CARROTS

CERCOSPORA LEAF BLIGHT: 

Symptoms:  Leaf spots caused by Cercospora carotae first appear along the margins of the leaves, often causing the leaves to curl. Spots inside the leaf edges are small, roughly circular, and tan or gray to brown with a dead center (fig. 1).  As the lesions increase in number and size, the entire leaflet withers and dies. The fungus attacks younger leaves and plants in preference to older ones. In heavily infested fields, however, both older and younger leaves are subject to attack. The pathogen also produces lesions on the petioles and stems (fig. 2), characterized by dark brown borders and tan to gray centers (fig. 3). The lesions may merge and girdle the stems, causing the leaves to die. Because Cercospora leaf blight develops rapidly in hot or humid weather, it is likely to occur in July and early August in New York State.

ALTERNARIA LEAF BLIGHT:

Symptoms:  Alternaria leaf spots first appear at the margin of the leaflets and are dark brown to black and irregular in shape (figs. 4, 5, 6). Lesions produced on the petioles and stems are dark brown and often coalesce and girdle the stems (fig. 4). As the disease progresses entire leaflets may shrivel and die, appearing scorched (fig. 7). Alternaria leaf lesions are generally more prevalent on older foliage and plants than on young foliage. The disease spreads rapidly on the older leaves of a maturing crop after the rows have closed due to poor air circulation.  Root lesions are irregular, dark brown to black, and appear as a shallow, firm area of decay. Cool weather is favorable for its development as well as when temperatures are warm or when moisture in the form of rain, dew, or irrigation water is persistent, the threat of infection and rapid spread of leaf blight organisms is high.

Control:  A two-to three-year rotation is recommended to allow for natural decline in the pathogen population. Fall plowing or disking is also recommended to hasten the decomposition of plant residues and to reduce the number of overwintering bacteria and fungal spores.  The use of disease-free seed is strongly recommended because all three pathogens can survive on or in the seed. Hot water treatment of seeds reduces bacterial leaf blight caused by Xanthomonas carotae.  Because Alternaria leaf blight destroys the mature foliage, it is important to encourage the continuous production of young vigorous leaves for gripping by a mechanical harvester. Research has shown that well-fertilized carrots are less likely to develop Alternaria leaf blight. A nitrogen application made in mid-August or early September may be beneficial in promoting foliage development. An alternative method of applying nitrogen is to add sprayable urea to each fungicide spray. Although only a small amount of the foliar-applied nitrogen is absorbed, the excess is washed to the ground and taken up by the roots.

CARROT POWDERY MILDEW:

Symptoms:  Fluffy white growth on stems and leaves.  Foliage becomes brittle, and may eventually shrivel and die.  Severe infection can result in foilage loss, making harvesting difficult and lowering crop yields.

Control:  Infection is favored by high humidity and moderate temperatures.  Carrots are more susceptible 7 weeks after planting.  Water stress will increase disease severity.  Preventative control measures include:  monitor young crops regularly, limit the movement of machinery and equipment from infected area, maintain good even crop growth with adequate fertilizer and irrigation program.

Treatment:  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.

ROOT DIEBACK (FORKING AND STUBBING):

Symptoms:  Root dieback of carrots produces excessively branched or stubbed roots. The fungus kills young tap roots less than 2 weeks after seed germination, reducing root length and/or stimulating multiple root formation (forking).  Can also cause damping-off.

Control:  Avoid overwatering and provide good drainage. Rotating to small grains might reduce soil populations of some Pythium spp.