Management of pink root disease in onion

Several diseases affect onion and green onion crops, however, the pink root caused by Setophoma terrestris (without. Pyrenochaeta terrestris; Phoma terrestris) has been easily detected in the root system of these crops. This disease is widely spread in countries where aliaceas are cultivated, and the resulting damage is intensified by the fact that they are always cultivated in the same area. In Brazil, the first occurrence of pink root was reported by Chaves & Erickson, in 1960. In onion cultivation, in the states of Minas Gerais and Santa Catarina, its record dates back to 1990 (Boff, 1990). Despite S. terrestris Being a pathogen of low specificity, found in soil with aliaceae cultivation, its occurrence intensifies in hot regions, where continued cultivation of the crop and conventional management intensify the damage. In southern Brazil, it has been observed at the end of the crop cycle due to an increase in temperature.

Setophoma terrestris is a low specificity pathogen that inhabits the soil and infects monocotyledons such as onions (A. cepa), green onion (A. fistulosum), shallot (A. cepa there. aggregatum), grass chives (A. schoenoprasum), garlic (A. sativum) and leek (A. ampeloprasum). In addition to these, corn, sorghum, wheat, cucumber and tomato can also be parasitized by the pathogen.

The main characteristic evaluated in a production system is yield, as the reduction in nutritional supply in the plant causes less bulb formation. When plants become ill, they lose their vigor, growth is paralyzed and the leaves wither, which leads to yellowing and subsequent death. 

Symptoms

The fungus affects all stages of plant development and the characteristic symptom is the pink, brown and brown color caused by the wrinkling of the tissues and death of the root. The pink color is due to the mycelial pigment of the fungus present in the infected root. After root death, the plant goes through a stage of reduced water and nutrient supply, which causes less vegetative development. Plants are easily pulled out of the ground due to root rot. However, the pink color and root rot are not standard for the pathogen and can be confused with a pest attack. Fusarium spp., however, it forms a white mycelial growth on the crown of the bulb. Affected roots are necrotic and invaded by secondary and saprophytic soil pathogens. In the same cultivation cycle, new roots can be emitted and infected by the fungus.

Correct diagnosis of the attack S. terrestris is confirmed by small blackish spots on the root, which indicates the presence of pcnidia or pycnidia primordia. The pink hue may not appear in young plants and may be inhibited if the plant has intense vegetative growth.

Epidemiology

The pathogen is present in most soils where alliaceas are cultivated and increases in successive crop cycles due to the increase in inoculum concentration. S. terrestris It can survive through dormant mycelium (chlamydospores) in the soil, in crop residues or freely in the soil. The initial infection occurs in the roots located in the center of the basal plate and symptoms in the roots become evident around ten days after infection. In the southern region of Brazil, the symptom occurs at the end of the crop cycle, due to the increase in temperature. However, infection can occur during the coldest period.

The pathogen is spread by soil movement, water runoff and, mainly, by the transport of bulbs, baby bulbs, mother bulbs and diseased seedlings. The wind has little interference in its dissemination.

The optimal conditions for the development of the disease occur when there is a temperature between 24ºC and 28ºC. However, soil moisture is not an essential factor for the establishment of the disease. However, there is a greater occurrence of the disease when the soil pH is close to or above 7. Soils with low organic matter register a greater intensity of the disease, due to less microbial competition in the soil.

Disease management

The onion

The cultivation of onions (Allium cepa L.) in Brazil ranks third among vegetables, after potatoes and tomatoes, and constitutes a socioeconomic activity of great relevance for the states of the Southern Region. Santa Catarina comprises the largest cultivation area of onions in the country and in the 2017/18 harvest, production reached 377.357,37 tons in a planted area of ​​19.596 hectares, with more than half of these areas concentrated in the Alto Vale do Itajaí region.
Leandro Luiz Marcuzzo,

IFC/Rio do Sul Campus

Growing Vegetables and Fruits February 2019

With each new edition, Cultivar Hortaliças e Frutas publishes a series of technical content produced by renowned researchers from all over Brazil, which address the main difficulties and challenges encountered in the field by rural producers. Through research focused on controlling the main pests and diseases in vegetable and fruit cultivation, the Magazine helps farmers in the search for management solutions that increase their profitability. 

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