Downy mildew epidemic on common ragweed in Hungary caused by Plasmopara halstedii

Common ragweed (Ambrosia artemisiifolia (L. ) Descourt) was introduced into Hungary almost a century ago and became the most important weed by the end of twentieth century. Its pollen causes a seasonal allergic reaction for ca. 10 % of the population of the country. In order to investigate pathogenic mycobiota as natural control organisms of common ragweed in Hungary, a country-wide disease survey was initiated in 1996. Six pathogens were reported for the first time from Hungary, among them Plasmopara halstedii (Far.) Berl. et De Toni (Bohár & Vajna 1996). This pathogen occurred sporadically and caused no severe damage to the common ragweed population. In the autumn of 2001, an epidemic occurred in large areas of central Hungary. Microscopic examination of 500 samples of infected plants collected randomly from eighteen locations of the central region proved that Plasmopara halstedii was the causal agent of the disease which caused mass death of common ragweed. Angular, pale green lesions with a white cover of sporangia were common on the lower surface of leaves which eventually became necrotic (Fig. 1 - 5).

Symptoms of systemic infection were not observed. In some cases there were mixed infections of plants by P. halstedii and Albugo tragopogonis. Incidence of downy mildew infection was 12 to 40 %, but in some locations it was as high as 95 to 100 %. This epidemic might be due to the unusual weather conditions of September and October 2001. In September, there were 22 days with a total precipitation of 102.1 mm. This is 243 % of the norm. In October there was an unusually warm period; the daily average temperature was 3.2 °C above normal and intensive dew formation was observed daily.

The pollen count from A. artemisiifolia in air samples from August 27 to September 2 was about 1000/ m³. From September 3 to 9 this number was still very high but declined to about 100/m³ from September 10 to 16. From September 17 to 23 pollen counts for A. artemisiifolia were ≤ 10/ m³ and reached 6/m³ from October 1 to 7, about 10 % of normal. Pollen counts were recorded by the Hungarian Aerobiological Service. The low pollen count is considered to be a consequence of the early mass death of ragweed plants caused by P. halstedii in connection with the period of abnormally high precipitation in September and above normal temperatures in early October. This is the first record of a downy mildew epidemic on common ragweed in Hungary and in Europe.