In keeping with a general tendency in plant pathology, most of the parasites able to attack the genus Populus are fungi, primarily Ascomycetes, but also many Mitosporic Fungi and Basidiomycetes1.
The number of fungi reported at least once on diseased poplars is so vast that to cite each one would itself be a chimerical task and in any event outside the scope of this work; new host-parasite associations, however, are being constantly described. Attention will thus be directed to the agents of the most important diseases on account of their incidence and geographical distribution, as well as of those known to be endemic in a given region or typical of certain soils and climates, and of those whose range is likely to extend in the future.
Generally speaking, the conventional distinction between root diseases, stem and branches diseases and leaf diseases is still useful, even though some parasites (such as Septoria musiva) are able to infect more than one organ, while leaf diseases, especially in intensive growing, result in poor trunk growth and hence in diminished production of wood. Even if it is not actually killed (an extreme event that occurs above all in the nursery or in the first 1 or 2 years after planting out), the whole of a tree suffers the consequences of infections of one of its organs, and it is also rendered more vulnerable to environmental stresses.
The fungi that cause leaf diseases are almost always primary parasites, i.e. able to attack healthy plants, whereas those that attack trunks and roots are mostly wound parasites, thus requiring breaches in the bark to be able to infect, and/or weakness parasites, which means that they are only aggressive on plants already debilitated for some other reason. Depending on the circumstances, therefore, they can take advantage of small more or less accidental injuries caused during the various cultural practices, or of a poor management, or of the unsuitability of the soil and/or bad climatic course. Their attacks may also follow previous infections of primary parasites or other weakness parasites on the same hosts. Therefore, a combination or succession of diseases is by no means uncommon in precarious and simplified ecosystems such as intensively cultivated poplar stands.
Although they are quite similar as regards their type of parasitism, root pathogens and stem pathogens give rise to different forms of damage: the former, through a physiologic impairment of the infected plants due to the rots of absorbing tissues, cause predominantly quantitative wood losses, the latter also result in a marked loss of quality, through the chromatic and/or structural alterations of the wood rings below the bark tissues affected by necroses or cankers2.
When compared with many more strictly forest genera, the large number of epidemic diseases to which Populus is subject may come as a surprise. Nevertheless poplar growing, whether in the nursery or in the plantation, involves the establishment of ecosystems that are extremely favourable to the rise of epidemics, either because plants are crowded together and often lack genetic diversity on vast areas due to the preferential employment of some clones, and because the array of fungous competitors that, in natural stands, help to maintain many diseases in a latent form here is very rarefied.
Lastly, it is interesting to notice that most pathogenic fungi on poplar display a certain specificity for section Leuce Duby3, or for sections Aigeiros Duby and Tacamahaca Spach together, in keeping with their high affinity (they only differ in some vegetative characters). Little information is as yet available concerning the diseases of poplars belonging to sections Turanga Bunge, Leucoides Spach and Abaso Ecken.
The taxonomy that is mentioned here for the various pathogens follows the classification reported in: Hawksworth D.L., Kirk P.M., Sutton B.C. & Pegler D.N. (1995). Ainsworth & Bisby's Dictionary of the Fungi (VIII edn.), xii-616 pp. International Mycological Institute, CAB International, Cambridge (U.K.); this even when it differs from that employed previously or commonly used at the informatory level. Consistently with these main lines, classes are not indicated for the Ascomycetes (phylum Ascomycota), because the whole question is still uncertain; besides, the pathogens whose teleomorph has not yet been found are ascribed to Mitosporic Fungi (the former "Deuteromycetes") with no further taxonomic specification. The current binomial for each pathogen is stated; in many cases, it has been thought appropriate to add the most common synonyms. When the teleomorph is infrequent in nature and the host's symptoms are usually connected with the anamorph, preference is given to the latter's binomial in the description of a disease (e.g. "leaf spots caused by Marssonina brunnea").
1 Some basic notions concerning the morphology, ecology and life cycle of Fungi are taken as read. Mention may be made of the current use of the term teleomorph for the perfect form or stage of a fungus, when spores resulting from the sexual process are produced; anamorph for the imperfect form, with asexual spores (conidia) or no spores at all; holomorph for the fungous entity as a whole.
2 A canker is a definite bark lesion, caused by parasitic infection, which progressively extends in association with host tissue necrosis; it is bounded by callus cells resulted from reaction of the still living contiguous tissues. The term necrosis refers to the temporary or permanent absence of this reaction on the part of the host.
3 The term Leuce has been retained here as more convenient for the purposes of explanation, although this section, which comprises the so-called white poplars and aspens, was recently renamed Populus Ecken. with aims of terminological exactness (Eckenwalder, 1996).
