Full text
| |
| Author(s): |
Cláudia Vieira Godoy
Total Authors: 1
|
| Document type: | Doctoral Thesis |
| Press: | Piracicaba. |
| Institution: | Universidade de São Paulo (USP). Escola Superior de Agricultura Luiz de Queiroz (ESALA/BC) |
| Defense date: | 2000-04-28 |
| Advisor: | Lílian Amorim |
| Abstract | |
ln order to assess the effect of weather variables on the development of southern corn rust, caused by the fungus Puccinia polysora, on maize crop, trials were carried out under controlled environment, on growth chambers, and in the field. Maize plants were inoculated with uredospore suspension of P. polysora and placed to growth chambers to assess the influence of temperature (5-35°C) and leaf wetness duration (2-24 hours) on the monocyclic components of the disease (incubation period, latent period and sporulation) and on disease severity, in two maize hybrids. The generalized beta function described the changes of incubation period, latent period and severity as a function of temperature. The optimum temperature estimated for the development of disease was 23°C. Symptoms were also observed at 10, 15, 20 and 30°C. Temperatures below the optimum increased the incubation (28 days at 10°C and 14 days at 15°C) and latent period (24 days at 15°C). Sporulation was smaller under 15°C. Disease severity increased with increasing periods of leaf wetness from 2 to 24 hours. The latent and incubation periods were not influenced by the different wetness periods at temperatures of 20, 25 and 30°C, remaining between 6-9 days (incubation period) and 8-12 days (latent period). The logistic model described disease severity as a function of wetness period. With the combination of the functions (generalized beta and logistic) a response surface of disease severity as a function of temperature and leaf wetness was obtained (R2=0,94). The minimum and maximum temperatures estimated by the surface to the development of the disease were 9.9 and 33.3°C, respectively. ln field trials, the development of southern com rust was monitored in plantings carried out between October and May in different locations (Castro/PR, Piracicaba/SP and Guaíra/SP), in two growing season (95/96 e 96/97). The logistic model described the disease progress. The components of the curves (maximum severity, area under disease progress curve - AUDPC - and rate of disease increase between assessments) were utilized for comparison of the epidemics on different locations and time and related with weather variables recorded during the trials. The functions determined in the controlled conditions were tested to verify their capacity of explain the development of disease under field conditions. Disease severity was high (severity higher than 30%) in Piracicaba and Guaíra, in plantings realized between December and February, and was inexpressive in later plantings. ln Castro, in all trials, disease severity was low (lower than 0,6%). The rate of disease increase, between assessments, showed positive correlation (p<0,05) with mean daily temperature and with the generalized beta function determined in controlled conditions to temperature and negative correlation with periods of relative humidity over 90% and with the logistic function determined in controlled conditions to leaf wetness. The AUDPC was negatively related to the model of weather favorability in function of leaf wetness. The increased importance of southern rust it is not only associated with the high amount of inoculum provided by successive plantings. In this work it was observed that the weather represents an important issue on epidemics development, because even in the presence of inoculum coming from previous plantings, epidemics were not observed in all trials. (AU) | |