Development and evaluation of Taqman assays for the differentiation of Dickeya (sub)species

Abstract

TaqMan assays were developed for the detection of seven Dickeya species, namely D. dianthicola, D. dadantii, D. paradisiaca, D. chrysanthemi, D. zeae, D. dieffenbachiae and D. solani. Sequences of the gene coding for dnaX were used for the design of primers and probes. In studies with axenic cultures of bacteria, the assays were highly specific and only reacted with strains of the target species, and not with non-target bacteria, including those belonging to other Dickeya species and other genera. The detection thresholds for DNA extracted from pure cultures of target strains ranged from 10 to 100 fg. The TaqMan assays for D. dianthicola and D. solani were more extensively evaluated as part of a method validation procedure. The threshold level for target bacteria added to a potato peel extract diluted ten-times in a semi-selective broth, was strain dependent and ranged from 1,000 to 100,000 cfu/ml. The coefficients of variation for repeatability and reproducibility were low and results were largely independent of the type of substrate, i.e. potato tuber or carnation leaf extracts. However, during routine testing of seed potatoes, false-positive reactions were found with the assay for D. solani. The use of the TaqMan assays for inspection of plant propagation material, ecological studies and studies on the effect of control strategies in disease management strategies is discussed.