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

Materials Approved for Organic Production:
Coniothyrium minitans (Contans). Contans is a fungus that, once applied and incorporated
into the soil, attacks and destroys the white mold sclerotia. Follow the directions on the label
carefully because success is dependent on eliminating near-surface sclerotia that are likely
to germinate and produce apothecia and spores. Using enough of the material to reach all
of the sclerotia in the soil profile would be cost prohibitive. Contans requires 3 to 10 weeks
to effectively colonize and destroy sclerotia. Apply Contans to a Sclerotinia-infested field
immediately following harvest at 1 lb/A, and incorporate the debris into the soil. Alternatively,
apply 3-4 months before the onset of disease at 2 lb/acre, followed by shallow incorporation (or
irrigate) to about a 1 to 2 inch depth. After application, do not till deeply, or sclerotia that are
deeper than the Contans treated zone will be brought to the surface.
CRATER ROT (Rhizoctonia carotae)
Crater rot is a post harvest disease of carrots in long-term storage. It is caused by a soilinhabiting
fungus that is thought to be able to survive indefinitely in the soil without
a host. Rhizoctonia carotae infection takes place in the soil or possibly in contaminated
storage containers. Very high humidity or a film of water on the carrots enhances disease
development.
The infection is most likely to take place in the field, though symptoms do not appear until
later in cool, high humidity storage. The first symptoms are small, white knots of fungal
growth. Small pits appear under these spots and enlarge into dry, sunken craters (Photo 9.11).
White patches of mycelia spread, and sometimes a whole crate of carrots may be covered with
a cottony mycelium that can be confused with white mold. Microscopic examination may be
required to positively identify the disease.
Cultural Control:
1. Cultural practices that encourage quick drying of the soil surface, such as wide
spacing, shallow cultivation, and good weed management, will lower incidence of field
infection.
2. After harvest, keep carrots cold (32 o F) with humidity no higher than 95%.
3. Clean and disinfect storage containers.
Materials Approved for Organic Production:
There are no materials approved for organic production.
CROWN ROT (Rhizoctonia solani)
Rhizoctonia solani may cause damping off of carrot seedlings shortly after germination when
the soil is wet and warm. It can cause sporadic crown rot infections, especially when conditions
are warm and wet at harvest. Petioles and crown tissues develop cankers that may penetrate
several millimeters into the root. The dark brown lesions are more common near the crown,
and later a dry rot may develop. A thin, white to tan layer of mycelia may grow near the crown,
and when carrots are pulled, it holds a clump of soil to the carrot. Secondary invasion by
bacteria may cause a soft rot.
This pathogen can survive many years in the soil, as a saprophyte or as sclerotia, and infect
carrots at any point in the growing season. Decay may not be noticed until the carrots are
harvested and in storage.
Cultural Control:
1. Avoid injury to the crown, or hilling soil over the crown when cultivating.
2. Encourage quick drying of the soil surface with wide row spacing, shallow cultivation,
and good weed management.
Materials Approved for Organic Production:
There are no materials approved for organic production.
Organic Resource Guide 79