On Crops: Most members of the carrot family, including carrots, celery, parsley, dill, chervil, and parsnip
Worldwide, wherever host crops are grown
White deposits develop on the top sides of leaves that give carrot leaves a ghostly grey appearance. As this fungal disease advances, the whitish parts of the leaves turn brown and shrivel, becoming dry and brittle. Powdery mildew is most likely to infect older plants that are beginning to decline. This fungus grows best under humid conditions with temperatures between 16-21C (60-70F).
Powdery mildew fungi clog up leaf pores and block light to photosynthetic cells, so the plants are weakened in their ability to use light as an energy source. New growth stops, old leaves fall off, and the plants struggle to stay alive.
Powdery mildew of carrots tends to occur in episodes, when dry soil conditions and high humidity come together in late summer. Spores that blow in on the wind are most likely to infect drought-stressed plants. Prevent drought stress by providing ample fertilizer to support strong foliar growth, and mulching to conserve soil moisture. Provide ample water using a sprinkler or hose during the morning hours, which can wash powdery mildew spores from leaves. Plant fast-growing varieties.
Pick off individual leaves that show powdery mildew as soon as you see them. If the carrots are mature, harvest them and store them in the refrigerator.