Powdery Mildew of Carrots

Causes
Powdery mildew of carrots in most parts of the world is caused by the fungus Erysiphe heraclei, but in some parts of Central Asia and the Mediterranean Leveillula lanuginose and L. taurica may be responsible. Besides carrots, these fungi can infect other umbelliferous crops and weeds such as celery, anise, coriander, dill, fennel, parsley, and parsnip.

Symptoms Powdery mildew symptoms appear first on older leaves and petioles as patches of white, powdery growth. The powdery growth can be very dense and noticeable. All above ground plant parts are susceptible to infection, including flower stalks and bracts. As the infection progresses, it moves onto younger plant tissue, where it cause chlorotic lesions. In California, powdery mildew is often found on the older foliage but rarely on the younger foliage. Although plants can survive heavy infections, if left unchecked, it can cause premature senesce. In carrot seed production fields, severe infections may cause flower distortions and reduced seed production.

Epidemiology Spores of the fungus can be carried long distances from other carrot fields, volunteer carrots, or other related host plants. Carrots are grown year-round in California so an inoculum source is always present. The spores do not require a film of water to cause infections like many other fungal pathogens. It only requires periods of high humidity and moderate temperatures of 50-90 degrees F for infection. Powdery mildew is most likely to occur in regions with warm dry climates. In California powdery mildew is more common in the fall when humidity rises and temperatures become more moderate.

Because infections begin on older plant parts, carrot fields become more susceptible as they age. The amount of yield loss depends on disease onset and severity. Early and severe infections can cause economic losses while late and mild infections may be totally insignificant. However, because most of the mechanical harvesting is done by lifting the carrots by the tops, a late infection may cause enough damage to the foliage to make mechanical harvesting difficult.

Control
Control is usually not required unless the disease appears early in the growing season or becomes severe late in the year. Sulfur is commonly used to control powdery mildew but other fungicides such as strobilurins are also effective. Some cultivars may be more tolerant than others. Nantes types appear to have more severe infections while Imperator types appear more tolerant. Avoiding water stress and maintaining plant vigor (without over fertilization) are ways to help prevent powdery mildew.

Source
The text and photos were provided by Joe J. Nunez, Vegetable/Plant Pathology Farm Advisor, Kern County, University of California Cooperative Extension.

Published
Winter 2004 issue of Carrot Country.

© 2004 Columbia Publishing