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| Author(s): |
Marisa Silveira Almeida Renaud
Total Authors: 1
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| Document type: | Master's Dissertation |
| Press: | Piracicaba. |
| Institution: | Universidade de São Paulo (USP). Escola Superior de Agricultura Luiz de Queiroz (ESALA/BC) |
| Defense date: | 2006-02-03 |
| Advisor: | Lilian Amorim |
| Abstract | |
The disease known as Alternaria Brown Spot of citrus, caused by Alternaria alternata pv. Citri, is recent in Brazil and only preventive methods of control have been adopted. ln order to better understand the pathosystem citrus - A. alternata pv. Citri , this study aimed to (i) generate and validate a diagrammatic scale to assess the severity of Alternaria Brown Spot of citrus; (ii) evaluate the effects of temperature (10 - 15 - 20 - 25 - 30 - 35°C) and wetness period (8 - 12 - 24 h) in in vitro germination of four fungi isolates; and (iii) evaluate the effects of temperature (20 - 25 - 30°C) and wetness period (8 - 24 - 48 h) on infection in 'Murcott' tangor fruits inoculated with two fungi isolates. Two types of symptoms were represented in the diagrammatic scale: small circular or irregular lesions in six levels of disease severity (0.1; 1; 2.5; 5; 11 and 25%) and large circular lesions in five levels (1; 2.5; 5; 10 and 17%). This scale was validated by four experts in disease quantification. The evaluation of the disease became more accurate and precise with the aid of the diagrammatic scale. The effects of temperature and wetness period was evaluated over conidia germination in vitro and on the incidence of the disease in 'Murcott' tangor fruits. Germination occurred under all combinations of temperature and wetness period. The greatest percentage of germination for all four isolates was with 24 wetness period on most temperatures for all isolates. The minimum germination temperature for the isolates, estimated by mathematical models fitted to the data, was from 7.1 to 9.9°C, while the maximum temperature ranged from 36.8 a 52.5°C. ln another test, the effect of discontinued wetness periods was tested at 25°C. Even though the greatest percentages of germination constantly occurred in the period of 24 h of continuous wetness, the germination ratios were not appreciably reduced with discontinued wetness periods. ln an in vivo experiment, ripe 'Murcott' tangor fruits were inoculated with a suspension of two A. alternata pv. Citri isolates and evaluated seven days later. On the tree tested temperatures, when fruits were kept under 24 hours of wetness after inoculation, disease incidence was above 70% to one fungus isolate and above 90% to another (AU) | |