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Three basic types of symptom can be observed on leaves depending on the host species; some features of more than one of these types are sometimes partially exhibited by one host at the same time.
The most typical symptoms on leaves of Aigeiros poplars, also reported on Populus balsamifera, consist of circular blotches from 1 to 10 mm diameter (mean 4-5 mm), brown or blackish, mainly epiphyllous, on which whitish punctiform acervuli are visible. Cellerino (1979) reported a clear outline on P. × euramericana [P. canadensis] and a dendritic pattern on P. nigra, whereas Spiers (1984) also observed drop-like and irregularly shaped blotches. The two authors agree on a progressive coalescence of blotches in the case of heavy infections, causing extensive necrotic patches of the leaf blade.
Another leaf symptom type, observed on some P. nigra and P. × euramericana clones and on P. simonii (Spiers, 1984), is small blackish spots (0.5-2.0 mm in diameter), circular or angular, each sprinkled with 1-3 acervuli; these spots may also coalesce to form necrotic patches.
The third symptom type, showed by infected leaves of P. tremuloides and P. angustifolia, consists of large, brown blotches with an indefinite dendritic outline. Conidial masses accumulate on the surface of radiating dendrites, that originate due to the development of subcuticular fibrillar mycelium.
Cellerino (1979) also reported large lozenge-shaped lesions on shoots of P. nigra and P. × euramericana, and Gremmen (1965) noted acervuli on dying twigs.
The control of D. populorum mainly follows that applied to D. punctiformis (better known as its anamorph, Marssonina brunnea) in intensive poplar culture, but it is rarely practised except in some countries of central and eastern Europe and for cases in which ornamental Lombardy poplars (Populus nigra var. italica) need to be preserved. No phytosanitary measure is specifically applied to D. populorum.
Cultural Control and Sanitary Methods
In situations where this kind of control is considered economically advantageous, the inoculum mass of D. populorum can be reduced by pruning infected shoots and burning them. Soil tillage can destroy the fallen leaves where apothecia are produced by the pathogen, thus eliminating primary infections. This method is particularly suitable for D. populorum, which does not overwinter as conidia.
Specific genetic control to obtain resistant poplar clones is only carried out locally, where the disease causes real damage. For example, in China P. simonii and P. canadensis are generally resistant to the pathogen.
Due to the variable regulations around (de-)registration of pesticides, we are for the moment not including any specific chemical control recommendations. For further information, we recommend you visit the following resources:
In Europe, D. populorum is not generally economically important, as in areas such as south-central Europe where poplar culture is most widespread, the clones most used are not very susceptible to this pathogen and the climate is not suitable for a consistent inoculum mass. However, it may cause considerable damage in the peripheral zones of the region of intensive poplar culture, especially in central and eastern Europe. Damage has been reported in Ireland (Riordan and Kavanagh, 1965), Russia (Sokolova, 1975), Bulgaria (Rosnev and Nardenov, 1986) and Finland (He and Kurkela, 1988).
In Europe, ornamental Lombardy poplars (P. nigra var. italica) are sometimes heavily infected, but in this case the impact is aesthetic in terms of landscape.