Several diseases affect onion and green onion crops, however, the pink root caused by Phoma terrestris (syn. Pyrenochaeta terrestris) has been easily detected in the root system of these crops. This disease is widely spread in countries where aliaceae 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 state of Minas Gerais and in Santa Catarina its record dates back to 1990 (Boff, 1990). Despite Phoma terrestris Being a low-specificity pathogen 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.
Phoma 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.
Symptoms
The fungus affects all stages of plant development and the characteristic symptom is pink, brown and brown coloration caused by tissue wrinkling and root death. 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, this forms a white mycelial growth on the crown of the bulb. Correct diagnosis of the attack P. terrestris is confirmed by small blackish spots on the root, which indicates the presence of pcnidia or pycnidia primordia. The pink hue may not be observed in young plants and may end up inhibited if the plant has intense vegetative growth.
Epidemiology
The pathogen is present in most soils where aliaceae are cultivated and increases in successive crop cycles due to the increase in inoculum concentration. P. terrestris can survive through dormant mycelium (chlamydospores) in the soil, in crop residues or freely in the soil and the initial infection occurs in the roots located in the center of the basal plate and the 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 colder 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 are temperatures between 24ºC and 28ºC, however, soil humidity is not an essential factor for the establishment of the disease. However, the disease has a greater occurrence when the soil pH is close to or above 7. In soils with low organic matter, the disease is more intense due to less microbial competition.
Disease management
Planting in disease-free locations is difficult to achieve, as the pathogen has a long survival period in the soil, in addition to being widely distributed among plant species.
The soil pH must be corrected to 5,5 - 6 to provide greater plant tolerance to pathogen attack. Soil coverage reduces water stress, waterlogging and temperature fluctuations, resulting in fewer infection conditions.
The use of green manure increases microbial biodiversity in the soil, resulting in competition with the pathogen. It is recommended to avoid surface runoff of rainwater and/or irrigation through soil conservation measures to prevent the spread of the pathogen within the crop;
Fertilization must follow what is recommended in the soil analysis to avoid imbalance between the root system and the aerial part of the crop and, thus, unfavorable infection. It is also important to produce seedlings in an area without the pathogen, as the symptom is not always detected in seedlings.
It is recommended to use healthy seedlings and avoid dense plantings in order to promote good plant development. Avoid the use of agricultural implements, as well as the movement of agricultural machinery and people who have passed through contaminated areas.
It is recommended to eliminate after harvesting any spontaneous plant that could serve as a host for P. terrestrials. Also remove any “guaxa” onion or green onion plant that remains vegetating in the field.
Crop rotation for at least three years reduces the inoculum present in the soil and the development of the disease, although it does not eradicate the pathogen. Non-host plants such as melons, legumes, potatoes, pumpkin alfalfa and beets can be used in crop rotation.
Among the resistant onion varieties are Baia Periforme, Excel, White Granex, Noda (Wordell Filho & Boff, 2006). However, at high temperatures the resistance can be broken. In green onions, Carvalho & Marcuzzo (2013) found that the Konatsu cultivar was more resistant to disease attack, presenting 10,81% and 12,8% less disease severity than Natsu and Nebuka, respectively.
There is no agrochemical registered with the Ministry of Agriculture, Livestock and Supply (Mapa) to control the disease and in experimental uses the results were not satisfactory for its control, as the pathogen may be deeper than the root system.
Solarization has provided a reduction of 73% to 100% in the incidence of the disease in Israel, however, in research carried out by Carvalho & Marcuzzo (2013) in the conditions of Alto Vale do Itajaí/Santa Catarina, an increase in the disease was found in chives- green with the use of solarization for 30 days before transplanting. In the region, the soil temperature with solarization reached a maximum of 36ºC, favoring the development of the pathogen.
Biological control with Trichoderma spp. has been used for several soil fungi, however, Marcuzzo & Carvalho did not find an effect on the biocontrol of Phoma terrestris when they used Trichoderma harzianum alone and a mixture of Trichoderma harzianum, Trichoderma viridae, Trichoderma sp. Clonostachys rosea, Bacillus subtilis e Paenibacillus lentimorbus applied to the soil in green onions.
In Brazil, although the pathogen is found in all regions that produce onions, direct losses caused by the disease have not yet been reported (Wordell Filho & Boff, 2006). However, Carvalho & Marcuzzo found a reduction in production in different cultivars of green onion caused by the pathogen. This was also confirmed by Maranhão et al (2003), who observed a 60% reduction in the root system in onions due to different isolates of the pathogen.
The measures aim to promote disease management and prevent other diseases from taking hold and increasing damage to the crop.
Onion culture (box)
Onion cultivation (Allium cepa L.) in Brazil occupies an area of 55.136 hectares, with a production of 1.426,192 tons and an average yield of 25.899kg/ha (IBGE, 2013). In Brazil, the crop ranks third among vegetables after potatoes and tomatoes and constitutes a socioeconomic activity of great relevance for the states of the Southern Region. The state of Santa Catarina comprises the largest area of onion cultivation in Brazil and in 2013 harvest, production reached 493.847 tons, representing almost 34% of the country's total production, in a planted area of 18.889ha, with more than half of this area concentrated in the Alto Vale do Itajaí region.
Green onion (box)
The production of condiments is an important source of income and sustainability for small agricultural properties. Green chives, also known as green chives, are one of the condiments in high demand by consumers, whether in the form in natura or even processed. Given the high demand for the industry, its cultivation has been carried out on a larger scale. The marketing in natura is made in fairs and supermarkets (Ferreira et al, 1993). According to Ferreira & Casimiro (2011), chives are an important crop that contributes to the quality of life of farmers, as the high added value and the involvement of family labor provide a condition of sustainability on the property.
Figure 1 – Pink root symptom on onion roots
Figure 2 – Symptom of pink root on green onion roots
Figure 3 – Symptom of dead roots caused by pink roots in green onion roots
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