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  2. Host and Disease Descriptions

Carrot (Daucus carota)-Alternaria Leaf Blight

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Latest revision: 
March 2024
Image related to Carrot (Daucus carota)-Alternaria Leaf Blight
Note the dark-brown to black lesions on the leaves.
Photo by Kathy Merrifield, 1990.
Image related to Carrot (Daucus carota)-Alternaria Leaf Blight
OSU Extension Plant Pathology
Image related to Carrot (Daucus carota)-Alternaria Leaf Blight
OSU Extension Plant Pathology

See:

Carrot (Daucus carota) - Black Rot

Carrot (Daucus carota) - Cercospora Leaf Blight

Cause Alternaria dauci, a fungus that overwinters in crop refuse or on alternate hosts. The disease also may be seedborne. A. dauci also can affect parsley. Older leaves are more susceptible to Alternaria than younger leaves. Leaf spots are common in carrots grown for seed production in western Idaho and eastern Oregon.

Symptoms Alternaria leaf blight usually appears first in early fall on older leaves (sometimes called late blight). When large amounts of infested refuse are present, spring infection can occur. Seedlings can develop a damping-off symptom from early infections. Lesions generally appear along leaf margins and are dark brown to black with a yellow border. Spots may coalescence, girdling the petiole and killing the leaf. The spots of this disease are easily confused with Cercospora leaf blight, but Alternaria leaf blight generally is more irregularly shaped and darker in color.

Cultural control

  • Irrigate so that carrots are dry by nightfall.
  • Rotation out of carrots for at least 1 year is important because refuse is the main way the organisms overwinter.
  • Plow under crop debris soon after harvest.
  • 'Orlando Gold' is resistant to Alternaria.

Chemical control

If the disease is common, spraying may be necessary. Formulas for timing sprays have been developed for this disease. Seed-to-seed crops-apply once in fall and once or twice in spring. Steckling production-apply once in fall and once to twice after transplanting in spring.

  • Carboxamide (Group 7) formulations are registered for use. Do not make more than two (2) sequential applications before alternating to a labeled fungicide with a different mode of action.
    • Endura at 4.5 oz/A on 7- to 14-day intervals. Preharvest interval is 0 days. 12-hr reentry.
    • Fontelis at 16 to 30 fl oz/A on 7- to 14-day intervals. Preharvest interval is 3 days. 12-hr reentry.
  • Cevya (Group 3) at 3 to 5 fl oz/A on minimum interval of 7 days. Preharvest interval is 7 days. 12-hr reentry.
  • Chlorothalonil (Group M5) formulations are labeled.
    • Bonide Fung-onil is available for home gardens. H
    • Bravo Ultrex at 1.4 to 1.8 lb/A on 7- to 10-day intervals. 12-hr reentry.
    • Echo 720 at 1.5 to 2 pints/A on 7- to 10-day intervals. Preharvest interval is 0 days. 12-hr reentry.
  • Copper products (Group M1) are not recommended as stand-alone materials.
    • Badge SC at 1 to 4 pints/A on 7- to 14-day intervals. Preharvest interval is 0 days. 48-hr reentry.
    • Bordeaux 6-6-100.
    • C-O-C-S WDG at 1 to 1.9 lb/A. 48-hr reentry.
    • Cueva at 0.5 to 2 gal/100 gal water on 7- to 10-day intervals. May be applied on the day of harvest. 4-hr reentry. O
    • Cuprofix Ultra 40 Disperss at 1.25 lb/A on 7- to 14-day intervals. 48-hr reentry.
    • Kocide 2000 at 1.5 lb/A or Kocide 3000 at 0.75 to 1.5 lb/A on 7- to 14-day intervals. 48-hr reentry. O
    • Previsto at 1 to 2 quarts/A on 7- to 14-day intervals. 48-hr reentry. O
  • Inspire Super (Group 3 + 9) at 16 to 20 fl oz/A on 7- to 10-day intervals. Make no more than two (2) sequential applications. Preharvest interval is 7 days. 12-hr reentry.
  • Iprodione formulations (Group 2) are labeled.
    • Iprodione 4L AG at 1 to 2 pints/A in a minimum of 10 gal/A water on 7- to 14-day intervals. Preharvest interval is 0 days. 24-hr reentry.
    • Meteor at 1 to 2 pints/A in a minimum of 10 gal/A water on 7- to 14-day intervals. Preharvest interval is 0 days. 24-hr reentry.
    • Rovral 4 Flowable at 1 to 2 pints/A in a minimum of 10 gal/A water on 7- to 14-day intervals. Do not make more than four (4) applications of this product per season. Preharvest interval is 0 days. 24-hr reentry.
  • Mancozeb formulations (Group M3) are labeled for seed crops only.
    • Dithane DF Rainshield at 2 lb/A on 7- to 10-day intervals can be used on seed crops only. Washington and Oregon only (SLN WA-220008, SLN OR-020030). 24-hr reentry.
    • Roper DF Rainshield at 2 lb/A on 7- to 10-day intervals can be used on seed crops only in Oregon (SLN OR-130003) and Washington (SLN WA-1300003) only. 24-hr reentry.
  • Merivon Xemium (Group 7 + 11) at 4 to 5.5 fl oz/A on 7- to 14-day intervals. Do not make more than two (2) applications before alternating to a fungicide in different FRAC groups (non-Group 7 and non-Group 11). Preharvest interval is 7 days. 12-hr reentry.
  • Miravis Prime (Group 7 + 12) at 6.8 fl oz/A on 7-day intervals. Do not make more than two (2) sequential applications before alternating to a labeled fungicide with a different mode of action. Preharvest interval is 7 days. 12-hr reentry.
  • OSO 5% SC (Group 19) at 3.75 to 13 fl oz/A on 7- to 14-day intervals. Can be applied the day of harvest. 4-hr reentry. O
  • Strobilurin fungicides (Group 11) are labeled for use. Do not make more than one (1) application of a Group 11 fungicide before alternating to a labeled fungicide with a different mode of action.
    • Cabrio EG at 8 to 12 oz/A on 7- to 14-day intervals. Preharvest interval is 0 days. 12-hr reentry.
    • Pristine at 8 to 10.5 oz/A on 7- to 14-day intervals. Preharvest interval is 0 days. 12-hr reentry.
    • Quadris Flowable at 6 to 15.5 fl oz/A or Quadris Opti at 2.4 pints/A on 7- to 14-day intervals. May be applied the day of harvest. 4-hr reentry for Quadris Flowable, 12-hr reentry for Quadris Opti.
    • Quilt Xcel at 14 fl oz/A on a 7- to 10-day interval. Do not apply within 14 days of harvest. 12-hr reentry.
  • Switch 62.5WG (Group 12 + 9) at 11 to 14 oz/A on 7- to 10-day intervals. No more than two (2) sequential applications alternating with another fungicide with a different mode of action for two (2) applications. Do not allow cattle or other livestock to feed upon leaves of vegetables. Preharvest interval is 7 days. 12-hr reentry.
  • Tilt (Group 3) at 4 fl oz/A on 7- to 10-day intervals. Preharvest interval is 14 days. 12-hr reentry.
  • Trilogy at 0.5% to 1%. Not labeled for use in Oregon. Do not use above 90°F or when plants are under heat or moisture stress. Do not use when foliage is wet as good coverage is essential. Poor control as a stand-alone product. 4-hr reentry. O

Biological control

  • Double Nickel LC at 0.5 to to 4.5 pints/A on 3- to 10-day intervals. Can be applied the day of harvest. 4-hr reentry. O
  • LifeGard WG (Group P6) at 1 to 4.5 oz/A on 7- to 14-day intervals for activating plant resistance. Gave good control when used in a fungicide program in rotation with other active ingredients while had fair control when used alone or in rotation with a copper formulation in studies conducted by the University of Georgia. Refer to label for appropriate rate per application volume. Preharvest interval is 0 days. 4-hr reentry. O
  • Serenade Opti at 14 to 20 oz/A on 7- to 10-day intervals. Applications can be made up to and the day of harvest. 4-hr reentry. O

References Dutta, B., Donahoo, W.M., and Foster, M.J. 2019. Evaluation of fungicides for managing alternaria leaf blight on carrot, Georgia 2018. Plant Disease Management Reports 12:V127.

Gugino, B.K., Carroll, J.E., Widmer, T.L., Chen, P., and Abawi, G.S. 2007. An IPM program for managing fungal leaf blight diseases of carrot in New York. Plant Disease 91:59-65.