The implication of a bacterium in a plant disease was first shown in 1878 for fire blight of apples and pears in the USA. Only a few years later, in 1885, this bacterium, Erwinia amylovora, was isolated from diseased plants and cultured. It was then inoculated in the host from which it was obtained to reproduce the disease and subsequently recovered from diseased tissue, fulfilling what is known as Koch's postulates. Isolation of bacteria on culture media and pathogenicity testing are also today basic techniques in diagnostic phytobacteriology.
Symptoms of plant disease caused by bacteria are varied: spots on leaves and fruits, wilting, tumor and canker, rotting and blight. Advanced stages of disease may result in plant death.
Bacterial densities in diseased plant tissue are usually high, reaching billions of cells, which may become visible in sticky droplets on the affected plant tissues.
Diagnosis of a bacterial plant disease depends on recognition of characteristic symptoms, isolation and identification of the presumed infectious agent. The most common bacterial diseases are not difficult to diagnose because the symptoms are very specific. However, divergence in disease symptoms can occur due to environmental conditions and different bacterial phytopathogens may develop similar symptoms.
Diagnosis starts with isolation of the bacteria by dilution plating on an appropriate culture medium to obtain well-separated colonies.
Dilution plating and bacterial cultures
Single colonies are cultured for identification. Cultural characteristics commonly give a general indication of the bacterium involved. The most common bacterial pathogens are readily identified using a commercially available serological assay or a simple molecular test such as PCR. For some plant pathogenic bacteria more explicit molecular analysis based on specific genomic sequences is required. These tests, known as DNA barcodes, allow to differentiate strains in a species complex or to identify closely related variants such as pathovars. A genomic fingerprint may provide information to trace the origin of introduction of the strain.
The final confirmation that the identified bacterial pathogen is the primary cause of the disease is obtained in a pathogenicity test. A susceptible host species is challenged with a cell suspension of a pure bacterial culture. Diagnosis is finalised when disease symptoms are reproduced and the bacterium is demonstrated in the affected tissue.
Plant pathogenic bacteria can be present as latent infection or contaminant in plant material such as scions, cuttings, tubers and seeds and in water used for irrigation. Detection tests are then performed on composite samples by selective plating, immunofluorescence and PCR (conventional and real-time assays, sometimes after selective enrichment).
The lab offers ISO 17025 accredited and standardised tests (EPPO, ISTA) for phytosanitary regulated bacteria. In case of a positive result the suspected occurrence is verified by identification tests and a pathogenicity assay.
Johan Van Vaerenbergh