Gianni Della Rocca

An increase in transmission-related traits and in phenotypic plasticity is documented during a fungal invasion

The adaptive rapid evolution of phenotypic traits is potentially a key contributor to invasiveness, but has been relatively little studied for the fungi, even though these organisms are responsible for devastating losses in agriculture and natural resources. In this study, we compare biologically relevant phenotypic characters of spore-generated individuals from two native and two invasive populations of the fungal pathogen Seiridium cardinale to infer which traits may be adaptive and rapidly evolving during an ongoing biological invasion. Results show that: (1) lower growth rate and smaller spore size are selected for in invasive populations, independent of the stage of invasion; (2) there is no selection evident towards increased rapid sporulation, but overall reproductive potential increases in later stages of the invasions; and (3) demographic plasticity of most traits increases during the initial stages of invasion, but decreases in a later phase. Comparisons against levels of neutral genetic variation (Qst-Fst comparisons) showed that the decrease in spore size is strongly adaptive, despite the trade-off of reduced viability. Lesion size of isolates inoculated on the naive Italian cypress host was not correlated with their growth rate, and was significantly lower in invasive than in native populations. This last result indicates that rate of host colonization is a complex trait affected both by host and pathogen, which may not be necessarily adaptive and/or which may not easily evolve. In summary, the success of S. cardinale as an invasive in the Mediterranean basin is associated with reduced spore size and increased plasticity of almost all traits in initial phases, followed by further decreased spore size, increased overall sporulation, and decreased plasticity in a second phase of the invasion. Interestingly, growth rate by population results show that invasive populations are well adapted only to moderate temperatures, while native populations fare well also when exposed to relative extremes in temperature. This different adaptation suggests a “master-of-some” specialization scenario for the invasion by S. cardinale in the Mediterranean.

Ecology of invasive forest pathogens

Invasive forest pathogens are a major threat to forests worldwide, causing increasing damage. The knowledge of both the specific traits underlying the capacity of a pathogen to become invasive, and the attributes predisposing an environment to invasion are to be thoroughly understood in order to deal with forest invasions. This paper summarizes the historical knowledge on this subject. Many aspects of the ecological processes underlying alien forest pathogens invasions are still unknown, which raises several scientific issues that need further study. The introduction of invasive forest pathogens to areas where naïve hosts are found, is mainly due to global plant trade. Rapid transportation and reduced delivery times increase the chances of survival of pathogen propagules and of their successful establishment in new environments. In forest pathogens, the reproduction mode seems not to be a crucial determinant of invasiveness, as highly destructive pathogens have a variety of reproductive strategies. The most important drivers of forest pathogen invasions appear to be (a) great adaptability to new environmental conditions; (b) efficient dispersal over long and short distances, possibly assisted by the capacity to form novel associations with endemic and/or alien insect vectors; (c) the ability to exchange genetic material or hybridize with resident or alien species. Moreover, these features interact with some key traits of the invaded environment, e.g. environmental variability and biodiversity richness. Host resistance and natural enemies may occur as a result of rapid selection/adaptation after the epidemic phase of invasion.

Terpene arms race in the Seiridium cardinale – Cupressus sempervirens pathosystem

The canker-causing fungus Seiridium cardinale is the major threat to Cupressus sempervirens worldwide. We investigated the production of terpenes by canker-resistant and susceptible cypresses inoculated with S. cardinale, the effect of these terpenes on fungal growth, and the defensive biotransformation of the terpenes conducted by the fungus. All infected trees produced de novo terpenes and strongly induced terpenic responses, but the responses were stronger in the canker-resistant than the susceptible trees. In vitro tests for the inhibition of fungal growth indicated that the terpene concentrations of resistant trees were more inhibitory than those of susceptible trees. The highly induced and de novo terpenes exhibited substantial inhibition (more than a fungicide reference) and had a high concentration-dependent inhibition, whereas the most abundant terpenes had a low concentration-dependent inhibition. S. cardinale biotransformed three terpenes and was capable of detoxifying them even outside the fungal mycelium, in its immediate surrounding environment. Our results thus indicated that terpenes were key defences efficiently used by C. sempervirens, but also that S. cardinale is ready for the battle.

Flammability of Two Mediterranean Mixed Forests: Study of the Non-additive Effect of Fuel Mixtures in Laboratory

In the Mediterranean region, wildfires are a major disturbance, determined by ecosystem and forest species characteristics. Both the flammability and resistance to fire of a mixed forest may vary from those of the individual species. Two mixed Mediterranean woodlands, a Cupressus sempervirens and Quercus ilex stand in Italy; and a Juniperus thurifera and Quercus faginea stand in Spain were investigated. Laboratory flammability tests were conducted on live foliage, litter samples and on litter beds from individual and mixed species to evaluate: (i) the flammability traits of the mixtures of live foliage and litter samples; (ii) whether the flammability of the two-species mixtures are non-additive, i.e., differ from expected flammability based on arithmetic sum of the single effects of each components species in monospecific fuel; (iii) the ignition success and initial fire propagation in litter beds. Flammability tests were also conducted on bark samples to estimate the resistance of the tree species to fire. The ignitibility of live foliage was lower and the combustibility was higher in Cupressaceae than in Quercus. Non-additive effects were observed in some flammability components of live foliage and litter, especially in the mixtures of C. sempervirens and Q. ilex. Ignitability and combustibility were higher and lower than expected, respectively, and tended to be driven by Quercus), while the consumability was lowered more than expected by both Cupressaceae. The ignition success in the litter beds was low, especially for the presence of Cupressaceae that increase the bulk density of the mixtures. Cupressaceae, which have a thinner bark, suffered more damage to the cambium after shorter exposure to the heat source than Quercus species. In all the species studied, time to reach lethal temperatures in the cambium was dependent on thickness rather than on flammability of the bark. The study findings revealed that tree species may influence flammability of mixed fuels disproportionately to their load. The studied species showed to exert a contrasted effect on flammability of the mixtures, increasing ignitability and decreasing combustibility and consumability well out of their proportion in the mixture. This may potentially influence fire dynamics in mixed forests.