Aleppo pine knot disease: histology of the knots, detection of casual agent and mode of transmission

Knot disease of Aleppo pine (Pinus halepensis subsp. halepensis) occurs in the western range of the host in the Mediterranean region. The disease, originally named Aleppo pine tuberculosis (from tubercle, i.e. knot) by Petri in 1924, causes a hyperplastic growth on the twigs and small branches, with knots that initially are small, green and lignified, and expand to 5-6 cm diameter at maturity. These knots have been associated with various bacterial species. In this study, only one bacterium was isolated from Aleppo pine knots, and this bacterium was constantly associated with the pine weevil Pissodes castaneus. The hyperplastic growth started either in the cortex or in the xylem, while the knot was formed in the cortical parenchyma. Bacteria were grouped in zoogloeae in the intercellular spaces or inside lysigenous cavities. Bacterial microcolonies have also been observed in the parenchyma cells. Inoculation of two of the bacterial strains on healthy Aleppo pine twigs did not induce knot formation. Knots did however form when adults of P. castaneus were externally contaminated with the bacteria and were then allowed to feed on pine saplings. These latter knots contained the same bacterial isolate as that which had been used to contaminate the weevils. All the isolates examined were tentatively assigned to the genus Erwinia. As this bacterium seems to be the causal agent of Aleppo pine knot disease, its complete identification and characterisation is needed.