Measures to control the spread of zebra chip disease should focus on the tomato / potato psyllid rather than preventing the export of fresh and seed potatoes, argues Dr. Munyaneza of the Agricultural Research Service at the US Department of Agriculture.
Zebra chip (ZC), a new and economically important disease of potato in the United States, Mexico, Central America, and New Zealand has caused millions of dollars in losses to the potato industry. ZC is characterized by a severe dark and light striped symptom pattern in raw and fried chips and fries processed from infected potato tubers, affecting their taste and making them commercially unacceptable.
The disease causes serious losses to fresh market, tablestock and export potato industry as well. Plant growth and yield are severely affected by the disease. Whole crops might be rejected because of high ZC levels, occasionally leading to abandonment of entire fields. However, consumption of products processed from ZC-infected tubers is not known to pose any human health issues.
ZC has been linked to the new bacterium “Candidatus Liberibacter solanacearum” (also known as “Ca. Liberibacter psyllaurous”), transmitted to potato by the potato / tomato psyllid Bactericera cockerelli. This bacterium also severely affects other solanaceous crops, including tomato, capsicum, eggplant, tomatillo, and tamarillo. The psyllid, the insect vector that can spread the disease from plant to plant, is native to North America and was accidentally introduced into New Zealand, apparently sometime in the early 2000s. Currently, all management tactics for ZC are targeted against the psyllid, which is controlled primarily by insecticide applications.
Liberibacter can be moved by its vectors (such as insects) or in live host plant material. The primary way that potato psyllids acquire and spread Liberibacter is by feeding on infected plants. Not only do psyllids become infective for life, they also transmit the bacterium to their offspring, which significantly contributes to ZC spread between geographic regions by dispersing psyllids and helps maintain the bacterium in the regions during the insect’s overwintering period.
A single adult potato psyllid is capable of inoculating the bacterium to potato within a period as short as six hours, leading to development of ZC. Disease symptoms generally develop in potato tubers about three weeks following exposure of plants to infective psyllids. Tuber development ceases upon the onset of ZC symptoms, resulting in substantial yield loss or lack of tuber set. In addition, levels of tuber solids decrease as soon as initial disease symptoms are observed, whereas reducing sugar levels in tubers increase dramatically, significantly lowering potato processing quality.
ZC-infected tubers potentially being a source of the disease spread is a major concern, especially for national and international trade of fresh and seed potatoes. Recent research conducted in the United States by Dr. Munyaneza and collaborators showed that potato seed quality of ZC-infected tubers is significantly diminished as the tubers generally do not sprout and if they do, produce hair sprouts and weak plants.
However, the study concluded that the risk of spreading ZC through disease-infected tubers is extremely low and not significant because the number of ZC-infected tubers giving rise to infected plants is generally negligible and these plants are short-lived. Most importantly, potato psyllids must be present to spread the disease.
The main pathway of introducing Liberibacter into potato and other solanaceous crops in regions where ZC is absent would be the introduction of infective potato psyllids, rather than infected seed material or fresh tubers. All life stages of the psyllid can easily be transported on live plant material that serves as hosts to potato psyllid, including produce for sale as well as plants meant for propagation.
Because potato tubers are not a suitable host of the psyllid, exported potato tubers are much less likely to contribute to psyllid movement. Therefore, more emphasis should be on developing strategies and phytosanitary measures to effectively exclude the potato/tomato psyllid instead of focusing on preventing export of fresh and seed potatoes.
Further Reading
Munyaneza, J.E. 2012. Zebra Chip Disease of Potato: Biology, Epidemiology, and Management. American Journal of Potato Research 89: 329-350.
