Abstract
In April 2019, hydroponically cultivated cucumber plants with characteristic symptoms of crazy root disease were found in two different commercial production cucumber greenhouses in Poland. Due to excessive and inappropriate root growth, this disease led to a reduction in yield and deterioration of the general conditions of infected plants. Bacteria isolated from the roots were subjected to a morphological evaluation, as well as molecular, biochemical and pathogenicity tests. To identify the bacteria causing the disease, Agrobacterium-like colonies were subjected to PCR with primers complementary to the pathogenicity-related genes located on the crazy root-inducing plasmid (Ri-plasmid): the virD2A + virD2E primers complementary to the virD2 gene and the rolBF + rolBR primers complementary to the rolB gene. The pathogenicity of the isolated strains was studied in sunflowers and cucumbers. Twelve strains positive for the Ri plasmid, as determined by PCR, and pathogenic to sunflowers were identified based on sequence analysis of the 16S rRNA and recA genes. One strain was classified as belonging to the genus Pararhizobium, three to Rhizobium, and eight to Agrobacterium biovar 1, with the highest similarity to genomospecies G3. The results of the analyses suggest that these strains may belong to a new, thus far, undescribed species. To confirm this hypothesis, further phylogenetic studies are required.
Highlights
The occurrence of crazy root disease, known as a root mat or hairy root, is responsible for significant economic losses, especially in hydroponic vegetable production
Out of the 51 strains isolated from the tested cucumbers, 12 that were preidentified as Agrobacterium/Rhizobium were pathogenic when tested on sunflowers, and after re-isolation, the identity of the inoculated bacteria was confirmed by colony morphology and PCR with primers complementary to the rolB gene
We used sunflower seedlings to test the pathogenicity of the bacteria because this plant species is very susceptible to a wide range of Agrobacterium and
Summary
The occurrence of crazy root disease, known as a root mat or hairy root, is responsible for significant economic losses, especially in hydroponic vegetable production. Based on reports of crazy roots in different plant species, it is known that the bacteria causing this disease carry root-inducing plasmids (Ri-plasmids) Bacteria that cause this disease and belong to the genus Agrobacterium have been classified mainly as Agrobacterium bv 1 genomospecies: G1, G3, G7, G8, and G9 (Sawada & Azegami, 2014; Bosmans et al, 2015, 2017; Ignatov et al, 2016). Among other species known to possess causal agents of crazy root disease, Rhizobium rhizogenes (formerly Agrobacterium biovar 2) and Allorhizobium vitis (formerly Agrobacterium vitis or Agrobacterium biovar 3) have been reported, and both species are known to be causal agents of crown gall disease (Weller et al, 2000)
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