Abstract
Aster yellows (AY) is an important disease of Brassica crops and is caused by Candidatus Phytoplasma asteris and transmitted by the insect vector, Aster leafhopper (Macrosteles quadrilineatus). Phytoplasma-infected Aster leafhoppers were incubated at various constant and fluctuating temperatures ranging from 0 to 35 °C with the reproductive host plant barley (Hordium vulgare). At 0 °C, leafhopper adults survived for 18 days, but failed to reproduce, whereas at 35 °C insects died within 18 days, but successfully reproduced before dying. Temperature fluctuation increased thermal tolerance in leafhoppers at 25 °C and increased fecundity of leafhoppers at 5 and 20 °C. Leafhopper adults successfully infected and produced AY-symptoms in canola plants after incubating for 18 days at 0–20 °C on barley, indicating that AY-phytoplasma maintains its virulence in this temperature range. The presence and number of AY-phytoplasma in insects and plants were confirmed by droplet digital PCR (ddPCR) quantification. The number of phytoplasma in leafhoppers increased over time, but did not differ among temperatures. The temperatures associated with a typical crop growing season on the Canadian Prairies will not limit the spread of AY disease by their predominant insect vector. Also, ddPCR quantification is a useful tool for early detection and accurate quantification of phytoplasma in plants and insects.
Highlights
Aster yellows (AY) disease in Brassica and in numerous other crops in North America has received considerable attention because of its significant negative economic impact on agriculture
‘Ca. Phytoplasma asteris’ by Macrosteles quadripunctulatus Kirschbaum was investigated at only four constant temperatures[9], while another study used two constant temperatures to report that the multiplication kinetics of Flavescence doree doubled at 25 °C compared to 20 °C10
This study represents the first use of Droplet digital PCR (ddPCR) to quantify the number of AY-phytoplasma in plants (B. napus and H. vulgare) as well as in the insect vector M. quadrilineatus
Summary
Aster yellows (AY) disease in Brassica and in numerous other crops in North America has received considerable attention because of its significant negative economic impact on agriculture. While there are more than 20 leafhopper species known to spread phytoplasma diseases, the Aster leafhopper, Macrosteles quadrilineatus Forbes (Hemiptera: Cicadellidae), is the major vector of AY-phytoplasma in Brassica crops[3,4,5] Biological parameters such as physiology, biology and behaviour of insects, pathogens and plants, will be affected by global climate change[6]. Experiments with fluctuating as opposed to constant temperatures may be a more accurate way to assess the effect of temperature on the biology of insects or pathogens[16] Another potential limitation of other studies is the investigation of a single organism instead of the complex of organisms that are involved in the epidemiology of a disease. No studies exist on the quantification of phytoplasmas in a vector insect using ddPCR
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