Effects of Temperature on ‘Candidatus Liberibacter solanacearum’ and Zebra Chip Potato Disease Symptom Development

Munyaneza, J. E., Sengoda, V. G., Buchman, J. L., and Fisher, T. W. 2012. Effects of temperature on ‘Candidatus Liberibacter solanacearum’ and zebra chip potato disease symptom development. Plant Dis. 96:18-23. Temperature has been shown to have a significant effect on development of liberibacter species associated with citrus Huanglongbing disease. ‘Candidatus Liberibacter africanus’ and ‘Ca. L. americanus’ are both heat sensitive, whereas ‘Ca. L. asiaticus’ is heat tolerant. The recently described ‘Ca. L. solanacearum’ is associated with zebra chip (ZC), a newly emerging and economically important disease of potato worldwide. This psyllid-transmitted liberibacter species severely affects several other solanaceous crops and carrot. Experiments were conducted to evaluate effects of temperature on development of ‘Ca. L. solanacearum’ and ZC disease. Potato plants were inoculated with ‘Ca. L. solanacearum’ by briefly exposing them to liberibacter-infective potato psyllids at various temperatures under laboratory conditions. Following insect exposure, the plants were maintained at selected temperature regimes in growth chambers, monitored for ZC symptom development, and later tested for liberibacter by polymerase chain reaction to confirm infection. Results indicated that temperatures below 17°C appear to slow development of ‘Ca. L. solanacearum’ and ZC symptoms, whereas temperatures above 32°C are detrimental to this liberibacter. Compared to Huanglongbing liberibacters, ‘Ca. L. solanacearum’ appears heat sensitive. The sensitivity of this bacterium and its insect vector to temperature may partially explain incidence, severity, and distribution of ZC in affected regions. The fastidious bacterium ‘Candidatus Liberibacter’, vectored by at least five psyllid species, has so far been associated with economically important diseases of citrus, solanaceous crops, and carrot. Citrus greening disease or Huanglongbing (HLB) is associated with three species of liberibacter, namely ‘Candidatus Liberibacter asiaticus’, ‘Ca. L. africanus’, and ‘Ca. L. americanus’. ‘Ca. L. asiaticus’ and ‘Ca. L. americanus’ are vectored by the Asian citrus psyllid, Diaphorina citri Kumayama, whereas ‘Ca. L. africanus’ is transmitted by the African citrus psyllid, Trioza erytreae Del Guercio. HLB is the most destructive disease of citrus in many parts of the world, including Africa, Asia, the United States, Mexico, the Caribbean Islands, and Brazil (4,10,11). Zebra chip (ZC), a newly emerging and very destructive disease of potato in the southwestern United States, Mexico, Central America, and New Zealand, is associated with ‘Ca. L. solanacearum’ (syn. ‘Ca. L. psyllaurous’) vectored by the potato/tomato psyllid, Bactericera cockerelli (Šulc) (8,9,12,14–16,20,22,23,26,33,35,36). This disease is causing millions of dollars in losses to the potato industry, often leading to the abandonment of entire fields (9,21,22,26,35). ‘Ca. L. solanacearum’ also severely affects several other cultivated and wild solanaceous species (15,28,29). Furthermore, ‘Ca. L. solanacearum’ has recently been detected for the first time in the carrot psyllid Trioza apicalis Förster and carrot plants damaged by this insect in Finland (20,24,25). T. apicalis is a serious pest of carrots in northern and central Europe, where it can cause up to 100% crop loss. Most recently, a new species of liberibacter designated ‘Ca. L. europaeus’ was described and detected in pear and the pear psyllid Cacopsylla pyri (L.) in Italy (32); this bacterium species apparently behaves as an endophyte in pear rather than as a pathogen. Development and severity of insect-transmitted plant diseases depend on the interactions between susceptible host plant, pathogen, vector, and a favorable environment. Little is known of environmental factors that influence development of different liberibacter species and symptoms of diseases they cause to their host plants. However, temperature has been shown to significantly affect the development of HLB-associated liberibacter species and disease symptoms (4,18). ‘Ca. L. africanus’ and ‘Ca. L. americanus’ have been described as heat sensitive, whereas ‘Ca. L. asiaticus’ appears heat tolerant (4,18,30). This pathogen sensitivity to heat may explain the geographical distribution of these different liberibacter species and their insect vectors as well as the diseases they cause in different parts of the world. Information on the effect of temperature on both ‘Ca. L. solanacearum’ and ‘Ca. L. europaeus’ is lacking. Herein, we report results of experiments designed to investigate the effect of temperature on development of ‘Ca. L. solanacearum’ and ZC potato disease symptoms under controlled laboratory conditions. Materials and Methods A series of experiments were conducted in 2009, 2010, and 2011 to evaluate the effects of temperature on development of ‘Ca. L. solanacearum’ and ZC disease. Potato plants were inoculated with ‘Ca. L. solanacearum’ by exposing them to liberibacter-infective potato psyllids at various temperatures under laboratory conditions at the USDA-ARS facility in Wapato, WA. Following insect exposure, the plants were maintained at selected controlled temperature regimes in growth chambers, monitored for ZC symptom development, and later tested for liberibacter by polymerase chain reaction (PCR) to confirm infection. Sources of potato plants and psyllids. Certified pathogen-free potato mini-tubers of the variety Atlantic were obtained from CSS Corresponding author: Joseph E. Munyaneza, E-mail: [email protected] Accepted for publication 14 July 2011. http://dx.doi.org/10.1094 / PDIS-03-11-0185 This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 2012.