Causes and Control Methods of Black Rot in Carrots

Carrots are one of the nutritious vegetables we commonly eat, and they are especially suitable for children and the elderly. Carrots are grown in all parts of China, and during cultivation they are highly susceptible to black rot, which leads to reduced yields and other problems. So what causes black rot in carrots? How can it be controlled?

I. What causes black rot in carrots?

Black rot in carrots shows different symptoms at different stages. Infection during the carrot seedling stage can cause damping-off of carrot seedlings that have not yet emerged or of seedlings at the seedling stage; older leaves of mature plants are susceptible, and when the disease occurs, black spots first form on the leaf stalks and then spread to the leaf tips, causing the entire leaf to die. In severe cases, it spreads to the crown of the carrot root, forming a black rotting ring (that is, black crown); after infection during storage, carrots develop dry, black, sunken lesions, and under warm, humid storage conditions the pathogen spreads rapidly among the carrots.

In the field, the primary sources of infection are contaminated soil, diseased plant residues, and contaminated seedlings. The pathogen can survive as spores in crop residues or soil for more than 8 years, and spreads through the movement of contaminated soil and diseased plants, through flowing water, or through the wind.

II. Symptoms of black rot in carrots

Black rot in carrots mainly harms the fleshy root, leaves, and stems, forming sunken black spots; in severe cases the spots rot, seriously affecting marketability and edibility. The infectious agent belongs to the fungi of the subphylum Deuteromycotina. It overwinters on the soil surface through mycelium and conidia along with diseased plant residues.

III. How can black rot in carrots be controlled?

Choosing rotation plots, avoiding continuous successive cropping, clearing diseased plant residues from the field, sterilizing the soil, choosing disease-resistant varieties, and coating seeds with fungicide for sterilization are effective ways to control black rot in carrots. The occurrence of black rot on the leaves during the carrot growing period not only causes yield losses but also accumulates enough pathogen to cause disease in the fleshy root during storage, and at the same time provides a pathway for infection of the carrot root crown. Therefore, chemical control at the early stage of leaf disease is an important means of controlling the disease.

Before storage, washing and removing damaged carrots to minimize the degree of damage can reduce the probability of black rot occurring in carrots during storage. Maintaining appropriate temperature (around 0°C) and humidity (less than 92%) during storage can prevent carrots from becoming infected and reduce the probability of disease spread. The sanitary conditions of storage after harvest are crucial for controlling the disease.

To control black rot in carrots, before or at the early stage of disease occurrence, use 200 ml of the protective fungicide 40% chlorothalonil suspension concentrate per mu, spraying once every 7-10 days for 3-4 consecutive applications, which can achieve a control effect with low input and high output.

The dicarboximide fungicide iprodione has a relatively good control effect on black rot in carrots. Before or at the early stage of black rot occurrence in carrots, use 75 ml of 500 g/L iprodione suspension concentrate per mu, spraying once every 7-10 days for 3-4 consecutive applications.