Some Common Problems with Delphiniums
On this page we have photographs of "Broad Mite", "Bacterial Blackspot" & "Sclerotinia rolfsii" -- also known as "Delphiniums Blight" plus some tips for dealing with these conditions.
This damage is caused by "Broad Mite" a tiny creature that you probably won't see but which sucks plant juices from the tender growing points. It can devastate your delphiniums. Note the wrinkled leaves and black, distorted centre growth. Spray with a miticide.
This is what Broad Mite does to the flowers. Note the red and dark purple colour splotches, and general distortion.
There are miticides available for the treatment of this pest. Destroying plants by burning is also a good option.
North Carolina State University has a good discussion of Broad Mite. The following link discusses cyclamen and broad mites: http://www.hort.uconn.edu
This page has a list of recommended chemicals to use for control of cyclamen mite - broad mite can usually be controlled with the same regime: http://www.ipm.ucdavis.edu
Unsightly, but rarely a major problem in the garden. Copper is a useful preventive.
Also known as Delphinium blight in the USA. It is difficult to see the small (1/16 inch dia), light brown fungal fruiting bodies, or sclerotes, lying on the top of the soil at the base of the plant in this bad scan. There is also a very vivid white fungal growth (mycelium) growing on top of and through the soil around the plant.
Collapse of the plant is rapid with this disease and irreversible. From first sign to total collapse -- just a day or two.
Destroy plants by burning and disinfect the soil. Grow your delphiniums elsewhere. This disease is often spread by dispersing infected soil via garden boots and implements.
Sclerotinia Rolfsii is very difficult to completely eradicate from the garden. The best control we have come across is to use Jeyes Fluid (which you may be able to source) as a soil fumigant. This works well where infection is limited to small areas. Solarisation in climates where you have plenty of hot sun can work too. This is a case of covering the ground with black polythene for several weeks over the hottest part of the summer. Don't expect to eradicate it though. The very best cure of all is to not grow plants that are susceptible.
I'm not sure how long S rolfsii will remain viable in the soil without a host but if anyone finds out, please let us know.
Leaf-tier ( Polychrysia esmeralda, the leaf-tier moth): please see this page for some excellent photos of this pest. It seems the best way to deal with it is to manually pick out the caterpillars.