Epidemic of Plasmopara halstedii on Ambrosia artemiisifolia in Hungary
Ambrosia artemisiifolia (candidate EPPO quarantine pest) was introduced into Hungary at the beginning of the 20th century and had become an important weed species in Hungary by the end of the century. Studies on potential natural control of A. artemisiifolia in 1996 identified 6 pathogens, including Plasmopara halstedii (causing downy mildew of sunflower), which occurred sporadically on A. artemisiifolia without causing severe damage. During autumn 2001, an epidemic disease was observed on A. artemisiifolia in large areas of central Hungary. Symptoms were angular, light green lesions with a white cover of sporangia on the lower leaf surface, becoming necrotic. P. halstedii was identified as the causal agent of this epidemic, and was sometimes found in combination with Albugo tragopogonis. Incidence of downy mildew infection was 12-40%, but could reach 95-100%. Mass mortality of A. artemisiifolia was observed. This epidemic might be due to unusually high precipitation in September and warm temperatures in October 2001. This is the first record of an epidemic of P. halstedii on A. artemisiifolia in Hungary and in Europe.
Vajna, L. (2002) Downy mildew epidemic on common ragweed in Hungary caused by Plasmopara halstedii. New Disease Reports, volume 6, August 2002 – January 2003. http://www.bspp.org.uk/ndr/jan2003/2002-22.htm