Search

The present-day phytosanitary disasters caused by biological invasions are afflicting urban and peri-urban forest stands worldwide, as well as the varied services they normally provide. In Europe, we are witnessing an alarming situation due to an increasing introduction of infectious diseases and pests. The authors present an up-to-date list of alien microbial pathogens and insect pests affecting urban greening that have been accidentally imported in Italy or that are likely to be introduced. Information about the biology, epidemiology, ethology, and control of these invasive organisms is provided. For each species, the current geographical distribution, including newly-colonized areas, is also given, as well as the chronological progression of its occurrence. Particular detail is used for describing symptoms—the key diagnostic elements for appropriate and timely phytosanitary management. This paper will benefit urban forest management, which is a crucial factor in maintaining the social and ecological viability of urban green spaces, as well as ecosystem services. The importance of engaging citizens in community-based monitoring of urban greenspaces for tracking the location, abundance, and pathways of invasive pathogens and pests will also be touched upon.

Many biotic and abiotic factors contribute to the onset of oak decline. Among biotic agents, a variety of fungi and insects cause extensive disease and insect outbreaks in oak forests. To date, research on fungus-insect interactions in Mediterranean forest ecosystems is still scarce and fragmentary. In this study, we investigated the assemblage of endophytic mycobiota and insect pests occurring in a declining oak stand, with the aim to explore if, and to what extent, the insect species were active vectors of fungal propagules. It emerged that some known latent pathogens of the Botryosphaeriaceae family, namely Botryosphaeria dothidea, Diplodia corticola, Diplodia seriata, Dothiorella sarmentorum, and Neofusicoccum parvum were isolated at high frequency from physiologically-impaired trees. In addition, propagules of these fungi were isolated from five insects, two of which (Cerambyx welensii and Coraebus fasciatus) are main oak pests. The life-history strategies of these fungi and those of wood-boring beetles were strikingly interconnected: both the fungi and beetles exploit drought-stressed trees and both occur at high frequency during hot, dry periods. This synchronicity increased their chance of co-occurrence and, consequently, their probability of jointly leading to oak decline. If these interactions would be confirmed by future studies, they could help to better understand the extensive decline/dieback of many Mediterranean forest ecosystems.

Summary The composition of the endophytic fungal assemblages in Quercus cerris, Q. pubescens and Q. robur, three oak species most susceptible to decline, was investigated in Fagare ` (Padova), Ulignano (Pisa), and Radda in Chianti (Firenze). The endophytic assemblages were studied as a function of health status of the whole tree and of twigs and leaves. The effect of station altitude was tested on Q. cerris. Samples were gathered from healthy and declining trees. In April (bud opening time) 20 twigs and 10 leaves were collected from each tree. Fungi were isolated from twig segments and leaf portions. A total of 23 fungal genera with 27 different species were isolated, in addition to a few sterile mycelia. Eleven species were found to be common to all three oak species, while other species were found only on one oak species, suggesting host specificity. Among the fungal species isolated, some were known to be pathogenic (Apiognomonia quercina, Colpoma quercinum, Diplodia mutila, Phomopsis quercina). The colonization frequency (CF) of pathogenic species varied between 0.9% for A. quercina in Q. cerris and 60.2% for P. quercina in Q. robur. Analysis of variance showed a statistically significant difference among the oak species tested. The CF was higher in declining trees in comparison with healthy trees, and also in twigs vs. leaves. The CF was found to be higher for Q. cerris trees growing at sea level as compared with those situated at an elevation of 350‐400 m. In stands where, on account of particular ecological conditions, pathogenic behaviour is displayed simultaneously by more than one endophytic fungal species, trees accentuate their decline.

Summary Soil-borne species of Phytophthora were isolated from 19 of 30 examined oak forest areas in Italy. The frequency of isolated Phytophthora spp. (35.2%) was significantly correlated with soil pH and longitude of the sites. Eleven Phytophthora species were detected. Phytophthora cambivora, P. cinnamomi and P. cactorum were recovered from sites in central and southern Italy whereas P. quercina was isolated in the northern and central part of the country. Phytophthora citricola occurred all over Italy. Phytophthora quercina was the only species significantly associated with declining oak trees.

Summary Forty isolates of Diplodia mutila, a dieback agent on oaks were characterized by their capacity to produce heterokaryons, by their vegetative compatibility, and by their pathogenicity tests on Quercus cerris. The forty isolates were collected from Quercus cerris, Quercus frainetto, Quercus ilex, Quercus pubescens, Quercus robur and Quercus suber in different regions in Italy. Three nit mutants (nitt, nit and nitM) were identified with different nitrogen sources: sodium nitrite, sodium nitrate, uric acid, ammonium tartrate and hypoxanthine. On the two nutrient media employed, a potato dextrose agar (PDA) medium with chlorate (PDC), and a minimal agar medium with chlorate (MMC), isolates obtained from Q. cerris yielded the greatest number of mutants: 254 on PDC (out of a total of 583) and 190 on MMC (out of a total of 440). Nine vegetative compatibility groups (VCGs) were identified. There was a strong association between these VCGs and host species. All isolates tested except one from Q. cerris were pathogenic.

Summary Tests in vitro and in situ showed that both mycelial and conidial suspensions of Acremonium mucronatum were antagonistic for Diplodia mutila, a common pathogen on the apical twigs of declining Quercus cerris, Quercus frainetto, Quercus pubescens and Quercus robur. Antagonism was greater with the mycelium than with the conidial suspensions, and was strongest at a test temperature of 20°C. The results suggest that pathogenicity of D. mutila on declining trees may be significantly reduced by the presence of A. mucronatum.

Summary The callus-fungal method was employed to test the response to C. flaccidum of the highly susceptible P. nigra var. laricio and the resistant P. sylvestris, and to ascertain whether results obtained with this method matched in planta observations. Calli were inoculated with axenie cultures of C. flaccidum obtained by incubating basidiospores on modified Schenk and Hildebrandt medium. Several parameters were evaluated. Colony growth was more rapid on P. nigra var. laricio. Colonies were dense on P. nigra var. laricio, but sparse on P. sylvestris. Aerial hyphae growth was abundant on P. nigra var. laricio, but less frequent on P. sylvestris. Hyphal branching began after 18 h on P. nigra var. laricio and after 45 h on P. sylvestris. Necrosis of the host cells set in after 24 h on P. nigra var. laricio, and after 70 h on P. sylvestris. The number of cells with plasmolysis was much larger in P. nigra var. laricio than in P. sylvestris. These results were consistent with the known resistance of the two species on whole plants.

The penetration of Cronartium flaccidum into the cotyledons, primary needles and secondary needles of Pinus pinaster and P. nigra var, laricio was investigated. SEM examination showed that the fungus did not form appressoria and that the germ tube penetrated directly into the stomata.

The authors have tested the behaviour of basidiospores of Cronartium flaccidum on needle surfaces of Pinus nigra var. austriaca, P. nigra var. laricio and P. sylvestris using Patton and Johnson fluorescence method. On P. sylvestris a species well-known for low susceptibility, the behaviour of basidiospores was different from the other two species. A resistance factor was identified at the stage of basidiospore germination. Summary Germination of C. flaccidum basidiospores was tested on the needles of seedlings of three pine species possessing different susceptibility: Pinus nigra var. austriaca, P. nigra var. laricio and P. sylvestris using Patton and Johnson fluorescence methods. Progress of germination was found to be different on P. sylvestris compared with that on P. nigra var. austriaca and P. nigra var. laricio in that its beginning is delayed to 20 hours after incubation as compared with the 10 and 5 hours in the cases of the latter two species. Percentages, 40 hours after incubation, are respectively 44, 93 and 83%. A resistance factor in P. sylvestris a species with well-known low susceptibility, was thus detected at the level of basidiospore germination. Resume La germination des basidiospores de Cronartium flaccidum sur des aiguilles de trois pins differents La germination des basidiospores de Cronartium flaccidum a ete observee sur des aiguilles des jeunes plants de Pinus sylvestris, P. nigra var. austriaca et P. nigra var. laricio qui montrent une resistance variee vis-a-vis de la rouille vesciculeuse. On note les differences suivantes: —– les basidiospores sur les aiguilles de P. nigra var. laricio debutent leur germination apres 5 heures et celles sur P. sylvestris apres 20 heures. —– apres 40 heures d'incubation 44% des basidiospores ont germe sur les aiguilles de P. sylvestris 93% sur P. nigra var. laricio et 83% sur P. nigra var. austriaca. Pinus sylvestris possede une resistance contre l'infection puisque la germination des basidiospores est moin et plus lente que sur les deux varietes de P. nigra. Zusammenfassung Keimung der Basidiosporen von Cronartium flaccidum auf den Nadeln von Kiefemarten Die Keimung der Basidiosporen von Cronartium flaccidum wurde auf den Nadeln der Samlinge von Pinus sylvestris, P. nigra var. austriaca und P. nigra var. laricio die eine verschiedene Kienzopfresistenz zeigen, kontrolliert. Zwischen den Kiefernarten bestehen folgende Unterschiede: — Basidiosporen auf den Nadeln von P. nigra var. laricio fangen nach 5 Stunden zu keimen an, wahrend die Basidiosporen auf den Nadeln von P. sylvestris nach 20 Stunden zu keimen beginnen; — nach 40 Inkubationsstunden ist der Keimungsprozentsatz: 44 fur die Basidiosporen auf den Nadeln der P. sylvestris 93 auf P. nigra var. laricio und 83 auf P. nigra var. austriaca. Die Untersuchung zeigt fur P. sylvestris Widerstandsfahigkeit gegen die Infektion wegen geringerer und spater einsetzender Keimung der Basidiosporen an.

The Authors observed, in Italy, over a number of years, isolated and grouped trees of the genus Quercus that declined over one or more vegetative seasons. The spread was very fast; in fact the trees declined in the course of 2–3 vegetative seasons. A series of symptoms was observed: production of epicormic shoots, yellowing of leaves, laceration of the bark, bleeding from the trunk. Many fungi were isolated: Armillaria spp., Cephalosporium spp., Cladosporium spp., Cylindrocarpon spp., Diplodia mutila, Hypoxylon mediterraneum, Phoma cavae, Pbomopsis quercina, Sporotrix spp. Future research will attempt to ascertain the possible causes of the decline.

An experiment on the effect of eight inoculum concentrations of basidiospores of Cronartium flaccidum, on development of symptoms on P. nigra spp. nigricans var. austriaca, P. nigra ssp. italica, P. nigra ssp. laricio var. calabrica (from Calabria and Corsica) showed that 60000 basidiospores/ml was the optimum concentration and P. nigra ssp. laricio was the most sensitive.

The effect of the fluorescent brightener Calcofluor (derivative of di-amine stilbene sulfonic acid) on the germination of basidiospores of Cronartium flaccidum (Alb. et Schw.) Wint. was tested by sowing spores on pine needles and then brightening them, and by labelling directly basidiospores on Calcofluor solution and subsequently sowing them on water agar. The first method enhanced germination most. It is recommended that 0.05% brightener in distilled water be used to maximise germination in pre-infection studies for resistance at the needle level.