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Plantwise Technical Factsheet

black rot of carrots (Alternaria radicina)

Host plants / species affected
Daucus carota (carrot)
List of symptoms/signs
Roots  -  necrotic streaks or lesions
Symptoms
This fungus causes a dry black rot of carrot, both crown and root. The primary lesions of older plants begin at the base of the petiole whence a dark, usually shallow, lesion spreads into the crown and sides of the root. Plants that are severely attacked wither completely and may become covered with olive-black patches of conidia. Secondary lesions develop below ground and are often coincident with cracks and splits. A dry mealy rot may develop in storage. On the flowering umbels the fungus produces a rot or drying such that seeding fails to materialize or the maturing seeds themselves are infected. On seedlings, it causes a damping off with the development of black moist rot of the root and hypocotyl. It also affects the seed, reducing germination and attacking the seed before germination (Neergaard, 1945).
Prevention and control
Cultural Control and Sanitary Methods

The black rot fungus can overwinter in debris in the soil from the previous crop and is thus potentially able to infect new plants in the following year, although Maude (1966) showed that this is not a significant source of infection for ware-carrots, but did affect the seed crop severely. How long the pathogen remains in the soil is not known; a rotation of 4 years is, however, recommended to reduce reinfection. The effect of temperature can also prove useful: Tahvonen (1978) noted that the incidence of damping off due to A. radicina and A. dauci could be reduced by germinating the seed at 13°C rather than at 25°C.

Host-Plant Resistance

A distinction is made by some authorities between the eastern and western races of the cultivated carrot. The eastern race (sometimes referred to as subsp. orientalis) have anthocyanins in the roots whereas the western race (subsp. occidentalis) have carotenes. The latter has a greater degree of resistance to A. radicina, however, there appeared to be some variation in the varietal types of the western race (Vlasova and Fedorenko, 1986). Particularly resistant to A. radicina are those belonging to the Amager varietal type (Vintsunas, 1980; 1986). Breeding work has been carried out to improve resistance in varietal types Nantes and Chantenay (Sycheva, 1985). Work on transferring chitinases to aid resistance to disease has been mixed: Punja and Raharjo (1996) found no detectable differences in disease resistance to A. radicina between transformed and control plants but Melchers et al. (1994) demonstrated, in vitro, that class V chitinases exhibited antifungal activity towards A. radicina.

Chemical Control

Due to the variable regulations around (de-)registration of pesticides, we are for the moment not including any specific chemical control recommendations. For further information, we recommend you visit the following resources:

Impact
The economic impact of A. radicina is in two areas: first, and most critically, in the storage of harvested carrots; second, in the reduction of seed quality due to infection. It was as a storage problem that the disease was first observed in the USA (Meier et al., 1922) and its economic impact noted by Lauritzen (1926) although Cam et al. (1993) rated it as only of secondary importance as a spoilage agent. The main difficulty is that the roots often show no sign of infection when lifted but the rot develops whilst in storage. Lauritzen reported that in one extreme case 62% of a 500 kg batch of carrots were infected by black rot. Neergaard (1945) also recorded the impact of black rot on seed production over a period of 10 years with an estimation of the monetary value of the losses. The disease attacks the flowers, thereby lowering seed production and it infects the seeds, lowering seed quality and reducing the percentage germination. There are, however, no recent figures on losses caused by this fungus.
Related treatment support
 
External factsheets
Pestnet Factsheets, Pestnet, English language
PlantVillage disease guide, PlantVillage, English language
TNAU Agritech Portal Crop Protection Factsheets, Tamil Nadu Agricultural University, English language
University of California IPM Pest Management Guidelines, University of California, 2012, English language
Department of Agriculture and Food Western Australia Farmnotes, Government of Western Australia, 2005, English language
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