Resource Guide for Organic Insect and Disease ... - Cornell University

Materials Approved for Organic Production:
Serenade and copper-based products offer some reduction in severity of the disease, but have
not been shown to reduce the percentage of plants that become infected.
CERCOSPORA LEAF BLIGHT (Cercospora carotae)
Cercospora leaf blight is often more destructive than Alternaria because it affects young tissue
as well as older tissue and may appear earlier in the growing season. The first symptoms are
spots that turn tan with a very dark rim and a chlorotic halo (Photo 9.9). Infections along the
edge of the leaf are somewhat crescent-shaped and follow the leaf margin while spots in the
center of the leaf are circular. As the spots enlarge, they coalesce, and the whole leaf turns
brown, withers, and dies. Both leaves and petioles are affected. The disease does not affect the
root. Cercospora is carried on infected seed, crop debris, and wild carrot weeds. The spores are
generally dispersed by wind but can also be transported in splashing water and on farm equipment.
Spore germination is dependent on at least 12 hours of leaf wetness.
Cultural Control:
1. Use clean seed.
2. Fall plowing of crop debris and a two to three year crop rotation is recommended if
Cercospora was a problem during the previous year.
3. Manage weeds to improve air flow and avoid alternate hosts.
4. Use less susceptible varieties of carrots if Cercospora leaf blight is a common problem.
Materials Approved for Organic Production:
Serenade and copper-based products offer some reduction in severity of the disease, but have
not been shown to reduce the percentage of plants that become infected.
SCLEROTINIA ROT/COTTONY ROT/PINK ROT (Sclerotinia sclerotiorum)
Sclerotinia sclerotiorum can affect all of the umbel crops. It is the same pathogen that causes
white mold, which affects many vegetables, including tomatoes, beans, lettuce, peppers, and
many weeds. It is the most destructive disease of stored carrots. Infected roots often show
no symptoms in the field, but the disease develops in storage. In stored carrots, Sclerotinia
causes a dark-colored, soft, watery rot that quickly becomes covered with a very white, cottony
growth of fungal mycelium (Photo 9.10). Black sclerotia form amid the mycelium. In the field,
the base of the plant may develop symptoms, and the crown, petiole, and leaves may become
dark and covered with the characteristic white, cottony mycelium and sclerotia.
Sclerotia can survive for more than a decade in the soil. When conditions are conducive
(saturated soils for more than two weeks), they may germinate and produce fruiting bodies
called apothecia, which produce millions of spores that become airborne. Spores that land
on senescing tissue germinate. Healthy tissue near the infected area can then be colonized if
wet conditions persist for more than two days. The infection can then spread throughout the
plant, including the roots. Roots are rarely directly infected by mycelium in the soil; however,
infection can take place in storage if healthy roots are in contact with infected roots, infested
boxes, or equipment.
Cultural Control:
1. Good weed management is essential since the high relative humidity under heavy
weed pressure is conducive to spore production, germination, and infection of crop
tissue. Furthermore, many weeds are hosts for Sclerotinia sclerotiorum.
2. Crop rotation for three years with non-host crops such as onions, beet, spinach, corn,
and grass family green manures will help to maintain low levels of sclerotia in the soil.
3. All cultural practices that reduce the duration of leaf wetness will reduce spore survival
and tissue infection.
4. The key to reducing the disease in storage is to harvest when cold, cull and clean crop,
sanitize containers, and keep storage temperatures at 32 o F.
78 Organic Resource Guide