Efeito da luz transmitida através de coberturas coloridas transparentes sobre a mancha-alvo do tomateiro

Abstract

Target spot, caused by the fungus Corynesposra cassiicola is the most important disease of the aerial parts of the tomato plants in tropical regions. Aiming to find an alternative management for the target spot disease, with low cost and low impact on the environment and easily adopted by small farmers, was evaluated the effect of light transmitted through transparent covers in different colors on the pathogen, the disease, and on the tomato production. This study was conducted at the Instituto Nacional de Pesquisa da Amazônia, Manaus, AM, Brazil. To evaluated the effect on the colonies diameter, mycelial mass and sporulation of the C. cassiicola, colony discs (0.5 cm in diameter) grown in PDA were transferred to the centre of polystyrene Petri dish (90 mm in diameter) containing PDA medium. The cultures were incubated for 10 days under 12 h photoperiod provided by fluorescent lamps located 50 cm above the plates, and at 25 ± 2 °C, within red, blue, yellow, green and colorless cellophane envelopes and under no cover. The conidia germination was also evaluated under these coverages. The effect of LED illumination (80-100 micromol m-2 s- 1) in red, blue, yellow, green, white and in the dark was evaluated on colony diameter and mycelial mass. The experiments were performed in randomized block design with three replications. The diameter, mass of the colonies and sporulation were inhibited under orange, blue e green and conidial germination was not affected. The colonies under green LED had significantly small diameter than the other treatments and equal diameter of the colonies without illumination. No difference in mass of the colonies was observed. To evaluate the effect of sun light transmitted through the colored plastics on the disease and the tomato productivity, plants were grown in pots (5 L of soil) and, 45 days after sowing, were inoculated by spraying the leaves with conidial suspension of C. cassiicola (4.75 x 104 conidium.mL-1) to the runoff point. Plants were transferred to individual greenhouses covered with transparent plastic in colors: red, blue, green, yellow and colorless. The experimental design was a randomized block with five replications, each plant in a greenhouse was an experimental unit. The temperature and relative humidity were recorded daily. The spectrum of the radiation reaching the plants, under each coverage, was also recorded. With the data of severity and disease index, measured twice weekly, disease progress curves were prepared and calculated the areas under the disease progress curves. These data were used in the statistical analysis. Plants in greenhouses received wavenlength 380 a 620 nm had the highest rates of photosynthesis, stomatal conductance, transpiration, CO2 uptake. Fruit production was significantly lower in plants under blue, green and violte roofs.