Psyllids TH E WOR LD OF “P syllids” or ‘jumping plant lice’ are small phytophagous, phloem feeding insects that are typically monophagous (feed on a single plant) or oligophagous (feed on a few related plants). Together with aphids, coccids and whiteflies they form the monophyletic group, Sternorrhyncha (Homoptera), which is considered basal within the true bugs (Hemiptera)”. This I copied from the Psyllids Home Page. The scientific name of the psyllids that attack potato and tomato is Bactericera (previously Paratrioza) cockerelli. When the insect feeds on potato it causes the symptoms “potato yellows’. The symptoms on the potato plant are an upward stretching of the top leaves that also show a marked yellowing. In severe cases plants die prematurely. Tubers remain small and have odd shapes; sometimes they start sprouting before harvest but when used as seed they often do not sprout or only give weak sprouts. In Mexico and southern parts of the United states the insect and its symptoms have been known since early last century. The symptoms presumably are caused by the psyllids toxic saliva but researchers are still looking if not the insect is a vector of a virus or other organism that is responsible for potato yellows. In the last decade of the last century, however, psyllids affected plants in Mexico produced tubers that internally showed brown discoloration. When processed into chips these chips showed brown stripes (zebra chips) and could not be sold. Presently the losses to the industry are many millions of dollars per year and the cause has only recently been elucidated and described by JM Crosslin and VG Sengoda and colleagues in Plant Health Progress and the American Journal of Potato Research in 2010. It reads like a detective story. For many years it was not clear why psyllid affected potato field that always showed potato yellows and also often showed leaf rolling, purple colors and leaf scorch increasingly started to show zebra chips (ZC) symptoms. Grafting a stem of a plant producing ZC on a healthy plant also caused ZC symptoms in this plant so researchers knew there had to be an agent but which one? Grafting healthy plants with stems from a plant showing potato yellows only did not lead to potato yellows in the recipient plant. Only in 2008 did researchers in America and New Zealand detect a novel bacterium “Candidatus Liberibacter solanacearum” in potato that showed zebra chips symptoms but not in potato with potato yellows symptoms only. The researchers planted healthy minitubers of the very susceptible to ZC chipping variety Atlantic in pots in greenhouse compartments. They identified psyllids colonies that either (from a ZC affected field in southern Texas) or not contained the newly discovered bacterium and verified their presence and absence in the laboratory and let them feed on the Atlantic plants. The plants were subjected to the following treatments: POTATO R ESEARCH Adult potato psyllids (courtesy InsectImages.org) without psyllids (controls), with bacteria free psyllids for 30 days and then cleaned, with bacteria free psyllids until the end of the experiment and with Liberibacter infected psyllids for 30 days when the psyllids and their eggs and larvae were removed. All plants exposed to psyllids after 30 days showed the typical potato yellows symptoms such as stunting, yellowing and curling leaves and side shoot growth. The plants with the clean psyllids removed after 30 days recovered and looked much like the controls, where not removed the potato yellow symptoms were severe and the plants died around day 75. The contaminated psyllids – even though removed after 30 days – did not recover and died 45 days after the beginning of exposure. All plants derived from minitubers produced tubers of which only from plants subjected to infected psyllids showed zebra chips symptoms. Laboratory test only found the bacteria in these tubers. So research has established that potato psyllids are capable of causing two disorders in potato: potato yellows and zebra chips but the second only as a vector of the newly discovered bacterium Candidatus Liberibacter solanacearum. Understanding all this may help in eradication programs including trapping and spraying, avoiding sources of the bacterium and avoidance of overwintering in greenhouses. Anton Haverkort anton.haverkort@wur.nl Potato World 2010 • number 3 25 Pagina 24
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