Septoria musiva Peck is a leading bark canker agent and one of the pathogens for which the European Union has promulgated quarantine measures designed to prevent its introduction into the Old Continent. Nevertheless, it can also attack the foliage; besides, the approximately ten other Septoria Sacc. species reported on poplars, all with teleomorphs referable to Mycosphaerella Johan. (fam. Mycosphaerellaceae, ord. Dothideales, phylum Ascomycota), only attack leaves.
In view of the composite symptomatology induced by the species of greatest interest, the Septoriae are here treated separately, including a few mentions on the most important leaf parasites.
1.3.1.1. Symptoms and the damage caused - S. musiva attacks young poplars in nurseries, coppices for the production of biomass, windbreaks in the North-American Plains, as well as in intensive plantations for the production of wood. Although its compatibility with the host is in part genetically controlled, distress of the plant seems to be a predisposing factor for infection, though not as decisively as for Discosporium popoleum.
Attacks begin in late spring [e.g. not before the end of May in north-eastern U.S.A., where they often follow and overlap those of the leaf parasite Marssonina brunnea (Ell. et Ev.) P. Magn.] and continue throughout the summer. The initial signs are irregular brown leaf lesions, with a whitish or yellowish centre, that are usually larger than those caused by Septoria populicola Peck (see § 1.3.2) and eventually join together to kill extensive portions of tissue. These symptoms at first appear on the lower foliage, but soon spread to the whole crown and cause moderate defoliation.
The disease goes no further on many poplar genotypes which are autochthonous of North America, and does not extend to other organs. On various introduced hybrids (not of P. × euramericana), instead, it involves the cortical tissues of current-year shoots and new branches, after penetration of the parasite through discontinuities in the tegument (leaf scars, lenticels, wounds) or connections with the leaves (petioles). The infected areas, which are visible owing to their brown colour and their sunken surface compared to the surrounding healthy bark, crack as the result of dehydration, while swellings and calluses form at their edges as the host seeks to stop the necrosis from spreading. If this reaction fails to prevent annular colonisation of the bark, the parts of the plant above succumb; even if it is successful, the large cankers thus created are sources of new inoculum and are often invaded by weakness parasites, as well as being sites of poor mechanical resistance.
In the plantations where attacks of S. musiva are recurrent, the many bark cankers induced by the same and by secondary parasites, together with inevitable wind breakage, generally result in the death of the trees within four years; according to some estimates, the loss of biomass on the part of very susceptible clones may exceed 60%. Stricken nursery plants are rendered unmarketable.
1.3.1.2. The pathogen - Septoria musiva Peck is the anamorph of the ascomycete Mycosphaerella populorum Thompson.
It spread from its original areale, including north-central U.S.A. (North and South Dakota, Minnesota, Wisconsin, Michigan) - where the resistant genotypes are more frequent but intense attacks are still constant, especially against those clones that have since been introduced -, to the whole of the central and east of North America, as shown by reports from Mexico and some Canadian States. Observed more than twenty years ago in the Crimea and the Caucasus, it has acquired a considerable incidence on wide poplar regions in Argentina, but has not yet appeared in Europe, Africa and Oceania.
Species belonging to the Aigeiros, Tacamahaca and Leuce (Trepidae only) sections are attacked by S. musiva in various ways:
_ with few exceptions, P. nigra and P. × euramericana are resistant to the disease, both in the form of leaf spots and bark cankers;
_ P. tremuloides is resistant, but its leaves are occasionally attacked (as reported in Mexico);
_ P. deltoides, P. balsamifera and P. tacamahaca are moderately susceptible to the leaf lesions;
_ several Aigeiros × Tacamahaca hybrids are susceptible to bark cankers, which become limiting on P. nigra × P. trichocarpa, P. nigra × P. laurifolia and especially P. deltoides × P. trichocarpa clones.
Recent studies have found evidence of different levels of aggressiveness in S. musiva populations; a statistical support for the identification of true pathotypes with specific degrees of virulence, however, has not yet been produced.
1.3.1.3. Biology and relations with the host - In the form of cortical parasite, S. musiva requires breaches in the bark for its penetration, on condition that the potential host is young. Distress on the part of the host due to stresses of various kinds, especially drought and ozone pollution, while not decisive, is a predisposing factor to the appearance and the evolution of cankers. On the contrary, ageing seems to confer resistance, as it is supported by the disease incidence in the field, which is almost exclusive in nurseries and recent plantations.
Even on susceptible young trees, new cankers form only on each year's shoots and not on the two-year-old branches, which indeed appear colonised, sometimes heavily, by S .musiva, but after infections started the previous year. On these older cankers, attacks of secondary parasites, e.g. Cytospora spp. and Fusarium spp., are often observed.
Sporulation is thus entrusted to the mycelium in colonised leaves and freshly formed cankers, where small blackish pycnidial conidiomata differentiate, each containing hyaline and elongated conidia (17-56 × 3-4 µm), with 6 or 7 septa, that are then released in whitish mucilaginous cirri and dispersed by the rain.
During the winter, the teleomorph appears on fallen leaves and sometimes on overwintering cankers in the form of blackish pseudothecial ascomata, in which cylindrical asci (51-73 × 12-17 µm) are produced, each containing eight hyaline uniseptate or pluriseptate ascospores (15-27 × 4-6 µm). Their release late in the following spring causes the primary infections, which seems to be confined to the leaves; however, it is possible that a certain share of these infections is due to conidia overwintered in the cankers. One or two weeks after the appearance of the first leaf lesions, the new conidia ripen and bring about the secondary infections on other leaves and on the sprouting shoots.
1.3.1.4. Control strategies - Countries in which the accidental introduction of S. musiva is a potential threat must apply severe quarantine measures and set strict limits on the exchanging of biological material. There is a real danger in the case of north-central Europe, where much use is made of many of the Aigeiros × Tacamahaca hybrids that have proved susceptible in North America, whereas in Spain, Italy and the Balkans the prevalent species is the more resistant P. × euramericana.
With reference to this, as for the other pathogens, selection of resistant clones must take due account of their possible susceptibility to other diseases now widespread: particularly, many Euramerican genotypes are exposed to attacks by the leaf parasite Marssonina brunnea, whereas the intersectional hybrids just mentioned would be better chosen in the effective control of this pathogen.
All the agronomic measures indicated against Discosporium populeum are equally effective for S. musiva, even though their adoption does not totally ensure freedom from the leaf syndrome and the eventual transition to bark canker, when the fungus passes to some branches via the petioles. In nurseries where heavy attacks are likely, it is thus advisable to apply a chemical control. Three spring-summer treatments with benomyl-based products considerably reduce the incidence of the disease.
Biological control with the mitosporic fungus Phaeotheca dimorphospora Des Rochers et Ouellette seems a promising prospect, since preventive spraying of both conidial suspensions and culture filtrates has proved an effective way of limiting the onset and progress of cankers.
Two of the species that only attack leaves are of particular phytopathological importance:
_ S. populicola Peck (teleomorph: Mycosphaerella populicola Thompson), a leaf spot agent with a considerable incidence in North America, where its attacks sometimes overlaps those of S. musiva; it has recently been reported in southern Africa, though not as responsible for substantial damage;
_ S. populi Desm. [teleomorph: M. populi (Auersw.) Schroet.], the equivalent of S. populicola in Europe (mainly reported in France and Russia), but also observed sporadically in Asia (Turkey, Iran, India), U.S.A. and Argentina.
These two species differ from each other and from S. musiva in some microscopic characters, in their hosts and in some peculiarities of the symptoms they cause. Both seem less polyphagous than S. musiva: S. populicola attacks P. trichocarpa and P. tacamahaca only, and S. populi is observed on P. nigra and P. × euramericana, though in India it was also reported on P. ciliata and P. alba.
The Septoriae require rather high temperatures and humidity, therefore they usually appear in late spring or early summer in nurseries, mini-rotations for the production of biomass or young plantations. S. populicola and S. populi induce a syndrome quite similar to the leaf symptoms described for S. musiva, except that the spots they cause are generally a little smaller; those provoked by the European species are also slightly paler and concave with respect to the surrounding leaf blade. Severe attacks result in yellowing of the crown and premature defoliation.
