Answer: Delphiniums are susceptible to a disease called aster yellows, and your description makes me think that's what's wrong with your plants. The pathogen is spread by leafhoppers. Aster yellows can infect over 170 different kinds of plants. On flowers such as asters, chrysanthemums and delphiniums, flowers are nonexistent, or small and deformed, often turning a greenish yellow. Sometimes flowers simply remain green. Flower spikes can be twisted and deformed and the entire plant is usually small and spindly. You can't eliminate aster yellows, but you can keep the disease under control. Start by uprooting and destroying infected plants and remove any weeds that may be harboring leafhoppers or carrying the disease. You can try to lure leafhoppers away from your plants with yellow sticky traps, or you can spray your plants with insecticidal soap to control the population. Good luck with your delphiniums!
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