Cause There are many different organisms that can cause leaf spots on penstemon. Puccinia palmeri and Puccinia pentstemonis are rust fungi that has been reported on penstemon throughout the PNW. A few other Puccinia species have also been reported from WA. Ramularia nivosa is common on penstemon species and reported throughout the PNW as well.
The OSU Plant Clinic has also found a Colletotrichum sp. (fungus) which caused a leaf and stem blight, impatiens necrotic spot virus, causing leaf spots and twisting, and a downy mildew.
Several other less common leaf spots have been reported from the PNW such as Alternaria sp., Stemphylium sp. and Septoria sp.
Symptoms Ramularia nivosa leaf spots are necrotic, concentric, and bounded by a darker periphery.
Cultural Control Penstemon does well in dry, sunny well drained sites.
- Avoid overhead irrigation that keeps plants wet for extended periods of time.
- Remove all infected plant parts during the growing season.
- Remove crop residue and old or low-quality plants that will not be marketed as they can become sources of inoculum.
Chemical control Penstemon may not be on all of these labels so be sure to try on a few plants first before general use.
- Compass 50 WDG at 2 to 4 oz/100 gal water is labeled for penstemon. Do not use organosilicate additives. Group 11 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.
- 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.
- 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 plus another fungicide. 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.
Reference Newcombe, G., Gaylord, R., Yenish, J. P., Mastrogiuseppe, J. and Dugan, F. M., 2009. New records for pathogenic fungi on weedy or non-indigenous plants. North American Fungi, 4:1-12.