First hybrid plane trees to show resistance against canker stain (Ceratocystis fimbriata f. sp. platani)

Canker stain, caused by the ascomycete Ceratocystis fimbriata f. sp. platani, is a major threat to plane trees in Europe. The American plane tree (Platanus occidentalis) carries some genetic resistance, but this species is not adapted to the climatic conditions in Europe. Therefore, hybrids between susceptible oriental plane trees (Platanus orientalis) and resistant P. occidendalis were screened for disease resistance in France. Among 960 hybrids, we found 18 individuals which survived two successive inoculations. These putatively resistant trees were cut back and, after 18 months, on each tree 12 of the newly produced shoots as well as two roots, were inoculated again. From these trees, only one showed complete and another one partial resistance. This selection process resulting in the resistant clone ‘Vallis clausa’ is described in detail.     

Screening of Metrosideros polymorpha (‘ōhi‘a) varieties for resistance to Ceratocystis lukuohia

Stands of the landscape‐dominant tree, Metrosideros polymorpha (‘ōhi‘a) on the Island of Hawai‘i, HI., USA, are dying from a phenomenon known as rapid ‘ōhi‘a death (ROD). Approximately 180,000 acres of forest have already been impacted by the disease, the majority of which is attributed to C. lukuohia, the more aggressive of the two Ceratocystis species responsible for ROD. Three isolates of C. lukuohia were compared for differences in aggressiveness. Mean disease severity and mean number of days to death did not differ among the three isolates of C. lukuohia. A single isolate was selected for use in a resistance screening of 128 plants across four varieties of M. polymorpha occurring on Hawai‘i Island: incana, glaberrima, polymorpha and newellii. Disease severity was lower in early‐successional var. incana (38.84%) and riparian var. newellii (36.11%) compared to late‐successional var. glaberrima (61.27%) and high‐elevation var. polymorpha (70.27%). Var. incana and var. newellii also had the lowest mortality (63% and 77%, respectively) while var. glaberrima (86%) and var. polymorpha (100%) had the highest mortality. Eighty‐five per cent of the plants that died did so within 49 days post‐inoculation and mean number of days to death was significantly higher for var. glaberrima (46) compared to var. polymorpha (31). The results of this preliminary study suggest that some form of resistance might be present in natural populations of at least one variety of M. polymorpha on Hawai‘i Island and should be investigated more intensively.     

The role of free proline in the sensitivity of poplar (Populus ‘Robusta’) plants to the action of SO11

The action of SO₂ caused an increase in the content of free proline in the leaves of Populus ‘Robusta’ plants. A protective role of proline pretreatment was demonstrated against injury, changes in water content and chlorophyll loss in the leaves of plants exposed to the gas.     

Molecular characterization of a phytoplasma from the aster yellows (16SrI) group naturally infecting Populus nigra L. ‘Italica’ trees in Croatia

Leaf and branch samples were collected from 10 Populus nigra L. ‘Italica’ trees found in the Zagreb urban area. One of the P. nigra L. ‘Italica’ trees exhibited leaf yellowing, overall sparse foliage, stunting and decline. Two methods for the nucleic acid extraction in the phytoplasma detection from P. nigra were compared. A phytoplasma from the aster yellows group (16SrI) was detected by PCR in the symptomatic as well as in four apparently asymptomatic plants. The pathogens are classified, by restriction fragment length polymorphism (RFLP) analysis of the 16S rRNA gene plus the spacer region, as members of a newly described subgroup 16SrI‐P. Phylogenetic analysis of 16S ribosomal and spacer region sequence confirmed their close relationship with the other members of the aster yellows group. However, RFLP analyses of other conserved genes such as tuf, BB88 and ribosomal protein rpL22 gene, clearly confirmed that this is a molecularly distinguishable phytoplasma belonging to a new ribosomal protein subgroup designated rp‐O.     

Seedling response of two trembling aspen (Populus tremuloides) families to infection by Hypoxylon mammatum

Open-pollinated progenies from two trembling aspen (Populus tremuloides) families with a 30-year field history of 91% (putatively susceptible, PS) and 45% (putatively resistant, PR) mortality, respectively, caused by Hypoxylon mammatum, were grown in a greenhouse for 3 and 9 months. Seedlings were inoculated with two isolates of H. mammatum, representing high and low aggressiveness, through a stem wound. Beginning 4 days after inoculation, host responses were examined on 312 seedlings per family for a period of 5 weeks on 3- and 9-month-old seedlings. Significant differences in host responses were observed between the two isolates, with the more aggressive isolate allowing less callus formation, causing more tissue necrosis and producing longer cankers than the less aggressive isolate on both aspen families. Between-family difference was much greater than within-family difference for tissue necrosis, callus formation and canker length for both seedling ages. When the 3-month-old seedlings were challenged with H. mammatum, the PR family showed greater tissue necrosis, longer cankers, and less callus formation than the PS family. However, when the more lignifìed 9-month-old seedlings of the same families were inoculated, the PR seedlings had much less tissue necrosis, shorter cankers, more callus formation and less seedling mortality than the PS family. The results indicate that artificial inoculation of seedlings in a greenhouse can differentiate two aspen families at the seedling stage, but their correlation to field performance is highly dependent upon the seedling age. Host responses of older seedlings may be a better indicator of field performance.     

Phytoalexin production by Aspen (Populus tremuloides Michx.) in response to infection by Hypoxylon mammatum (Wahl.) Mill and Alternaria spp.

Freshly wounded stem sections of P. tremuloides Michx. produce phytoalexin when inoculated with mycelium of Hypoxylon mammatum (Wahl.) Mill., and Alternaria sp. These exudates are inhibitory against spore germination of these two fungi and, although inhibitory against mycelial growth of Alternaria sp., have no effect on the mycelial growth of H. mammatum. It was possible to correlate the amount of phytoalexin elicited by different strains with the inhibitory activity.     

The effect of Phlebiopsis gigantea and urea stump treatment against spore infection of Heterobasidion spp. on hybrid larch (Larix × eurolepis) in southern Sweden

The efficacy of stump treatment with 40% urea solution and spore suspension of Phlebiopsis gigantea against primary infection by Heterobasidion spp. on hybrid larch (Larix × eurolepis) was tested in two field studies in southern Sweden. In the first study, stumps treated with urea or P. gigantea were sampled by cutting a cross‐section disc after 3 months following exposure to natural conditions, and in the second study, stumps treated with only P. gigantea were sampled after 2 months. Spore traps made from hybrid larch, Norway spruce (Picea abies) and Scots pine (Pinus sylvestris) were used to estimate the abundance of ambient spores of Heterobasidion spp. in the second study. Urea significantly reduced infection frequency of Heterobasidion spp. compared to the control. Phlebiopsis gigantea was less effective at reducing infection frequency in the first study than in the second study. Infections were caused by both H. parviporum and H. annosum sensu stricto. The amount of H. annosum infection in proportion to Heterobasidion spp. was significantly lower on the stumps treated with P. gigantea compared to the control; however, the proportion of H. parviporum on the treated stumps was not higher than the control in the first study. Spore traps made of Scots pine had a significantly higher frequency of infection than Norway spruce and hybrid larch. Only the spore traps made of hybrid larch showed significant correlation with the control stumps in terms of relative infected area. Conclusively, it seems prudent to protect hybrid larch stumps from primary infection by Heterobasidion spp., and both urea and P. gigantea can be recommended as stump treatment agents on hybrid larch, even if urea seemed to present more stable results.