Disc-shaped lesions can also enlarge during storage. 1,4 Sometimes lesions are not visible when the carrots are harvested but may cause losses later on in storage. 1,2,3 There are no foliar symptoms, and the disease may not be detected until the roots are near marketable size, as the roots must be pulled and examined to see the symptoms. Secondary pathogens can colonize the affected tissues and increase the size of the cavities. Initially, the lesions appear gray but become darker as they enlarge and develop into cavities. The lesions can develop on any part of the taproot, but they most commonly occur on the upper third of the root. These spots develop into elliptical or irregular, depressed lesions that are a half-inch or larger in diameter and oriented across the taproot. 1,2Ĭavity spot symptoms start as yellow, pinpoint spots on the taproot about twelve weeks after planting. Incidence rates of 10 to 20% can cause whole loads to be rejected during grading, and severely affected fields are sometimes abandoned. The disease may not have a major impact on total yield, but it can reduce marketable yield resulting in substantial economic losses. Cavity spot can occur in fields with no prior history of carrot cultivation. violae is the most common and has a broader host range than P.sulcatum. Carrot varieties with some resistance to black rot are available.Ĭavity spot is caused by the water mold organisms Pythium violae, Pythium sulcatum, and other Pythium species. Fungicides can be applied to plants in the field however, the applications are often only marginally effective because of the difficulty in reaching the target areas of the plant with the fungicide sprays. Burying inoculum through deep tillage can help lower rates of infection. Prompt destruction of crop debris after harvest can help speed the decomposition of the infested tissue and help lower inoculum levels. 1,2,3 Once the pathogen is present in a field, three to four-year rotations to non-host crops can help lower inoculum levels and prevent a buildup of the pathogen. Treating seeds with fungicides, such as iprodione, can also help eliminate seedborne inoculum. This is especially important when planting in fields with no history of the disease to prevent the introduction of the pathogen into the field. Older, senescing tissues are also very susceptible to infection.Ī primary method for managing black rot is to plant disease-free seeds. Infection is favored by wet conditions (rain or overhead irrigation) when temperatures are over 68☏ (20☌). 1,2,3 Carrot plants can be infected at any stage, but young seedlings are particularly susceptible.

The fungus can also survive for several years in infested crop debris and in the soil. The black rot fungus can be seedborne, mainly on the seedsurface. The black lesions reduce the marketability of the harvested roots. The rot can continue to spread downward on the root, with black, sunken lesions developing below the soil surface. 1,2,3 The dark rot spreads to the crown and the upper part of the taproot. Some fields are abandoned before harvest if disease severity is high. The infected petioles can break off when the carrots are mechanically harvested, leaving the taproot in the ground, thus reducing yield. The spots enlarge, and the affected tissues darken and decay. Small, chlorotic spots first appear on the petioles. 1,2īlack rot causes necrosis of the roots and crown, and infection usually starts where the leaf petioles attach to the crown.

While the disease mainly affects the crown and upper taproot, the pathogen can also cause seedling damping-off and blighting of the leaves and umbels. Black rot, caused by the fungus Alternaria radicina, occurs worldwide in most carrot-producing regions.

Carrot seedlings damping off