Hello,

In viewing your pictures and researching delphiniums, it does not appear that you have a common problem listed at this website:

https://extension.psu.edu/delphinium-diseases

There are many reasons that the leaves may be black and crispy. The most common things to consider are the growing conditions. Delphiniums need moist but well-drained soil in which to thrive. Are your plants sitting in waterlogged soil? If so, add some sand or perlite to increase the drainage. Perhaps you’re giving the plants too much water too often. Cut back on your watering amount/and/or schedule. Check for the moisture level in the soil at about 3cm down. If the area is dry, then water it sparingly. 

The next thing to consider is whether you have given the plants enough nutrients. Are the leaves turning yellow, then black? Or are they emerging black? Yellow leaves indicate a nutrient deficiency. It could also mean that there is too much phosphorus in the soil. As a matter of course and to avoid planting new plants in inadequate soil, I would strongly recommend testing your soil for nutrients. The URL below will give you all the information you need to test you soil. Michigan State University has the most thorough and quick return of test results.

https://ohioline.osu.edu/factsheet/hyg-1132

Here is what I recommend:

• Thoroughly inspect the plants from the ground up. Look at the root zone, the stems, and the underside of the leaves. If you have a magnifying glass, the better the inspection will be.
• When was the last time these Delphiniums were fertilized? They are very heavy feeders and may not be getting enough nutrients, hence the black leaves.
• Remove a bud and cut it open. Examine the inside for insects, worms, and kinds of moving insects. It may be a mite infestation, but again, it is very selective, and you would see the damage on the outside as well.
• Make sure to sterilize your garden tools after working and before using them on a new plant.
• Watchful waiting. If you can find no evidence of insects, no other areas of black spots, no discolorations, then I would wait a few weeks to see if the problem is solved by proper watering. It appears that these plants are a bit crowded, perhaps they are in competition with the other plants for nutrients and water. A bit of time-release fertilizer may help them flourish. Keep an eye on the plants and check frequently for signs of infection and insect infestation. (See the delphinium diseases website.) If you see insects, stem damage, any of the signs indicated in the website, then the plants will have to be sacrificed to prevent continued damage to surrounding plants. If you sacrifice them, do not compost. Put them into a garbage bag and discard with your trash. If this is an infection or insect infestation, a compost pile will never get hot enough to kill the problem.