Drought and Heat Promote Macrophomina phaseolina Infection in Impatiens hybrida

Impatiens rank second only to petunias in bedding plant sales and are worth $170 M wholesale in the US. Although impatiens have traditionally had little disease pressure in the US, in 2004 downy mildew (Plasmopara obducens J. Schröt.) had been reported on Impatiens walleriana Hook.f., and by 2011 it had spread worldwide and presented a major threat to the commercial production of impatiens. From 2009 to 2014, total sales for all impatiens dropped nearly $30 M, likely in response to disease reports. Growers were encouraged to shift production from susceptible I. walleriana to resistant New Guinea impatiens and their interspecific hybrids (Impatiens hybrida Hort.). Since then, US sales for New Guinea impatiens have increased from 34.5% of the total impatiens market in 2009 to 46.2% in 2014. Here we report a new devastating disease affecting hybrid New Guinea impatiens. Macrophomina phaseolina (Tassi) Goid. devastated eight cultivars of New Guinea-type impatiens at the Truck Crops Branch Experiment Station in Crystal Spring, MS. Plants performed well during the season; however, near the end of July they developed stem cankers and died within a 2­- to 3-­week period. Plants tested negative for Phytophthera, but Koch’s postulates and DNA ITS sequencing confirmed M. phaseolina as the causal pathogen. Subsequent growth chamber studies of inoculated impatiens demonstrated that environmental factors of heat and drought promote the development of disease symptoms. Fourteen-hour daytime temperatures of 36.7 °C and ten-hour nighttime temperatures of 24 °C induced stem canker growth and expansion. At low day and night temperatures of 22 °C and 20 °C, stem cankers did not develop at inoculation sites. Macrophomina phaseolina was able to infect both wounded and unwounded stems, but stem cankers only developed on unwounded stems when grown at high temperatures. Drought conditions also played a role in promoting pathogenicity, but to a lesser extent than temperature. Further research will be needed to determine management practices that are suitable to landscape settings. This fungal pathogen is found worldwide in various agricultural and horticultural settings. The Systematic Botany and Mycology Laboratory Fungus-Host Distributions Database has over 1,500 reports of charcoal rot worldwide on nearly 300 genera including corn, cotton, soybean, petunia and begonia. As global temperatures rise, the occurrence of this disease may become more prevalent and problematic to impatiens and other horticulturally important crops.