Population dynamics in tobacco blue mold incidences as a consequence of pathogen control and virulence performance of Peronospora tabacina phenotypes

Peronospora tabacina is a major disease in tobacco cultures in many countries. Fungicide treatment with phenylamides is often used to control the pathogen, but resistance occurs and continuous application may cause selection pressure which influences the pathogen's population structure. Field isolates collected from blue mold incidences in Germany and other countries of the European Mediterranean zone over a period of 10 years were screened for the presence of the two currently recognized phenotypes of the pathogen, one of which is fully tolerant to mefenoxam (metalaxyl-M). Besides the geographical distribution of the fungicide-tolerant strain in France, Italy and Germany, the long-term monitoring revealed a significant phenotype shift in the population of P. tabacina in Germany. This coincided with the alteration of disease management with respect to the fungicides recommended for use. While between 2002 and 2004 only metalaxyl-M-tolerant isolates were found in field accessions from Germany, the metalaxyl-M-sensitive strain occurred in 2005 and its ratio in the population increased gradually. Vice versa, the tolerant strain was not found anymore in samples 2010 and 2011. Infection experiments under controlled conditions with genetically homogenous strains of the two phenotypes revealed a significant higher virulence of the sensitive strain which is expressed in a two day reduction of the time period required from inoculation to sporulation. The consequence of fitness differences between the two phenotypes was tested in dual infection experiments starting with a 1:1 mixture of sporangia. When propagating the offspring by using the early produced sporangia for the next infection, the sensitive strain dominated already in the F2 generation and no tolerant phenotype was detected with specific primers in PCR with DNA from sporangia after four infection cycles. In contrast, the tolerant strain could survive when late produced sporangia were used for the next round of infection. These results suggest that the dominance of the metalaxyl-M tolerant phenotype of P. tabacina in field samples is due to selection pressure caused by continuous intensive use of the fungicide rather than by natural selection criteria.