Studies of the interaction between horse chestnut leaf miner (Cameraria ohridella) and bacterial bleeding canker (Pseudomonas syringae pv. aesculi)

The white flowering horse chestnut tree (Aesculus hippocastanum L.) was first introduced to the UK ca. 500 years ago. Over the past eight years however, this tree has suffered from severe attack by a mining insect pest known as the horse chestnut leaf miner (HCLM; Cameraria ohridella) and, concomitantly a gram negative bacterium (Pseudomonas syringae pv. aesculi; Pae). Although studies have investigated the influence of each problem individually on tree growth and vitality the interaction between HCLM and Pae on horse chestnut remains unknown. For this reason four year old horse chestnut seedlings were artificially inoculated with Pae in the presence and absence of HCLM. Effects on tree vitality were assessed by monitoring alterations to leaf chlorophyll content, leaf chlorophyll fluorescence (F_v/F_m ratios) as well as key defensive enzymatic activity (β-1,3-glucanase, peroxidase) of woody tissue at the site of Pae infection. With respect to mean lesion size, the main proxy of Pae success or aggressiveness, lesion size was significantly increased in the presence of HCLM compared to trees inoculated with Pae but where HCLM was controlled using insecticide sprays. Results of this study also indicate that suppression of two key defensive enzymes, β-1,3-glucanase and peroxidase, within woody tissue at the site of Pae infection, possibly as a result of reduced photosynthetic productivity caused by HCLM defoliation, maybe the underlying reason as to why Pae severity is greater in the presence of HCLM.     

Ecosystem functioning in cities: Combined effects of urbanisation and forest size on early-stage leaf litter decomposition of European beech (Fagus sylvatica L.)

Environmental changes associated with urbanisation can affect the functioning of ecosystem processes. In cities, forests are among the most frequent types of green areas and provide a wide range of ecosystem services including air cleaning, decomposition of leaf litter and recreation. The European beech (Fagus sylvatica) is a frequent and widespread deciduous tree in temperate forests in Central Europe. In this study, we examined the effects of urbanisation on decomposition processes of F. sylvatica leaves in different-sized forests in the urban region of Basel, Switzerland. We used standardised litterbags (mesh size: 2 mm) with F. sylvatica leaves to assess the impact of degree of urbanisation (indicated by the percentage cover of sealed area in the surroundings) and forest size on the early stage of leaf litter decomposition and seasonal microbial activity. We found combined effects of degree of urbanisation and forest size on the decomposition rate of leaf litter (k_litter). Large forests showed the highest k_litter in areas with sparse settlements and the lowest k_litter in densely settled areas, whereas the opposite pattern was recorded for small and medium-sized forests. This indicates that abiotic and biotic forest characteristics of forests of similar size differently influenced k_litter depending on the degree of urbanisation. Moisture content of litter was the best predictor of microbial activity, followed by forest size. We assume that factors acting at the landscape scale such as the degree of urbanisation might be too coarse to detect any differences in microbial activity. Our results revealed that even small urban forests contribute to this important ecosystem function. As decomposers are at the bottom of the food chain, management actions that support the biological activity in soil might be also beneficial for species at higher trophic ranks.     

Foliar fertilization of peach (Prunus persica (L.) Batsch) with different iron formulations: Effects on re-greening,iron concentration and mineral composition in treated and untreated leaf surfaces

A trial to assess the effects of applying several Fe-containing formulations on Fe-deficient (chlorotic) peach leaves was carried out under field conditions. Solutions consisting of an Fe-containing compound (FeSO₄·7H₂O, Fe(III)–citrate, Fe(III)–EDTA, Fe(III)–DTPA or Fe(III)–IDHA) and one of five different surfactant treatments (no surfactant, an organo-silicon, an ethoxylated oil, a non-ionic alkyl polyglucoside and a household detergent) were applied to one half of the leaf via dipping, first at the beginning of the trial and then after 4 weeks. The re-greening of treated and untreated leaf areas was estimated with a SPAD apparatus, on a weekly basis, during 8 weeks. At the end of the experimental period, leaves were detached, and tissue Fe, N, P, K, Ca, Mg, Mn, Zn and Cu concentrations were determined in Fe-treated and untreated leaf areas. Treatment with Fe-containing solutions always resulted in leaf chlorophyll (Chl) increases, which however significantly depended on the Fe-source, the surfactant-type and the combination between both formulation components. Untreated leaf zones experienced a Chl increase only in some cases, and this depended on the type of surfactant used. Iron application significantly increased the Fe concentration of treated and untreated leaf areas, especially with some formulations. Foliar treatment with Fe-containing solutions induced significant changes in the concentration of several nutrients as compared to those found in Fe-deficient peach leaves, with changes being similar in treated and untreated leaf areas, although in some elements the extent of the changes was of a different magnitude in both materials. This indicates that some leaf mineral composition changes typical of chlorotic leaves are dependent on leaf Fe concentration rather than on leaf Chl levels. Results obtained are relevant to help understand the factors involved in the penetration and bioavailability of leaf-applied Fe, and to assess the potential of foliar Fe fertilization to control Fe deficiency in fruit trees.