[Carrot leaf blight]
This is one of the most important seed-borne diseases. It is common in carrots and causes major problems in both conventional and organic crops in wet weather conditions, which are conducive to epidemic development.
Black rot is an important disease in carrot cultivation worldwide. In cooler regions, there may be only limited damage if the outbreaks do not occur until late in the season.
Symptoms and diagnosis
The initial symptoms are difficult to detect, as they often occur on young plants the moment the first leaves begin to die off. Emerging seedlings incidentally display signs of infection and there is some loss of seedlings with 'white mycelium’ symptoms. Angular, watery or brown lesions develop along the leaf edges and sometimes have a yellow halo. The lesions increase in size and number, leading to necrosis of part of the leaf followed by the whole leaf dying off. Fairly large dark lesions develop on the petioles and can result in the leaf dying off. In the case of an epidemic, noticeable areas with brown, yellow and dead leaves are visible in the crop, which eventually grow towards each other and attack the entire crop. It is important to check the young green leaves for characteristic brown lesions, as there are other possible causes of general browning of a crop. The diagnosis should be confirmed through microscopic examination of the lesions for spores, as Cercospora (Cercospora carotae) can cause practically identical symptoms. The roots are usually not infected. In seed crops, symptoms will spread to the top of the plant, also infecting the flowers and seeds.
Conditions for disease development
Infected seed is often the most important source of infection. In the case of sowing seed, sometimes more than 50% of the seed is infected. The disease is spread via spores on the surface of the seed and as mycelium in the seed coat. It is also spread via crop residue in the soil. During the growing season, dispersion is also possible by means of airborne spores. Spores need a wet surface to germinate, and at temperatures of between 16-25 °C can infect a leaf within 12 hours. However, at lower temperatures (<10 °C), it is only after a leaf wetness period of several days that the disease really develops well. At temperatures of >20 °C, a period of 5 to 10 hours will suffice. The disease cycle lasts just 8-10 days and epidemics can develop quickly during hot, wet weather.
Impact and importance
Leaf drop in the growing season reduces the yield. Losses will increase if the carrots cannot be harvested with a top-lifting harvester. Problems in one seed lot can lead to problems in other carrot crops if dispersion to other crops occurs during the growing season. The disease can be controlled with foliar fungicides, but that will require a programmatic approach with various active ingredients. Fungicide resistance problems are a source of concern.
Controlling seed-borne Alternaria dauci by means of seed testing and seed treatments should deliver immediate results for carrot growers.