SCLEROTINIA HEAD ROT OF CABBAGE Marc A. Cubeta, Extension Plant Pathologist Bryan R. Cody, Agricultural Research Technician Joseph Hudyncia, Research Technician USDA/ARS, Raleigh

Vegetable Disease Information Note No. 25
Department of Plant Pathology
North Carolina State University

INTRODUCTION

Head rot of cabbage, often referred to as "white mold", is caused by the soil fungus Sclerotinia sclerotiorum. In North Carolina, Sclerotinia head rot may occur during the fall and spring production season when cool, wet weather conditions favor disease development. Sclerotinia head rot can limit the production of healthy cabbage and result in serious economic losses by reducing the marketability of cabbage heads. Although losses to this disease occur primarily in the field, losses also can occur in storage and during transport to market.

LIFE HISTORY

The fungus S. sclerotiorum causes disease when air temperatures are cool (55 to 77 F) and periods of wet weather associated with rain, high relative humidity and heavy dew occur together. In addition to cool, wet weather conditions, the fungus requires readily available source of nutrients to cause disease. Nutrients for fungal growth, infection and disease development are often obtained from dead (senescent) or wounded cabbage leaves. Wounds can be caused by chemical injury (fertilizers, insecticides and herbicides), farm cultivation equipment, frost and insect damage.

The fungus survives from season to season as hard, black, "raisin-like" structures called sclerotia that are usually produced on infected cabbage heads during the growing season. As infected leaves of cabbage heads dry out, mature sclerotia fall to the ground and become incorporated into soil during cultivation. Once formed, sclerotia can survive many years in soil. Sclerotia can cause disease in two ways in the following spring and fall. Dead or wounded lower leaves of cabbage that are laying on the soil surface near overwintering sclerotia can become infected by the fungus. A second, more common way that the fungus causes disease is when overwintering sclerotia located near the soil surface form small, tan to orange colored fruiting structures called apothecia. Each apothecium can produce millions of spores, called ascospores, that are released, carried by wind and deposited on cabbage leaves. Spores of the fungus are usually present during most of the growing season. When cool, wet weather conditions occur, spores germinate and usually infect wrapper leaves on the top and sides of a cabbage head. These spores also are capable of infecting many plants other than cabbage, particularly weeds. Ragweed flowers and stems infected with the fungus can fall on or come in contact with cabbage heads and frequently cause infection in commercial production fields in North Carolina. More than 408 different plant species can be infected by S. sclerotiorum.

SYMPTOMS AND SIGNS

Symptoms often first appear as water soaked spots on lower or upper cabbage leaves. As water soaked spots enlarge, infected tissue becomes soft, and some outer leaves begin to wilt. A white cottony growth becomes evident on the leaves as the disease progresses. Eventually the entire cabbage head is covered with this white cottony growth and the overwintering sclerotia begin to form. Initially sclerotia are white and round and later become black, hard and irregular in shape. The black sclerotia become visible on the outside of the cabbage head, but they also form inside the head.

MANAGEMENT

Sclerotinia head rot is difficult to manage and requires an integrated approach. Sclerotinia head rot resistant cabbage varieties are not available, and no fungicides are currently registered for use on cabbage in North Carolina to manage this disease. A 3 to 4 year rotation with a nonsusceptible corn or small grain crop, which is practiced by most North Carolina cabbage growers, may provide some relief. However, rotation is often of limited value because sclerotia can survive up to eight years in soil and spores may be blown by wind from nearby fields. In cabbage production areas outside of North Carolina, flooding fields for several months has been a very effective practice for reducing the survival of sclerotia. If possible, select fields without a history of severe Sclerotinia head rot disease.

Proper management of weeds and insects can reduce the occurrence of Sclerotinia head rot. Wounds associated with insect feeding, herbicides and cultivation should be minimized. Since the fungus can infect weeds, particularly ragweed, good weed management is also recommended. Cultural practices such as planting in a well-drained soil away from the edges of a tree line and orienting rows in the direction of prevailing winds will help to reduce production of apothecia and spores by creating drier soil moisture conditions.

MANAGEMENT SUMMARY:

1. Rotation, minimum of 3-4 years.
2. Minimize wounding.
3. Plant in well-drained soil.
4. Avoid fields with a severe disease history.

This page (http://www.ces.ncsu.edu/plymouth/pubs/headrot.html) was created by Marc A. Cubeta, Ph.D. Extension Plant Pathologist and Bruce A. Emmons, Area Specialized Agent, Information Management. The authors would like to acknowledge Drs. David Shew and Steve Koenning, Department of Plant Pathology, North Carolina State University, Raleigh, NC for reviewing this publication and their many valuable comments and suggestions.

Date Created 03/21/97.
Last revised on 08/09/01.