Soil fungi combined with bacteria and other organisms create rich soil and contribute to plant health. Occasionally, one of these common fungi is a bad guy and causes disease. Cotton root rot of carrots stems from one of these bad guys. The villain in this story is Phymatotrichopsis omnivora. There are no existing chemicals for treating carrot cotton root rot. Carrot cotton root rot control starts at the time and manner of planting.
Symptoms in Carrots with Cotton Root Rot
Carrots grow easily in loose sandy soil where drainage is excellent. They are one of the mainstays of salads, side dishes, and even have their own cake. However, several diseases can wreck the harvest. Carrots with cotton root rot are victims of one of the more common types of diseases, fungal.
There are many host plants to the fungus, including alfalfa and cotton, and causes high economic losses in these and more crops. While there is no listed carrot cotton root rot control, several cultural and sanitation practices can keep it from infecting your plants.
The initial symptoms may be missed because the fungus attacks roots. Once the disease takes hold on roots, the vascular system of the plant is compromised and leaves and stems begin to wilt. The leaves may also become chlorotic or turn bronze but remain firmly attached to the plant.
The plant will quite suddenly die. This is because the attack to the root system has interrupted normal exchange of water and nutrients. If you pull up the carrot, it will be covered in soil that is stuck to it. Cleaning and soaking the root will reveal infected areas and mycelial strands on the carrot. Otherwise, the carrot will appear healthy and undecayed.
Causes of Cotton Root Rot of Carrots
Phymatotrichopsis omnivora is a necrotroph which kills tissue and then eats it. The pathogen lives in soil in the southwestern U.S. to northern Mexico. Carrots that are grown in the warmest parts of the year are particularly susceptible. Where soil pH is high, low in organic matter, calcareous, and moist, the incidence of the fungus increases.
It is estimated the fungus can survive in soil for 5 to 12 years. When soils are 82 degrees F. (28 C.), the fungus grows and spreads rapidly. This is why carrots planted and harvested in the hotter parts of the year are most susceptible to cotton root rot.
Treating Carrot Cotton Root Rot
The only possible treatment is fungicide, however, this has little chance of effectiveness because the sclerotia the fungus produces goes very deeply into soil – much more deeply than a fungicide can penetrate.
Crop rotation and planting to time the harvest during the cool part of the season will help minimize the disease. Using non-hosts in areas previously infected can help prevent the fungus from spreading too.
Perform soil tests to ensure a low pH and add copious quantities of organic matter. These simple cultural steps can help reduce the incidence of carrot root rot.