Wheat dwarf - an old disease with new outbreaks in Sweden

The wheat dwarf geminivirus (WDV) which is transmitted by the leafhopper, P sammotettix alienus, has occasionally shown up as a severe disease on winte r wheat in certain parts of Sweden. Characteristic symptoms for wheat dwarf are dwarfing, yellowing and no or e mpty heads which frequently stay entirely or partly in the sheaths. These k inds of symptoms, which are mainly caused by secondary infections in spring or early summer, seem to be identical to those of the disease called "slid sjuka" and found already in the early parr of the century in Sweden. This d isease was disastrous in certain years (e.g., 1915 and 1918) in many wheat fields bur disappeared and has not been a problem since the 1940's until it showed up as wheat dwarf last year (1997). Some field observations and tests are reported. ELISA was found to be an ex cellent method to detect and confirm occurrence of WDV both in plants and i n hoppers, usually giving reliable results also when testing old dry plants . The damage caused by WDV varied considerably. Mostly, only a low frequency of infected plants was found except in minor spots but on some farms also d isastrous damage occurred. Spring-sown barley did nor show any symptoms or damage. In fact, several observations and findings in 1997 suggest that the barley strain of WDV does not occur in Sweden. Attempts to infect two comm on barley cultivars (cv. 'Baronesse' and cv. 'Etu') with WDV also failed Further evidence was obtained that the common couch-grass (Agropyron repens ) cannot act as a host for WDV, which is very important from an epidemiolog ical point of view as this grass is vegetatively propagated. Causes of the new outbreaks of wheat dwarf are discussed and it is conclude d that agricultural practices have changed in a way favourable for the dise ase. Avoidance of "EU set-aside fields" after WDV-infected wheat crops stem s to be crucial for a successful control of the disease.