Aug 25, 2021

Some tips to control fungal leaf blights of carrot

Powdery mildew, alternaria and cercospora are three important fungal foliar pathogens that can cause early defoliation in carrots, thus reducing yields and making harvest difficult. Each pathogen produces distinct symptoms.

Powdery mildew causes characteristic white, powdery lesions on foliage. Symptoms of alternaria include irregular, dark brown to black spots which typically show up on older leaves first. Cercospora leaf spots are round, grayish-brown and are more prevalent on younger foliage. Both leaf blights typically start at the margins of leaflets and as more spots develop leaflets begin to wither and die. Symptoms similar to leaf infections can develop on stems and petioles.

Control of both diseases begins with regular scouting and preventative fungicide applications on susceptible varieties. For processing crops or situations when the crop is not being marketed with its foliage, a 25% disease incidence threshold may be used to time the first fungicide application. Scout carrot fields by variety. While walking across the field in a ‘V’ or ‘W’ shaped transect, evaluate disease incidence on five leaves from five adjacent plants in a minimum of 10 locations.

A leaf is infected if one or more fungal leaf blight lesions are observed. When twelve of the fifty leaves scouted show symptoms (~25%) then apply the first fungicide spray. Subsequent sprays can be based on the label recommended spray interval or on increased disease severity.

Under severe defoliation, add urea (10.0 lbs/A) to encourage new leaf growth. Alternate Fontelis1.67SC (penthiopyrad, 7) at 16.0 to 30. fl. oz/A, azoxystrobin 2.08F (11) at 9.0 to 15.5 fl. oz/A, or Cabrio 20EG (pyraclostrobin, 11) at 8.0 to 12.0 oz/A, or Pristine 38WG (pyraclostrobin + boscalid, 11+7) at 8.0 to 10.5 oz/A with chlorothalonil (M5) at 1.5 to 2 pt/A. For Alternaria only tank mix one of the following with 1.5 pt/A chlorothalonil (M5): Endura 70W (boscalid, 7) at 4.5 oz/A, Inspire Super 2.82EW (difenoconazole + cyprodinil) at 16.0 to 20.0 fl. oz/A, Merivon 2.09SC (fluxapyroxad + pyraclostrobin) at 4.0 to 5.5 fl. oz/A, or Switch 62.5WG (cyprodinil + fludioxonil, 9 +12) at 11.0 to 14.0 oz/A.

Do not make more than one sequential application of Quadris, Pristine or Cabrio (FRAC code 11). For more information on tolerant varieties and control please see the 2020/2021 Mid-Atlantic Commercial Vegetable Production Recommendations.

For organic growers, there are several varieties such as Bolero, Calgary, Carson that exhibit tolerance to leaf blight and should be grown if available. If foliar blights have been a problem in the past, avoid seeding carrots in those areas of the farm. Seeding rates that allows for more air flow and space between plants will help foliage dry out much quicker may help reduce potential problems. Keep rows free of weeds. Organic copper applications following the above scouting guidelines may also help suppress foliar blight development.

– Andy Wyenandt, Rutgers University