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Your delphinium leaves turning yellow and brown may be due to quite a few causes. These include fungal infections, bacteria, infestation of bugs and other insects, and poor location, growing conditions and inadequate care. Here are the most common causes for you to check out.
Growing conditions and care
Delphiniums need moist but well-drained soil in which to thrive. The main environmental cause of yellow leaves on this plant is too much water. Is your plant sitting in water-logged soil? If so, add some sand or perlite to increase the drainage. Perhaps you’re giving the delphinium 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 it’s dry, water you plant.
While you’re messing around in the soil, consider if you’ve given the plant enough nutrients. Yellow leaves can signify a nutrient deficiency. On the other hand, they can also mean that there’s too much phosphorous in your soil. You may need to buy a soil kit to check this out.
Providing the right growing conditions and care can help avoid most of these issues so I recommend also reading my growing delphiniums guide here.
Delphinium crown rot
This is the most common disease that creates yellow leaves. The “crown” of the plant is really the base, so this is when it rots at the base of the stem and in the root system. This is usually the case if you notice that the leaves at the base of the plant are the first to change colour. There may be an unpleasant smell as well. And you may notice a white cottony mass around the base of the plant. You need to remove the infected plant from your flower bed.
Note that rot may also appear in the stem and in the buds of the plant. Check there also.
These are thin, unsegmented roundworms that you can’t see unless you have a magnifying glass. They suck all the juice out of the plant, so your delphinium starts to look wilted and poorly and becomes stunted. The leaves turn yellow and the plant dies. There’s nothing you can do to stop this once it starts. Pull up the plant and burn it to prevent it spreading to other plants.
With this disease, the lower leaves of the delphinium start to brown as flowers start to appear. The brownness moves to the stems and creeps upward. There’s nothing you can do but pull out the plant and safely dispose of the remains.
The delphinium leaves turning yellow or brown can indicate the presence of assorted viruses. Instead of a full color change, you may notice yellow rings on the surface of the leaves. Remove and burn any diseased plants.
While delphiniums like the sun, they can’t handle full, direct sunlight. Brown leaves may indicate that they’ve been scorched by the sun. erect a screen of some kind to give your plant some shade while not shutting out the sunlight.
For more information about the problems that can happen with delphiniums, please read my guide on Problems with delphiniums, pests and diseases.