Biogeosciences and Forestry Biogeosciences and Forestry Lenticel infection in Fraxinus excelsior shoots in the context of ash dieback Miguel Nemesio-Gorriz (1) , Brian McGuinness (2) , Jim Grant (3) , Luke Dowd (4) , Gerry C Douglas (1) Common ash (Faxinus excelsior L.) in Europe is declining on a continental scale due to the action of Hymenoscyphus fraxineus, an invasive forest pathogen that causes ash dieback disease leading to the collapse and eventual death of ash trees through shoot infection in the crown and through stem collar infection. This study confirms for the first time lenticels as entry points for pathogens to enter shoot bark. Results show the impact of lenticel infection at a very early stage of invasion by H. fraxineus in a F. excelsior provenance trial and its correlation values with other factors such as shoot dieback, canker-like lesions and bud burst. No significant provenance effects were observed for incidence of shoot dieback, lenticel necrosis or canker-like lesions on shoots, but provenance effects were significant for bud burst phenology. The strongest correlation was observed between lenticel necrosis and canker-like lesions on the lenticels of shoots. Boheremia spp. were most frequently isolated from necrotic ash lenticels and confirmed by ITS sequencing, but also species of Diaporthe, Epicoccum, Aspergillus, Neonectria, Didymella and Hymenoscyphus fraxineus. Finally, lenticel density was similar in sets of ash genotypes that were characterized as having a high and low susceptibility to ash dieback.

A survey of bleeding canker disease, caused by Pseudomonas syringae pv. aesculi, was undertaken across Ireland. Incidence has become severe and can be considered epidemic, as 61% of the 1587 horse chestnut trees surveyed showed symptoms of the disease. Bacteria were isolated from a sample of trees and characterized using gyrB DNA sequencing. DNA was also extracted directly from wound tissue. The Irish P. syringae pv. aesculi genotype was identical to genotypes previously sequenced with gyrB from the UK and some other locations in Europe. Real-time PCR, using existing primers and a newly designed, more pathovar-specific primer set, was assessed for use in disease screening. With molecular screening, a total of 11 trees from a sample of 55 tested positive for P. syringae pv. aesculi in Ireland. It was more efficient to extract DNA directly from wound tissue, especially fresh bark, for disease detection than to undertake bacterial isolation with subsequent molecular analysis. A further set of sequencing primers was developed for the amplification of the gyrB gene from P. syringae pv. aesculi and their specificity was shown using a diverse sample of bacterial isolate DNAs. The study also isolated and identified other bacterial species from diseased material; some of these are known pathogens (Brenneria nigrifluens, P. marginalis and P. syringae) or have previously been identified as potentially beneficial endophytes of host trees (Erwinia billingiae, E. tolentana, P. fluorescens, P. putida and Raoultella).

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Lecanicillium fungicola causes dry bubble disease of the white button mushroom and produces masses of sticky conidia. Humans are an important vector in the spread of this disease in mushroom farms. Three hand cleaning treatments (tap water, liquid soap and alcohol-based hand sanitisers (ABHSs)) were evaluated for their effectiveness at eliminating conidia of L. fungicola from a contaminated index finger. The hand sanitisers were highly efficacious in reducing the number of viable L. fungicola conidia on contaminated fingertips, although some variability was encountered. The tap water and liquid soap treatments had little effect. An in vitro test confirmed that the log10 reduction in viable conidia after 1 min exposure to the different treatments was ≤0.15 for tap water and soap and >4.5 for the ABHSs, which is similar to what is achieved in the medical care field for many bacteria and viruses. Thus, regular use of ABHSs by staff on mushroom farms may help to reduce the incidence of dry bubble disease. Their use could also be beneficial in other areas of intensive horticulture or agriculture where human hands are known to transmit plant pathogens to uninfected plants.