Black Rot (Black Crown)

Black rot of carrots caused by Alternaria radicina starts in the crown with black lesions on the base of the petioles. The lesions may extend down into the root, causing the characteristic “black crown”. The upper part of the carrot root will have a dry, dark rot, which occasionally may extend into the lower part of the carrot root. Affected tops often break during mechanical harvesting leaving the roots in the ground and reducing yields. Although all crops growth stages can be infected, older, mature carrots are particularly susceptible. Black rot can also be a problem in storage.

Under certain conditions, the fungus also can cause a leaf blight, which is characterized by dark brown lesions along the leaf margins. The fungus can also cause a pre- and postemergence damping-off of carrot seedlings.

Alternaria radicina is seedborne and may be spread on carrot seed. The fungus is favored by sprinkler irrigation or rainy weather and high temperatures, which may predispose tissue to infection. It also survives in carrot debris and in the soil for several years.Black rot is important only in certain areas or regions in California. It is common in coastal mountain valleys, such as the Cuyama Valley.

Plant Alternaria-indexed seed or treat seed in a hot water dip; this is especially important in fields where black rot has not been identified or carrots have not been grown.

Furrow rather than sprinkler irrigation may reduce disease development. Crop rotation is highly recommended to prevent buildup of the fungus in the soil. Deep tillage may provide some control by burying inoculum of the fungus away from the carrot crown.

Use cultural controls in a certified organic crop.

Assay your seed, if pathogen is found, treat the seed. Foliar applications of fungicides to control black rot are marginally effective.