Cruciferous clubroot is one of the rare cases of plant diseases caused by a protozoan - Plasmodiophora brassicae Woronin. This pathogen forms resistant spores that persist for a long time in the soil and can be easily dispersed by seedlings, trays, soil dragging through erosion, dust, and soil adhered to machines and implements. Reports indicate that the half-life of spores is 3,6 years, but can reach more than 18 years, even if subsequent cultivation of host species, species from the Brassicaceae family, is not carried out (Dixon, 2014). This characteristic implies the need for attention to preventive measures to avoid the introduction of these pathogens in brassica production areas in general.
The first observation of P. brassicae in Brazil it was carried out at the end of 1934, in the state of São Paulo, on common cabbage (Viegas & Teixeira, 1943). Currently, its presence has already been confirmed in several other states, such as Rio de Janeiro, Paraná, São Paulo, Minas Gerais, Distrito Federal, Rio Grande do Sul, Santa Catarina and Espírito Santo. However, losses have been increased in areas where the cultivation of susceptible vegetables is carried out intensively and continuously, such as in the metropolitan region of Curitiba, Paraná, in the Serrana region of Rio de Janeiro (Nova Friburgo) and in the green belt of São Paulo. Paul. In these situations, losses have been frequent and exceed 60%. In some cases, when the quantity and distribution of inoculum in the area are high, brassica cultivation becomes unfeasible for a period greater than or equal to two years. This period, however, does not guarantee the elimination of the pathogen, only the reduction in the quantity or viability of resistance spores in the soil.
All members of the Brassicaceae family are potential hosts of P. brassicae, as they allow the pathogen to complete its entire life cycle (Dixon, 2009). Among the vegetables in this family, due to their economic importance and susceptibility, greater losses have been recorded in cauliflower and broccoli, especially when grown in the summer. The higher temperatures and greater humidity during this period favor the development of disease and losses. But unfortunately for producers, this is the period in which they achieve better sales prices.
The typical symptom of the disease is the formation of galls (hernias) in the root system, as a result of the rapid and disorderly multiplication of cells, in size and number. These galls impair the absorption of water and nutrients, causing symptoms of nutrient deficiency, yellowing and wilting of the leaves, especially during the hottest periods of the day. All these disorders cause the underdevelopment of host plants which, as a result, present inflorescences, heads or leaves of low commercial value, affecting productivity and profitability. In conditions more conducive to the development of the disease, premature death of plants may occur.
The resistance spores of P. brassicae they remain dormant in soils until they are stimulated by the presence of exudates released by the roots of host plants. The exudates stimulate spore germination and the release of biflagellate zoospores. With the help of these flagella, the zoospores move through the soil water film until they reach and penetrate the root hairs. Soil moisture is a key factor in the success of the pathogen in this early phase of the cycle. After multiplying in the root hairs, the pathogen migrates and penetrates the cortical tissues of the root, where it causes a series of physiological and anatomical changes that culminate in the disordered multiplication and enlargement of the cells and, consequently, the formation of galls or tumors. Resistant spores are formed inside these galls, released into the soil during the gradual rotting of the roots and, from then on, dispersed throughout the area or to new areas.
The conditions that contribute to the development of the disease are the presence of a high concentration of resistance spores (≥106 units g-1 of soil), acidic soils (pH ≤ 6,2), with low calcium levels, high humidity, mainly when it exceeds 70% of field capacity, and temperatures close to 25°C (Gossen et al., 2014). More recently, it was reported that losses can be increased due to the toxicity caused by the aluminum ion (Al3+) in the soil (Bhering et al., 2017) due to the damage it causes to plant roots in acidic soils.
The pathogen can be dispersed by the loading of spores contained in the soil during the plowing and harrowing processes and in surface water runoff. Contaminated seedlings are also agents for spreading the disease (Gossen et al., 2014). Therefore, the use of machinery, agricultural implements and any other materials containing the pathogen can be a potential source of inoculum and contribute to its introduction into new brassica cultivation areas. Continuous and intensive cultivation favors the multiplication, dispersion and survival of the pathogen in the areas.
Because disease control options are restricted, prevention should be seen as the main strategy. Recommendations such as the use of healthy seedlings, acquired from reputable nurseries, correct soil management, the use of good quality water free from contamination, in addition to washing and sanitizing tractor tires and agricultural implements, are essential to avoid the introduction of the pathogen into the area.
Once present in the area, disease control must be seen under an integrated concept. Unlike other pathosystems, the availability of resistant cultivars is extremely limited, being restricted to only a few Chinese cabbage materials. Furthermore, for its chemical control, only one active ingredient, cyazofamide, is registered with the Ministry of Agriculture, Livestock and Supply (Mapa) (Agrofit, 2019). Therefore, improvements in the chemical and biological conditions of the soil are basic strategies that must be considered. Liming is the most traditional and important strategy for disease and crop management. Elevating pH, supplying Ca and reducing toxic Al3+ can reduce losses caused by the disease (Bhering et al., 2017). Results obtained by Santos et al. (2017) indicate a 17,8% reduction in the severity of the disease, and increases of 26,5%, 32,5% and 37,78% in the volume of healthy roots, total root dry mass and fresh mass of kale inflorescences -flor, respectively, with the application of 4 t/ha of limestone compared to the control.
Other points that must be considered are the application of good quality organic matter, crop rotation with non-host species and fallow. In addition to this, it is speculated that the cultivation of some non-host plants, such as leeks, ryegrass and rye, could reduce the inoculum potential in the soil by acting as bait and not allowing the pathogen to complete its cycle (Donald and Porter, 2009).
Although these are costly practices, the uprooting and destruction of the roots of plants containing clubroot, at the time of harvest, are also recommended. Pulling them out and burning them prevents the inoculum from being replaced in the soil.
In studies carried out by UFRRJ, in Nova Friburgo, Rio de Janeiro, the largest national producer of cauliflower, it was found that, in addition to the intensive cultivation of this species for more than 30 years, other conditions have favored losses due to cruciferous hernia, such as : a) steep terrain and low adherence to the use of soil conservation practices, such as planting on contour lines, terracing or water drainage channels to reduce and/or avoid the carryover of contaminated soil; b) failure to carry out liming and other technical recommendations in soil fertility management (Bhering et al., 2017; Santos et al..
Despite exhaustive investments made worldwide, there is still no single control strategy that results in efficient control of the disease. Therefore, the adoption of preventive and prophylactic measures is the main strategy for controlling this disease, such as the use of healthy seedlings, care and cleaning of machines and implements, and correct fertility management, including correction of soil acidity, crop rotation and the removal and destruction of diseased plants. Practices that encourage plant root development can also help reduce losses.
Research underway in Brazil and in countries such as Canada, Australia and Germany has been carried out in search of understanding the interactions between the pathogen, soil factors and host plants. It can be concluded, therefore, that the management of cruciferous hernia is complex and that, for now, preventive measures are the most important to avoid the risk of loss due to the disease.
Carlos Antônio dos Santos, Aline da Silva Bhering, Nelson Moura Brasil do Amaral Sobrinho and Margarida Goréte Ferreira do Carmo, UFRRJ
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