Alternative inoculum sources for fire blight: the potential role of fruit mummies and non‐host plants

Abstract

Fire blight is the most damaging bacterial disease in apple production worldwide. Cankers and symptomless infected shoots are known as sites for the overwintering of Erwinia amylovora, subsequently providing primary inoculum for infection in the spring. In the present work, further potential sources of inoculum were investigated. Real-time PCR assays covering a 3-year-period classified 19Á9% of samples taken from fruit mummies as positive. Bacterial abundance in fruit mummies during autumn, winter and spring was up to 10 9 cells per gram of tissue and correlated well with later infection rates of blossoms. Blossoms of non-host plants growing close to infected trees were also shown to be colonized by E. amylovora and to enable epiphytic survival and propagation of bacteria. The results indicate a potential role of fruit mummies and buds in overwintering and as a source of primary inoculum for dissemination of the pathogen early in the growing season. Non-host blossoms may also serve as an inoculum source in the build-up of the pathogen population. Both aspects may contribute significantly to the epidemiology of E. amylovora. The significance of infected rootstocks as an inoculum source is also discussed. Fruit mummies might be used to determine pathogen pressure in an orchard before the beginning of the blooming period.

Real‐time PCR detection of Erwinia amylovora on blossoms correlates with subsequent fire blight incidence

“…The modified protocol was successfully applied to blossoms from different host plants, i.e. Malus domestica , Pyrus communis , Cydonia oblongata and Crataegus sp., for blossoms of various non‐host plant species (Weißhaupt et al ., ), bark (not shown), cankers (not shown), buds and fruit mummies (Weißhaupt et al ., ).…”

“…To overcome the limitations for quantification and simultaneously allow for on‐site examination and large‐scale monitoring for the presence of E. amylovora , the authors have improved the real‐time PCR approach introduced by Salm & Geider (); the procedure now allows the use of whole bacteria without prior sample processing (Voegele et al ., ) on a variety of different plant tissues including non‐host plants (Weißhaupt et al ., ). Using a mobile laboratory with a portable SmartCycler II system, the method allows a quantitative screening of E. amylovora and on‐site analysis of samples in less than 2 h directly in the field (Voegele et al ., ).…”

“…In these and the present studies (Weißhaupt et al ., ), an ‘absolute’ quantification by means of standard curves (Larionov et al ., ) has been used, yielding correlations with an R 2 value of 0·98. In previous studies (Larionov et al ., ) a value of at least 0·99 was considered necessary; however, these studies referred to triplicate single serial dilutions of DNA, with lower dilution factors, in contrast to the investigations of the current authors in which whole bacteria, decimal dilution steps and a combination of four to seven serial dilutions were used to produce one standard curve.…”

“…However, so far, real‐time PCR assays have mostly focused on the analysis of pure DNA extracted from a variety of samples. Based on the real‐time PCR approach described by Salm & Geider (), the present authors have improved the method to be used directly on whole bacteria in the field on a variety of different samples (Weißhaupt et al ., ). The present study focuses on the early detection and quantification of fire blight bacteria on apple and pear blossoms in the area around Lake Constance in the northern alpine forelands of southern Germany, Switzerland and Austria, a region severely afflicted by fire blight (Moltmann, ).…”

Development of a viability digital PCR protocol for the selective detection and quantification of live Erwinia amylovora cells in cankers

“…While a role of other reservoirs in fire blight epidemics has been discussed [4][5][6] , cankers are widely considered one of the main sources of E. amylovora cells for the spread of the disease. However, knowledge of E. amylovora population dynamics in cankers through time and the impact of environmental and/or host-specific factors on E. amylovora survival in cankers is scarce, partially due to limitations of classical microbiology detection methods employed in plant disease diagnostics.…”