BIOFERTILIZER AND SILICATE CLAY AS RESISTANCE INDUCTORS TO THE BLACK SPOT OF PAPAYA DISEASE

ABSTRACT

Black spot of papaya disease is one of the most serious problems of the papaya culture and its control is based on the excessive application of chemical products. The goal of this study was to evaluate the effect of the application of biofertilizer and silicate clay in the resistance induction to the black spot of papaya disease, being evaluated the incidence and severity. The treatments received road leaf applications of biofertilizer (T2), silicate clay (T3), biofertilizer plus silicate clay and water on the witness (T1). The evaluations were made 5 days after to 6th, 9th and 12th road leaf application of the products. The treatments 2, 3 and 4 provided significant reduction of the disease’s incidence and severity. However, the incidence and severity reduction were larger with biofertilizer plus silicate clay associated. The application of these products is a effective measure and economically viable method for the black spot management in papaya cultivations. Key word: Asperisporium caricae, Carica papaya, induction resistance, sustainable agriculture.

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INTRODUCTION

Brazil is the world's largest producer of papaya, with a production of 1.5 million tons per year (Martins & Costa, 2003). However, papaya can be affected by several diseases, which is the most important economic factor in reducing the production and export of fresh fruits (Nishijima et al., 1994). Among the diseases, there is smallpox or black spot, caused by Asperisporium caricae (Speg) Maubl. et al., 2000). To circumvent such problems, the control of this disease is mainly based on the application of fungicides (Rezende et al., 2005). However, the intensive use of these fungicides can cause resistance of the pathogen to them, as well as affect human health, both the consumer and the professionals involved in the production processes, and cause negative effects on the environment (Tuzun & Kuc, 1991). .

Aiming at alternatives to chemical control, the use of genetic resistance has been one of the most efficient practices within integrated management (Torres & Garcia, 1996). However, a genetic improvement program is expensive and time-consuming, not always responding quickly to the needs of agriculture (Cavalcanti et al., 2005). However, an easy-to-manage and low-cost alternative is induced resistance, which consists of increasing the resistance level of the plant through the use of external agents (inducers), without any alteration of the plant genome (Stadnik, 2000).

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Silicon has been reported as one of the elements associated with inducing resistance in plants (Savant et al., 1999) and its absorption can increase this resistance, especially for crops that accumulate it (Mauad et al., 2003). Research carried out with several cultures has confirmed the potential of silicon in reducing the intensity and severity of diseases (Menzies et al., 1991; Datnoff et al., 1997). Rice plants (Oryza sativa L.), for example, cultivated with increasing doses of this element had reduced sheath blight (Rhizoctonia solani Kühn) severity (Rodrigues et al., 2002). However, for papaya the effects of silicon on diseases have not yet been tested. The mechanism by which silicon affects the development of diseases in plants is possibly the result of the action of this element on the host tissue, providing physical impediment and a greater accumulation of phenolic compounds and lignin at the site of injury (Chérif et al., 1992). This structural function provides anatomical changes in tissues, such as epidermal cells with a thicker cell wall due to the deposition of silica in them (Blaich & Grundh Fer, 1998), favoring a better architecture of plants, in addition to increasing photosynthetic capacity and resistance to diseases (Bélanger & Menzies, 2003). Among the sources of silicon, calcium silicate (CaSiO3) is the most used form in most commercial products (Barbosa Filho et al., 2000). Among the products sold are silicate clay, whose trade name is Rocksil®. Another example of a commercial product is the organomineral fertilizer, whose trade name is Micromix®, which acts by providing a rapid assimilation of available nutrients, increasing the production of plant mass, a fact that can make the plant resistant to pathogens (Tecnobiol, 2005). Thus, the objective of the present work was to evaluate the effect of the use of organomineral fertilizer and silicate clay, isolated or associated, in the reduction of the incidence and severity of A. caricae in papaya.

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MATERIAL AND METHODS

The experiment was carried out at the Center for Scientific and Technological Development in Phytosanitary Management of Pests and Diseases (NUDEMAFI) of the Center for Agricultural Sciences of the Federal University of Espírito Santo (CCAUFES), in Alegre - ES. Papaya plants (Carica papaya) var. formosa were grown in plastic bags (16cm x 34cm) and kept under field conditions. The application of organomineral fertilizer and silicate clay began 60 days after sowing, at 10-day intervals. Applications were performed via foliar, with the aid of a manual mini-spray. The sprays with organomineral fertilizer were based on 2.0 ml/L of water, silicate clay at 15.0 g/L of water and in the association of the products, 2.0 mL of organomineral fertilizer per liter of water were used more 4.0g of silicate clay per liter of water. As a control, water was used in the spraying of the plants. Quantifications of incidence and severity were performed five days after the 6th, 9th and 12th application of the products. The incidence was expressed as a percentage, which was obtained by counting the number of leaves that showed symptoms of the disease, divided by the total number of leaves of each plant and multiplied by one hundred. The severity of the disease was quantified by removing two leaves with the same physiological age, in the lower part of each plant, which were scanned and submitted to the QUANT (1.0.1) program (Vale et al., 2004), obtaining the injured area of ​​each leaf. The experiment was carried out using a completely randomized design with a 3×4 split-plot factorial, with one factor being the number of applications and the other being the source of induction. Four replications were performed per treatment, each repetition consisting of a papaya plant. The means were compared by Tukey's test at 5% probability, through the SAEG 9.0 Software.

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RESULTS AND DISCUSSION

Significant interactions were observed between treatments and number of applications, for incidence and severity of lesions caused by A. caricae. The use of organomineral fertilizer and silicate clay provided a lower incidence of lesions caused by A. caricae on the leaves of papaya plants in relation to the control, highlighting the treatment in which the association of the two products was used, which had a lower incidence in all applications (Table 1). It is worth mentioning that no incidence values ​​lower than 50% were observed. Thus, to try to differentiate the treatments, the severity data was also used. From this analysis, it was found that the treatments that received application of organomineral fertilizer and silicate clay, the severity of the disease was significantly lower (Table 2). However, the smallest area under the disease severity progress curve (AACPD) was observed in the treatment that received application of organomineral fertilizer plus associated silicate clay (Table 2, Figure 1). Similar results were observed by Santos (2002), who found a linear decrease in the incidence and severity of brown eye spot with the use of calcium and sodium silicates in coffee seedling substrates. Possibly, the mechanisms by which silicon can confer resistance to a certain disease can be due to structural barriers such as the accumulation of this element in the cell wall of the epidermis and cuticle or accumulation at the site of penetration of the pathogen (Rodrigues et al., 2003) or by activating chemical and biochemical barriers in plants (Bélanger et al., 2003). In the same sense, Pozza et al., (2004) observed that the increase in coffee resistance to brown eye spot was due to the greater thickness of the cuticle and the increase in the absorption of micronutrients by plants treated with silicon. This hypothesis was confirmed through scanning electron microscopy images, in which they observed the presence of a thicker cuticle partially covering the stomata on the lower surface of the leaf of coffee seedlings treated with calcium silicate in the soil. The authors observed that the thickening of the cuticle, mainly due to the formation of a thicker epicuticular wax layer, made it difficult for the pathogen to penetrate directly through the cuticle or through the stomata. This layer of epicuticular wax may have made the surface more hydrophobic, preventing the formation of a water film, which is important for vital pathogenic processes, such as germination and penetration, in addition to allowing the accumulation of antifungal substances in the cuticle (Pascholati & Leite , 1995). At the end of the evaluations, it is noted that the (AACPD) of severity in the control was significantly higher than in the other treatments.

However, in the treatment in which organomineral fertilizer was associated with silicate clay, the lowest (AACPD) was observed (Figure 1). Thus, the association of organomineral fertilizer plus silicate clay can be considered as a viable alternative within the integrated production of papaya. However, to obtain good results from the effect of silicate sources in increasing the resistance of plants to pathogens, a continuous supply of these elements is necessary (Bélanger & Menzies, 2003). Therefore, in order to reduce the severity of smallpox in papaya, a periodicity of applications must be maintained, together with other viable control alternatives, which allow the sustainability of the agroecosystem involved.

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CONCLUSION

Foliar application of 0.5% organomineral fertilizer, 1.5% silicate clay and the association of 0.5% organomineral fertilizer with 0.4% silicate clay diluted in water, reduces the incidence and especially the severity of the disease. papaya smallpox (A. caricae), showing potential for managing papaya resistance to this pathogen.

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