Regalia (Group P5) at 1 to 4 quarts/A is labeled for other diseases and may help with controlling this disease in Oregon. Does not benefit from the addition of an adjuvant. Preharvest interval is 0 days. 4-hr reentry. O
  • Switch 62.5WG (Group 12 + 9) at 11 to 14 oz/A on 7- to 10-day intervals. After two (2) applications of Switch (Group 9 and 12 fungicide), rotate to another fungicide with a different mode of action for two (2) applications. May be applied the day 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