Efeito da temperatura sobre o período de incubação, período latente e produção de esporos de Amphobotrys ricini em frutos de mamoneira

Abstract

Castor plant (Ricinus communis L.) is a tropical oilseed species. The oil extracted from its seeds is regarded one of the most versatile of nature because of its many industrial applications. Like all species of cultivated plants R. communis is affected by several fungal diseases. Among those, the gray mold, caused by the fungus Amphobotrys ricini, is considered one of the most destructive, accounting for losses of up to 100 % of production. For the development of epidemics is necessary, beyond the pathogen and susceptible host, the occurrence of favorable climatic conditions for disease development. Among the climatic factors that affect the development of plant diseases, the temperature is often considered one of the most important, since it directly affects the duration of the incubation and latent periods. However, for the pathosystem R. communis x A. ricini, the effect of this variable on the duration of the latent period (LP), incubation period (IP), or on the spore production (SP) of the fungus had not been investigated so far. Thus, green fruits of castor varieties BRS Paraguaçu and IAC Guarani were harvested, surface sterilized in alcohol and sodium hypochlorite and then inoculated with a suspension of 2 x 10 5 spores per mL of A. ricini. After inoculation, the fruits were disposed on acrylic boxes and kept under different temperatures for seven days. Temperatures ranging from 16 to 32 °C were evaluated. The PI and PL evaluations were made by visual inspection every eight hours, while the SP was evaluated at the end of seventh day, by washing the fruits in a 50% ethanol solution. The fungus was able to infect the castor fruits, i.e. complete its IP, both at 16 ºC and 32 ºC. However, at temperatures of 31 and 32 ºC, the fungus was not able to complete the PL during the trial. The shorter incubation and latent periods were observed at 24 and 25 ºC. At this same temperatures the highest values of SP, ranging from 1.35 to 3.28 x 10 spores per mL, were observed. At temperatures above 31 °C SP was considered null once the fungus has not able to complete the PL. The relationship between temperature and PI, PL and SP was described based on quadratic equations, with r 2 ranging from 0.85 to 0.99. Based on the first order derivative of the regression equations the optimum temperature for the development of the fungus ranged between 22 and 26 ºC.