Postbloom fruit drop: temporal and spatial dynamics, sensitivity of Colletotrichum acutatum to fungicide and disease control

Author(s)

Geraldo José da Silva Junior

• Document type: Doctoral Thesis
• Press: Piracicaba.
• Institution: Universidade de São Paulo (USP). Escola Superior de Agricultura Luiz de Queiroz (ESALA/BC)
• Defense date: 2011-08-04

Examining board members

Lilian Amorim; Armando Bergamin Filho; Nelson Gimenes Fernandes; Marise Cagnin Martins Parisi; Marcel Bellato Sposito

• Advisor: Lilian Amorim

Abstract

Postbloom fruit drop (PFD), caused by Colletotrichum acutatum, was firstly observed in Belize during 1956-1957 and subsequently reported in 1979. In the same year the disease was also reported in Brazil. PFD causes severe damage when bloom period coincides with rains. The disease is controlled primarily by preventive fungicide sprays. The repeated use of fungicides with the same mode of action can select resistant individuals. The knowledge of PFD epidemiology can generate precise information to develop efficient control strategies and improve the efficacy of chemical control. Thus, this study aimed to: i) characterize the temporal and spatial dynamics of PFD in young sweet orange orchards, ii) evaluate in vitro the baseline sensitivity of C. acutatum to fungicides and, iii) evaluate the efficacy of different fungicides, spray intervals and fungicide application schedule for PFD control. The temporal and spatial PFD-dynamics was characterized in three 2-to-4-year-old orchards with 500 trees each. High daily disease progress rates (r) described by the logistic model of 0.55 were observed after rainfall and prolonged leaf wetness. The spatial pattern of diseased trees varied considerably, initially at random and latter moderately aggregated, suggesting the contribution by other pathogen spread mechanisms, beside rain with wind. There was no correlation between the incidence of diseased flowers and the incidence of persistent calyxes. These calyxes are not important sources of inoculum. In vitro, assays demonstrated that the isolates of C. acutatum from São Paulo State were not resistant to fungicides difenoconazol and carbendazim. In the greenhouse, carbendazim and the trifloxystrobin + tebuconazol mixture showed significant effects when applied prior to inoculation or 24 h after inoculation, however only the mixture was effective 48 h after inoculation. In the field, the fungicide mixture was more effective for PFD control in orchards with high disease incidence. When the proportion of symptomatic flowers reaches 100%, yield reduction can be as high as 70% in Pera sweet orange trees. Frequent sprays may be required during bloom period to control the disease, although sprays were more important when two or more rainy days with prolonged leaf wetness occurs, especially during petals expansion and flowers opening. (AU)