Penstemon-Root Rot

Latest revision: 
March 2024

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Cause The OSU Plant Clinic has found Phytophthora spp., Rhizoctonia spp., and Fusarium spp. associated with root rots of Penstemon spp. grown in Oregon. Nursery stock grown in California were found to have P. cryptogea complex. Native plant nurseries in Idaho have reported problems with root rot on this crop as well. These soilborne fungi are favored by cool, wet soil and/or media. They can become a problem if growing media becomes contaminated or a pathogen is introduced on seeds or in irrigation water. These fungi are able to survive for various periods as resting spores or as mycelium in soil and residual host tissues, and they can be seedborne. They also can persist in old, reused containers.

Symptoms A root rot caused by Phytophthora citrophthora had stunting, leaf discoloration, wilting and rotting of roots and the root crown. Plants eventually died.

Cultural Control Penstemon does well in dry, sunny well drained sites.

  • Use new pots, trays, or treat any reused items with a disinfectant, use steam, or solarization techniques (such as with anti-condensation film) to clean them.
  • Use soilless growing media that is stored under cover in a concrete bay or in some other clean and sealed container kept dry, cool, and in a low-light environment.
  • Growing media could be treated by steam (140°F for 30 minutes), solarization (double-tent at 160°F for 30 minutes or 140°F for 1 hour), or chemically. Storage area surfaces should be easy to clean and disinfest. This could include metal or plastic bins, trailers, trolleys, in bags on a sealed surface, or racks under cover.
  • Use ground sources for irrigation and avoid surface water sources for propagation of the crop.
  • Good drainage and water management are essential components of a management plan. Ensure growing media and holding areas are well drained.
  • Monitor soluble salt concentrations regularly adjusting media to an EC of 1.0 to 1.25 and keep pH between 5.8 and 6.2.
  • Remove diseased plants immediately and carry out rigorous hygiene measures to prevent further spread.
  • Remove crop residue and old or low-quality plants that will not be marketed as they can become sources of inoculum.
  • Replace wood benches or potting tables for ones made of plastic or wire that will facilitate cleaning and disinfestation.

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. Penstemon is not on any of these labels so be sure to try on a few plants first before general use.

For Rhizoctonia:

  • Banrot 40WP at 4 to 8 oz/100 gal water. Group 1 + 14 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.
  • Spirato GHN at 1 fl oz/100 gal water. Use with oils or adjuvants may cause plant damage. Group 12 fungicide. 12-hr reentry.
  • Thiophanate-methyl-based products. Inconsistent efficacy. 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 Phytophthora: 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 at 1.25 to 4 lb/100 gal water. Do not use with adjuvants. Group P7 fungicide. 24-hr reentry.
  • Banol at 2 to 3 fl oz/10 gal water. Group 28 fungicide. 12-hr reentry.
  • Banrot 40 WP at 4 to 8 oz/100 gal water. Group 1 + 14 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 P7 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. No restrictions on reentry when used as a soil drench or media incorporation.
  • 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. No restrictions on reentry when used as a soil drench or media incorporation.
  • Proplant at 2 to 3 fl oz/10 gal water. Group 28 fungicide. 12-hr reentry.
  • Spirato GHN at 1 fl oz/100 gal water. Use with oils or adjuvants may cause plant damage. Group 12 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. No restrictions on reentry when used as a soil drench.
  • Terrazole 35 WP at 3.5 to 10 oz/100 gal water. Group 14 fungicide. 12-hr reentry.

For Fusarium:

  • Use thiophanate-methyl products.

Reference Brooks, F.E., and Ferrin, D.M. 1995. First report of phytophthora root and crown rot of Penstemon species. Plant Disease 79:212.