Experimental testing of rust fungus‐mediated herbivory resistance in Betula pendula

Correlative evidence suggests that pathogenic leaf rust fungus, Melampsoridium betulinum, in late summer may negatively affect folivorous insects in the following summer. Correlative association does not necessarily, however, reveal causality. Alternatively, other interconnected plant characters may determine rust densities and herbivore performance. In this study, we used birch clones and rust fungus inoculations to manipulatively test the effects of birch rust and birch genotypes on the growth performance of folivorous moth larvae of Epirrita autumnata (Lep. Geometridae) in the subsequent year. The inoculation treatment increased rust densities (three‐ to 60‐fold) compared with natural infection levels. E. autumnata performance varied among birch clones and showed 4% lower growth performance on rust‐inoculated trees. However, the larval performance did not differ between rust‐treated shoots and untreated control shoots and the use of tree‐specific rust densities as a covariant in statistical analyses failed to reveal any negative association between rust fungus and larval performance. As the slight difference in larval growth performance also levelled until pupation, we propose that rust infection has biologically insignificant importance to the performance of E. autumnata.     

Variation in Melampsoridium resistance among European white-birch clones grown in different fertilization treatments

Resistance of 2-year-old plantlets of seven European white birch, Betula pendula, clones to birch rust, Melampsoridium betulinum, was studied in field experiments and in a leaf-disc bioassay. In addition, rust resistance of plantlets growing in a nursery under three fertilization treatments was tested. The birch clones clearly varied in their levels of resistance. One of the clones was consistently the most resistant, and two were very susceptible. Plantlets growing in the lowest fertility treatment were the most resistant. Clone × fertilization interaction was small. The plantlets grown in the lowest fertilization treatment, in particular, deviated from the other treatments. Generally good rust-resistance correlations were obtained between different experiments. The leaf-disc bioassay was an effective way of determining the field rust resistance of birch clones. The possibility of trade-offs between rust resistance and tree growth is discussed.     

Differences in the rust resistance of greenhouse and outdoor-grown white birch species,Betula spp.

The 2-year-old seedlings of five different white birch species (Betula platyphylla, Betula papyrifera, Betula pubescens, Betula pendula (two types) and Betula resinifera x Betula pendula) grown both in a greenhouse and outdoors, were inoculated in a leaf disc assay with two different birch rust (Mel-ampsoridium betulinum) isolates from B. pendula and B. pubescens. The resistance of these birch species varied significantly. Resistance to the B. pubescens rust isolate was not related to the resistance of the B. pendula rust isolate. The behaviour of a birch genotype grown in the greenhouse did not correspond to the behaviour of the same genotype grown outdoors. The outdoor growth environment greatly increased the contents of soluble proteins, rubisco, chloro-phyll and nitrogen in the leaves of diploid birch species (B. platyphylla, B. pendula and B. resinifera x B. pendula). For tetraploid and pentaploid species (B. pubescens and B. papyrifera, respectively) there was no such clear difference in the leaf physiological status between the seedlings grown outdoors and in the greenhouse. The C:N ratio was higher for the greenhouse-grown seedlings in all the birch species, but the difference was significant only with the diploid species. The incidence of rust in the birch species did not correlate with any of the leaf physiological parameters studied. The adaptability of birch genotypes to the environment in relation to their resistance to birch leaf rust is discussed.     

Screening of clones of Betula pendula and B. pubescens against two forms of Melampsoridium betulinum leaf rust fungus

Leaf discs from seedlings of six clones of B. pendula Roth and two clones of B. pubescens Ehrh. were inoculated with two urediniospore isolates of M. betulinum Kleb. The field collection iso-lates were obtained from B. pendula and B. pubescens growing in the field. The B. pendula rust was more specialized than the B. pubescens rust. The clones of B. pubescens showed partial resis-tance against the B. pendula rust, while the B. pubescens isolate was compatible to both birch species. Some interclonal variation was also found in both birch species. The results support an earlier suggestion by Klebahn that M. betulinum has two formae speciales.     

Effect of Betula pendula clone and leaf age on Melampsoridium betulinum rust infection in a field trial

Ten 5‐year‐old Betula pendula clones were studied for their rust resistance in the field. The trees were treated by inoculating 10 leaves on a shoot with Melampsoridium betulinum urediniospore suspension or spraying the control leaves with water. The birch clones differed significantly in their resistance to M. betulinum leaf rust fungus and the clones also varied in their responses to the local rust strain and the inoculated rust strains. However, natural rust infections and inoculation treatment were positively correlated. The older leaves had fewer infections than the younger ones on the tip of the shoot in the control trees, but in the inoculation treatment no significant correlation was found between the leaf ages and rust infection. The factors behind the different leaf susceptibilities are discussed.     

Occurrence of the moth Cydia pactolana is associated with the spruce canker fungus Neonectria fuckeliana

Several young damaged Norway spruce stands in eastern and central Finland were observed from 2013 to 2016. The damage included trees with heavy resin flow, necrotic foliage, stem and branch cankers and dead trees. Pest identification resulted in the tortricid moth Cydia pactolana whose occurrence was always associated with the presence of the ascomycete pathogen Neonectria fuckeliana. Both the insect and the disease contributed to the extent of the damage, but it is not possible to say in which order they had attacked the trees. Apparently, changed climate has affected the increased occurrence of both the fungus and the moth. However, the characteristics of the insect–fungus interaction and the factors contributing to the coincidences are unknown. Emerging coexistence or potential symbiosis of the two damaging agents is a serious threat for Norway spruce cultivation. Understanding the biology of this fungus–insect interaction is important for controlling them.     

Pre-penetration behaviour of the urediniospores of birch rust,Melampsoridium betulinum,on greenhouse-grown and microcultured leaves of Betula spp.

In two different experiments the urediniospores of Melampsoridium betulinum from Betula pendula and Betula pubescens germinated both on mature leaves of greenhouse-grown plants and on sterile in vitro leaves of micropropagated plantlets, which were cloned from the same B. pendula and B. pubescens genotypes. The urediniospores and germ tubes were more easily detached from the leaf surfaces of in vitro leaves. If germination took place on a leaf vein, the growth continued across the veinal ridges; otherwise, no determined growth towards the stomata could be observed with either of the leaf types studied. In both experiments on the in vitro leaves of B. pubescens clone V5944, the germ tubes of the rust isolate from B. pendula mislocated appressoria significantly more often than the germ tubes of the rust isolate from B. pubescens. On the mature leaves of B. pendula clone E4214 and B. pubescens clone V5940 there were also significant differences in appressorial locations between the two rust isolates but the clonal responses were inconsistent. The results of the inoculations suggest that the incompatibility of the rust isolates from B. pendula on the leaves of B. pubescens is not related to the significantly higher ratio of failures in locating appressoria in this host-rust combination.