Introduction
This is an important seed-borne disease in Umbelliferae vegetables that damages carrots, parsnips, celery, celeriac and various related herb crops. The disease can also be soil-borne. N.B.: other Alternaria species may also occur in carrots, since incidence of A. carotiincultae (which has larger spores) in carrots has also been reported in the US. In celery, A. petroselini is possibly more virulent than A. radicina.
Geographic distribution
Black rot is an important disease in all parts of the world where carrots are grown, especially when harvested crops are stored.
Symptoms and diagnosis
The most important symptoms are black lesions at the base of the petioles and at the crown of the carrot. This can cause the foliage to die and incidentally make harvesting with a top-lifting harvester impossible. In storage, the disease causes dry black rot that can spread over practically the entire surface of the carrots and to healthy carrots. Similar symptoms are also visible on the crown, the underside of the petioles and the roots of celery and celeriac. The disease causes white mycelium in seedlings, which causes the roots and stem to turn black. Small brown spots with chlorotic edges appear on the leaves. These spots spread and coalesce until the leaves are completely black. Severe infections of seed crops can cause umbel blight, impeding seed development.
Conditions for disease development
Infected seed is often the most important source of infection. The disease is spread via spores on the surface of the seed and as mycelium in the seed coat. It is also transmitted via the soil, where it can survive in crop residue or as microsclerotia or spores. As with other Alternaria species, long leaf wetness periods and hot temperatures (>20°C) are conducive to infection via airborne spores. Older leaves are the most vulnerable. Infection on the underside of petioles can lead to infection of the crown. In the field, carrots often show little sign of infection, but they can be seriously affected when stored in cold places. It is important to minimise damage during harvesting and to store at low temperatures (0-1 °C) with a high relative humidity.
Impact and importance
This disease has the greatest impact on stored carrots. Damaged carrots are unsaleable. Some loss can also occur in other Umbelliferae vegetables, such as celery or herbs, particularly if there are high levels of the pathogen on seeds. A. radicina can survive in the soil for years and the use of infected seed can lead to recurrent problems with crop rotation. Eliminating any sources of Alternaria spp. on seeds is extremely important if carrots are grown to be stored after harvesting.
