A Detailed Guide to Controlling the Three Major Diseases of Carrots
Carrot, alias red radish, clove radish, Hulufujin, also known as Hulufu, red beet, yellow radish, etc., is an umbelliferous biennial herbaceous plant, with its fleshy root eaten as a vegetable.
The three most common diseases of carrots: carrot black spot , carrot leaf spot , and carrot soft rot. These three major diseases severely affect the production efficiency of carrots, so what are the symptoms of the three major diseases of carrots, what are the causes, what are the conditions for onset, and how should they be controlled? Let us introduce them below.
Carrot black spot
Carrot black spot is one of the major diseases nationwide. When seed plants are affected, it causes reduced yield and poor quality. Symptoms identification: onset on leaf stalks and stems. Leaves are mostly affected starting from the leaf tip or leaf margin, producing small brown spots with yellow halos. After expanding, they become irregular dark brown, with light brown spots inside; after covering the leaves, the leaf margins curl upward and turn yellow and dry from the lower part. When humid, the spots are densely covered with black mold (conidia). When stems and flower stalks are affected, oblong dark brown slightly sunken spots are produced, making them easy to break at this point; seeds are not plump and carry the pathogen.
Cause: caused by Alternaria dauci. The pathogen overwinters on seeds and diseased residues. The seed-borne rate is high, causing seedling damping-off. The following year, spores growing on new spots are spread by airflow and directly invade through stomata, wounds, or the epidermis.
Conditions for onset: the optimum temperature for onset is around 28°C; disease does not occur below 15°C or above 35°C. In the south, onset occurs in July and August. Lack of fertilizer and weak growth aggravate the disease.
Control methods:
1. Treat seeds with 0.3% of 50% iprodione wettable powder, or 50% thiram wettable powder, or 40% seed dressing dual, then sow after seed dressing.
2. Implement a 2-year crop rotation.
3. Increase base fertilizer and topdressing.
4. In the early stage of onset, spray 50% iprodione wettable powder at 1500 times dilution, or 75% chlorothalonil wettable powder at 600 times dilution, or 70% mancozeb wettable powder at 600 times dilution, once every 7-10 days, for 2-3 consecutive treatments.
Carrot leaf spot
Carrot leaf spot is a common disease on carrots, sometimes epidemic during the high-temperature period in summer and autumn. When seed plants are affected, it affects seed yield and quality.
Symptoms identification: onset on leaves, leaf stalks, and stems. Young leaves initially produce circular or nearly circular 2-4 mm brown spots, which expand into spindle-shaped or polygonal forms, with a grayish-brown center and slightly raised edges. When humid, gray mold appears on both the front and back. When severe, the spots connect into patches, and leaves die from the lower part. Brown sunken spots are produced on both leaf stalks and stems. This disease produces gray mold on the spots, which can distinguish it from black spot. In the field, it often occurs mixed with black spot.
Cause: caused by Cercospora carotae. The pathogen overwinters inside and outside seeds and on diseased residues on the soil surface. The following year, spores are produced and spread by wind and rain.
Conditions for onset: occurs during high-temperature periods in summer. With early lack of fertilizer and drought, plants grow weakly, and afterward, onset is severe when there is rain, heavy fog, or poor drainage.
Control methods:
1. Treat seeds with 0.3% of 50% iprodione wettable powder, or 50% thiram wettable powder, or 40% seed dressing dual, then sow after seed dressing.
2. Implement a 2-year crop rotation.
3. Increase base fertilizer and topdressing.
4. In the early stage of onset, spray 50% iprodione wettable powder at 1500 times dilution, or 75% chlorothalonil wettable powder at 600 times dilution, or 70% mancozeb wettable powder at 600 times dilution, once every 7-10 days, for 2-3 consecutive treatments.
Carrot soft rot
Carrot soft rot, also called rot disease, is a common disease, occurring more in the south. Onset is severe in seed plants.
Symptoms identification: mainly harms the fleshy root. In chronic onset in the field, some leaves first turn yellow and then wilt. In acute onset, the whole plant suddenly wilts and turns green and withered. Examining diseased plants, the root crown shows water-soaked softening, the fleshy part turns grayish-brown, softens, and rots, and the exuding juice gives off a foul odor.
Cause: caused by Erwinia carotovora, the same as cabbage soft rot.
Conditions for onset: the pathogen invades through natural wounds, mechanical wounds, insect wounds, and other openings. Wounds are often caused by heavy rain after long-term drought, storms, intertillage loosening of the soil, and damage by underground and aboveground pests. Onset is severe in summer and autumn with high temperature and abundant rain and in poorly drained land.
Control methods:
1. Implement a 2-year rotation with field crops, or a paddy-upland rotation.
2. Use high-ridge direct seeding cultivation; it should not be too dense, ventilation should be good, and drainage should be strengthened,
3. Control aboveground and underground pests early.
4. After onset, appropriately control watering.
5. Remove and dispose of diseased plants when found.
6. In the early stage of onset, spray 72% agricultural streptomycin sulfate soluble powder at 4000 times dilution, or 14% copper ammonia complex aqueous solution at 300 times dilution, or Fengling at 200 times dilution, once every 7-10 days, for a total of 2 sprays.