Pests

Aphids:  Aphids are, of course, one of the most common pests in any garden but from my experience they are not really a major pest in penstemons.  You may find some residual damage such as leaf curling but they only really pose a threat during propagation where they can severely damage seedlings.  Chemical control is not really necessary and treatment with soapy water (use real soap not washing-up liquid) should suffice.

Chrysanthemum Eelworm:  This is probably the most important pest in penstemons in Europe.  It is most serious if an outbreak occurs in a nursery where it can be devastating.  Chemical control is available to the commercial grower but not to the home gardener and the most effective control is prevention.  Symptoms begin as leaf spots consisting of yellow-green blotching which results in leaves turning brown and falling off.  Because the eelworms tend to move upwards from the base of the plant the browning will move upwards and some plants may only have green tips at the top of the plant.  Because eelworm can be confused with die back (see diseases) you should look for leaves which have begun to curl and are brown along the inner side.  (I will put up some photographs of this condition later).  The 1mm microscopic nemetodes feed and breed in buds and leaf axils as well as internally entering through cuts or breaks.

Always check new purchases of plants by keeping them separate from established stock for at least a month to see if they show any of the above symptoms.  Suspect plants should be destroyed by burning.  If an attack occurs in a border then the affected plants should be removed and destroyed and the ground left weed free and not planted with susceptible plants in the same growing season.  The nemetodes can survive for as much as three months in garden debris but not so long in open weed free soil.

The Chrysanthemum eelworm (Aphelenchoides ritzemabosi) also attacks asters, chrysanthemums, dahlias, doronicums and some common garden weeds such as groundsel, sow thistle, buttercup and chickweed.  An attack is more likely in mass plantings in a border than in pot cultivation.

Diseases

Powdery Mildew:  Mildew will affect a wide variety of plants in the garden but I have not yet found it in penstemons although some varieties are apparently susceptible.  Proximity to cornflower could be a problem as this is highly susceptible.

Die Back:  Can be confused with Chrysanthemum Eeelworm.  The cause of this condition is not really known but may appear in large clump forming penstemons resulting in the loss of large sectors of the plant.  If found try pruning back the affected parts but if the plant is in danger of loss then take cuttings from sound non-flowering tips and propagate.

Mould:  Normally only found in the propagation stage.  Characterised by white fur on the plants.  Treat with a recognised fungicide or destroy affected seedlings.

If you find that I am innacurate in my descriptions or remedial action please let me know.  I will not be offended.  Honest!!