The main limiting factor in cauliflower productivity is a bacterium called Xanthomonas campestris pv. campestris, popularly known as black rot, which can occur in all growing regions and affects the plant at any stage of development. This pathogen spreads easily and manifests itself across a wide temperature range. In more advanced stages, wilting and premature leaf fall may occur, which generates significant losses.
This bacterium infects the interior of the host plant, mainly through stomata, hydathodes and/or injuries, presenting characteristic symptoms such as yellowing of the leaf edge, progressing to necrosis in a characteristic “V” shape, with vertices facing the central veins. As it progresses, it manages to invade the conductive vessels, causing darkening and limiting the circulation of sap, which causes leaf wilting. In cases of severe attacks, the plant dies even before the “head” forms.
Few pesticides have been studied to control this pathogen. The complexity of factors involved in the development of the disease requires more than one control method to be successful. By evaluating the mode of action of several active ingredients, a useful and effective combination can be reached.
Alternative disease control can be carried out through the use of biological control strategies and the induction of resistance through the use of molecules or elicitor substances, such as plant and mineral extracts. The use of minerals has also been proven to be effective in combating diseases in different cultivars, and can strengthen the cell walls of plants. The application of these products can be carried out on the ground or in the air.
With the aim of evaluating the efficiency of different chemical products in preventing and controlling the appearance of black rot, as well as their influence on cauliflower productivity, an experiment was carried out on a commercial farm in the city of Guabiju, Rio Grande do Sul. The variety used was a hybrid with the commercial name Korlanu. In this area, plowing and harrowing are carried out in order to accelerate the decomposition of crop residues to shorten the life cycle of the pathogen. Millet (Pennisetum glaucum) was used as vegetation cover with the aim of reducing the incidence of guax and host plants.
On February 18, 2018, the soil received full correction and maintenance fertilizer, as per technical recommendations. The treatments were arranged in the blocks through a draw. Each block had seven treatments (Table 1) and three replications. Each experimental unit occupied an area of 13,5m², three meters long by 4,5 meters wide.
The cauliflower seedlings were purchased from a nursery with uniform size, approximately 15cm. These were produced with organic substrate in Styrofoam trays with 250 cells, where they were still treated with insecticide and fungicide in the nursery to ensure their health until the date of transplantation.
35 days after sowing, 630 cauliflower seedlings were transplanted (210 units each block and 30 each repetition), using a manual planter, observing a spacing of approximately 0,75m between rows and 0,6. .3m between plants. Four days after transplantation (DAT), specimens that died due to environmental factors were replanted. These specimens were not considered in the diagnosis and final evaluation, but received the same treatment as the others, with the sole and exclusive purpose of populating the unoccupied space. On the same date, treatments began in the blocks, incorporating the nutrients silicon (T5), copper hydroxide (T7) and sulfur (T1). These products were weighed with the aid of a precision scale and, according to dosages obtained in the literature and determined by the manufacturers, a dose of 3g/m² was used. The application was carried out around the plants, approximately 24cm away, and the incorporation was carried out using a hoe. Foliar application of commercial products was carried out at 39, 54, 69, 89 and 1 DAT, as described in Table XNUMX, equally on all dates. All treatments, including the control, received chemical insecticide control from the carbamate chemical group, in a homogeneous way.
To apply foliar treatments, a backpack machine with fan nozzles was used and, concomitantly, an adhesive spreader was used to facilitate homogenization on the leaf surface. The distribution of treatments in the blocks was done by draw. Surface sprinkler irrigation was carried out whenever soil moisture was low. At 39 DAT, the pathogen was inoculated in all plants. The inoculum was obtained by grinding leaves from plants diagnosed with Black Rot. The plants came from another area, and part of the leaf blade with greater severity of the disease was removed. Grinding was carried out with the aid of a crusher, to disrupt cells and expose the pathogen. This process resulted in a pasty solution, where some pointed wooden sticks were later added, which remained in contact with the solution for approximately 30 minutes and were subsequently inserted into the limb of a leaf of each plant in the experiment.
At 84 and 97 DAT, the plants that had an acceptable size for commercialization were evaluated. From these plants, leaves were randomly removed from the middle/lower third in order to evaluate the leaf area affected by the pathogen (severity). The evaluation took place through a comparative table or diagrammatic scale, prepared by the authors, using seven leaves removed from the crop and affected by the disease, which presented different degrees of injury, allowing for establishing degrees of severity or grades from 1 to 7, where 1 presents no visible symptoms and 7, the maximum severity observed in the area. At 97 DAT, the weights of the inflorescences were also weighed and recorded, using a precision scale.
To assess the severity of each plant, three leaves were removed from the middle part that best represented the visual appearance or average severity of that plant.
The data obtained were subjected to analysis of variance, using the Sisvar software, and the means compared using the Tukey test at a 5% probability of error.
At 24, 39, 54 and 69 DAT, the plants showed no symptoms of the disease. From 84 DAT onwards, the first symptoms began to appear, triggering severity assessments. Two assessments were carried out at 84 and 97 DAT.
Regarding severity, at 84 DAT, the T3 treatment (silicon 68%) was the one with the highest score, even being slightly higher than the control, and the T2 treatment (N, C and amino acids) was the one with the lowest score (Table 2). This lower score is believed to be the result of the presence of amino acids that provide greater turgidity and vigor to the plant, even favoring the production of defense proteins.
At 97 DAT, the lowest score was presented by the T7 (sulfur) treatment and the highest score remained T3 (68% silicon). Silicon, despite being reported as important in the plant defense system, strengthening the cell wall and activating defense mechanisms, was not relevant in this interaction. However, there was no direct correlation between the grades and inflorescence weights, as, for example, T7, which presented one of the lowest severity grades, also demonstrated the lowest inflorescence and three inflorescence weight values (Table 2). Treatment T7 presented younger leaves than the others and with greater health. Authors mention that sulfur-based products have a curative and eradicating action, due to the emission of vapors that arise after application. By penetrating the cells, sulfur reduces energy production, preventing the multiplication of the pathogen's cells, causing their elimination. Despite the low severity in T7, this treatment showed low inflorescence weight. According to other studies, sulfur should be applied in phases close to the end of the crop cycle, as this nutrient is required for the formation of inflorescences. If applied before or in high doses, it can cause a reduction in the size and, consequently, the weight of the inflorescence, which justifies the results found in this work. Disease suppression by sulfur can also be explained by a reduction in soil pH when sulfur is oxidized. However, many crops cannot tolerate low pH.
Productivity, given by the weight of the inflorescence, indicates that the T6 treatment (phosphite 30% P2O5 and 20% K2O) was the best and presented intermediate severity scores to the others. Phosphite salts are another option in the integrated management of some diseases, such as root and foliar diseases, whose metabolic route has been studied in some crops. The development of the disease in plants can occur at certain levels and not affect productivity, which was possible to visualize in this treatment. Researchers report that this product can have direct and indirect action on microorganisms, acting as a toxin on bacteria and also stimulating the formation of self-defense substances in plants. The induction of resistance occurs more intensely in the young tissues of the plant, and applications must be preventive and directed with greater emphasis on the new leaves of the crop, in addition to favoring the absorption of Ca, B, Zn, Mn, Mo, K and other elements , improving the nutritional status of plants, especially in the most demanding phases such as budding, development and flowering. Furthermore, it presents rapid absorption, total assimilation and lower energy requirements for the plant. Research reports that both the application of calcium phosphite and potassium phosphite demonstrated satisfactory effects in controlling soft rot in peppers, and suggested that this control can be extended to several other diseases that affect vegetable crops. In tomato late blight, potassium phosphite, even in isolated form, was able to reduce the affected leaf area by 25%, compared to the control.
The antimicrobial action in T2 (N, C and amino acids) occurs due to the presence of citric acid and ascorbic acid (vitamin C) which break down fungal and bacterial cells. These elements also activate the plant's natural defenses through exogenous phytoalexins that have bacteria-inhibiting and antioxidant activities (flavanoids) that reinforce and protect the plant cell. Like T6, this treatment presented disease, but at a low level of severity compared to the control and with high inflorescence weight.
Reports with copper-based compounds such as T5 indicate that they can reduce the severity of black rot on leaves, stems and heads. However, cases of copper resistance by Xanthomonas have already been described and are associated with the types of plasmids present in these bacteria. To explain copper resistance in phytopathogenic bacteria, the use of mechanisms such as the control of copper permeability inside cells and also proteins present in the periplasmic space that act to sequester copper and in the outer membrane of the bacterial cell is related. Copper, although it does not kill the bacteria, has a bacteriostatic effect on it, reducing its activity and dangerousness. T5, in this work, presented intermediate severity and productivity in relation to the other treatments.
T4 (Ca 22% and Cl 58%) was the one with the lowest severity at 97 DAT after T7 (elemental sulfur 99%) and high inflorescence weight, as well as T2 (N, C and amino acids) and T6 (30% P2O5 and 20% K2O), indicating that it is a suitable product for disease control and productivity. The association of chlorine with calcium provides a biocidal action that effectively shocks fungi and bacteria. Calcium acts on the essential middle lamella and strengthens the cell wall of tissues. Plants that receive Ca in high doses, during the growth phase, presented a higher concentration of polysaccharides, resulting in greater efficiency and tissue resistance. The efficiency of chlorine, in turn, depends on several factors such as temperature, exposure time, pH and microbial load. However, its use in crucifers, during the vegetative period, for the treatment of Xanthomonas still lacks information.
An important factor, in addition to product efficiency, is related to cost-benefit, allowing us to evaluate which point is monetarily satisfactory for the application of a given input. Of the treatments used, T6 was the one that presented the best cost-benefit ratio (value R$ 2,08/kg), followed by T4 and T2 (Table 3).
Furthermore, the choice of appropriate treatment must consider the history of the crop, that is, presence of the pathogen, incidence and losses, as well as characteristics of the environment that may favor the appearance of the disease in the area.
Cauliflower (Brassica oleracea var. botrytis L.) is among the 15 most important vegetables, being one of the most cultivated in Brazil, mainly in the states of São Paulo, Rio de Janeiro, Rio Grande do Sul, Minas Gerais, Paraná and Santa Catarina. It is an inflorescence-type vegetable that belongs to the Brassicaceae family, just like cabbage, broccoli and other cabbages. It presents specific thermoclimatic and management requirements, requiring low temperatures to form “heads” (edible parts) of suitable size for commercialization, becoming restricted in regions with mild temperatures. Cultivation in this climate condition was only possible due to genetic improvement that developed hybrids with tolerance.
Neimar Cenci, Adjar de Oliveira, Igor de Sordi, Daniel Alves, Hugo Catapan, Gabriela Tonello, Rafael Goulart Machado, Katia Trevizan and Alice Casassola, Faculdade Ideau Inst. of Development. Education. from Passo Fundo
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