Effects of Frequency of “Extreme” Temperature Highs on Development of Soybean Rust

Abstract

Previously, we hypothesized that summer "extreme" diurnal temperature highs in the southeastern United States were responsible for the yearly absence or delay of soybean rust development until fall. Utilizing temperature-controlled growth chambers, a diurnal temperature pattern of 33°C high and 20°C low reduced urediniospore production by 81%. However, that study did not consider the influence of frequency of extreme temperatures on soybean rust. We now report that a temperature high of 35°C for 1 h on three consecutive days, initiated 15 days after inoculation, when lesions had formed, reduced urediniospore production by 50% and required 9 to 12 days for sporulation to resume once the extreme temperature highs ceased. Furthermore, three consecutive days in which the temperature high was 37°C, beginning immediately after inoculation and subsequent dew period, reduced lesion numbers by 60%. The combined effects of reduced numbers of lesions and urediniospores per lesion caused by extreme temperature highs can account for observed absence or delay of soybean rust development in the southeastern United States until fall. A comparison of frequency of extreme temperature highs with numbers of counties reporting presence of soybean rust from 2005 to 2012 verified that extreme temperature highs may be largely responsible for absence or delay of soybean rust development. This is the first report showing the effect of frequency of extreme temperature highs on development of soybean rust. Because the south-to-north progression of soybean rust is required for the disease to occur in the major soybean-production regions of the United States, temperatures in the southeastern United States have a major effect on the entire U.S. soybean industry.

Temperature at the early stages of Clavibacter michiganensis subsp. michiganensis infection affects bacterial canker development and virulence gene expression

“…Bonde et al . () reported that a high temperature of 35°C for 1 h on three consecutive days, initiated 15 days after inoculation when lesions had formed, reduced Phakopsora pachyrhizi urediniospore production by 50% and resulted in a requirement of 9–12 days for sporulation of soybean rust. It is possible that an increase of 7°C for 8 h is not enough to affect the cellular processes that typically occur at high temperature.…”

Observations on the gall rust fungus Prospodium transformans, a potential biocontrol agent of Tecoma stans var. stans (Bignoniaceae) in South Africa

“…& P. Syd. on soybean (Bonde et al, 2013). However, in general, during our field trials, P. transformans did not produce teliospores, regardless of temperatures after inoculation.…”

Soybean Rust Epidemics as Affected by Weather Conditions in Brazil

“…At different counties located in southern USA air temperatures above 33 °C decreased the SBR epidemics. Such effects were prominent whenever three consecutive days with air temperatures of 35 °C were observed, and in this case 50% fewer urediniospore were produced (Bonde et al, 2013).…”

“…At Pedra Preta low values of LWD at the beginning of the growing season were not sufficient to affect the SBR in soybean production fields (Table 3). During the period with a low LWD, soybean plants were in the vegetative phase, and at this time soybean plants had a low susceptibility to SBR (Bonde et al, 2013). Thus, throughout the crop growing season in Pedra Preta neither the weather conditions nor the host characteristics favored the SBR occurrence in soybean production fields.…”

“…occurrence (Bonde et al, 2013). Under Brazilian weather conditions Alves et al (2007) evidenced a more intense SBR infection at an air temperature around 20 °C, culminating in a lower SBR infection when the air temperature remained below 15 °C or above 28 °C.…”