Disease
A Populus nigra specific gall Pemphigus spyrothecae, Kew. © F Rumsey
Poplar Scab (Venturia populina/Pollaccia elegans)
Since around the summer of 2000 a virulent disease has hit Populus nigra and in particular the selected male clone known as the Manchester Poplar; this in most cases leads to death over a three year period. The infection is caused by a fungal disease called Venturia populina, or its asexual stage Pollaccia elegans . A number of Venturia species cause diseases on other species of trees, the most common being apple scab. Known here as Poplar Scab, the Italians have termed it 'Summer Leaf Drop'. Abroad it is principally infecting varieties and hybrids of black poplar all of which occur in Greater Manchester but there remain unaffected. It is suspected that a new strain of Venturia has either developed, or arrived from abroad. Why it suddenly appeared in Greater Manchester with such virulence remains a mystery. There is a strong suspicion that climate change is playing a role. It is known that 2 weeks of wet weather in spring are needed for infection and if then followed by a hot summer, conditions are perfect. Once infected the tree remains infected. The infection is often coupled with the presence of other pathogens such as the Poplar Leaf Spot (Marssonina brunnea). Because of its possible devastating consequences for the native Water Poplar populations the spread of the disease is being closely monitored. Initial symptoms are hard to detect; it is more readily detectable in groups or rows of poplars where some will appear to have a slight browning of the leaves compared to surrounding individuals and canopy leafing is reduced. In the following spring, trees infected the previous season leaf-out but they do not produce the normal very dense canopy. By late June to mid July, they can lose up to 90% of their leaves. Re-foliation is attempted but leaves continue to drop until by early October the infected trees have no leaves left. New leaves initially show black lesions followed by a light brown shrivelled appearance similar to brown paper. Leaf fall in the autumn is an active process with the tree forming an abscission layer at the base of the petiole (leaf stalk). Because infected leaves are killed, they cannot form an abscission layer and so hang brown on the tree for some time.
As the leading new shoots are also killed back, new growth must arise from a point lower down the twig. Infected trees begin to develop a dishevelled appearance, obvious even to casual observers. The blackened drooping leading tip is a good indicator.
At some point before the actual death of the tree, it has been observed that the thick bark becomes detached from the underlying wood of the stem. Following wet weather, damp patches appear on the stem giving the appearance of a weeping canker. This is probably caused by rainwater gaining access higher up the stem and lodging in the space between the wood (the xylem) and the loose but intact bark. When pierced with a sharp implement, the rainwater flows out with a rusty red appearance. The bark is easily removed at this point and the inner bark (the phloem) presents a curious dark red stringy appearance. Once the trees die, the loose bark dries, cracks, and can be easily pulled off the tree in large sheets.
Trees that have been pollarded over the last few years succumb very quickly presumably due to their already being under stress and the new young leaves are more susceptible to infection.
[www.aie.org.uk/trunkline/pests/aie_pd_popscab.html]
Many other fungal pathogens of Poplar species are reported, some with a preference, or strict specificity for P. nigra and its close relatives.
Other parasites
Mistletoe (Viscum album). This well known stem-parasitic flowering plant frequently attacks Populus species and is often found on Hybrid Black Poplars but, interestingly, is very rarely found on pure P. nigra.