Fire blight, a disease caused by the bacterium Erwinia amylovora (Burrill) Winslow et al., has been causing serious damage in Hungarian pear plantations since 1996. A prospective control measure could be the use of resistant cultivars. For that purpose ten pear cultivars have been tested under laboratory and greenhouse conditions for r...esistance to Erwinia amylovora strains collected in Hungary. Six of these cultivars are Hungarian ones of unknown origin, while four are traditional old varieties. Resistant cultivars should serve as germplasm for future breeding. Inoculations were made with a mixture of different pear isolates of the bacteria collected from various growing regions of Hungary (Ea 21, 23), at a density of 5x108 cells/ml. Susceptibility/resistance has been assessed on the basis of intensity of blight symptoms observed on shoots, flower parts and fruits. Cultivars were assigned to three susceptibility groups (low, moderate and high). Complete resistance was not found among the cultivars tested. The highest level of resistance was found in cultivar 'Kieffer', while the other cultivars displayed either moderate or high susceptibility to infection.
In the frame of the apple breeding program at the Department of Fruit Science old cultivars found in the Carpathian basin were involved in examination as well. Beside testing susceptibility to fire blight (Erwinia amylovora of old genotypes (historical, local and regional cultivars) originating from scattered orchards of Visk (foot of...the Carpathian Mountains), fruit quality was also evaluated and other open-field observations were made.
According to the results of inner content examinations, 'Szemes alma' and 'Sikulai alma', chosen as gene sources for breeding to resistance against fire blight, had inner content values worthy to mention in some aspects, and competing with those of the control cultivar 'Jonathan'. The quality of 'Pónyik alma', also recommended as a gene source is confirmed by the data of earlier literature. Further three cultivars ('Batul', 'Vilmos renet', 'Zöld sóvári') also proved to be good based on resistance against Erwinia amylovora, the first two cultivars are suitable from the point of view of fruit quality, too.
In our field observations, susceptibility to apple scab and powdery mildew was evaluated. Among cultivars selected from the foot of the Carpathian Mountains on the base of resistance against Erwinia amylovora, the production value of 'Szemes alma' is worsened by its high susceptibility to apple scab. Further selected cultivars ('Sikulai', 'Pónyik alma', 'Batul', 'Vilmos renet') are not too susceptible to fungal diseases and their fruit quality is also suitable, therefore they can be recommended for direct planting in ecological farming or in restoring scattered orchards.
Research project has been initiated in 1999 with the aim of evaluating the degree of susceptibility/resistance of pear cultivars grown in Hungary to fire blight disease caused by Erwinia amylovora. The recently selected promising cultivars were also examined. Inoculation experiments were conducted in controlled greenhouse conditions be...cause of quarantine regulations in Hungary. Following the disease process, development of symptoms of plant organs (shoots, flower parts, fruits) was observed. Suspension of two E. amylovora strains (Ea 21, Ea 23) isolated from pear was used in a mixture (5x108 cells x m1-1) for the inoculation. Twenty-six pear cultivars were examined and grouped into four categories: low susceptibility, moderately susceptible, susceptible and very susceptible. Most of the cultivars were susceptible or very susceptible while some promising 'Eldorado', 'Harrow Delight' and `Hosui' showed low susceptibility.
Floral biological characteristics that may influence cultivar susceptibility to fire blight were studied in 10 pear cultivars in two Hungarian orchards from 1999 to 2003. The receptaculo-ovarial, automorphic nectary is usually bigger in tolerant cultivars than in susceptible ones. Nectary stomata are meso- or xeromorphic. Susceptible cultivars...tend to have more xeromorphic stomata, where guard cells are located 1-3 cell rows below the epidermis. The size of nectar chambers is usually smaller in susceptible cultivars. Floral nectar, consisting mainly of glucose and fructose, is more abundant and less concentrated if the cultivar is susceptible to fire blight. The amount of chlorogenic acid was higher in the flowers of tolerant cultivars than in susceptible ones.
The aim of the Hungarian apple breeding program started in the Department of Fruit Science was to find resistant apple cultivars against major diseases (scab, powdery mildew, fire blight). The outbreak of fire blight (Erwinia amylovora) in 1996 motivated us to search new resistant sources principally from old traditional apple cultivar...s. First of all, cultivars have been gathered since 1997 from Carpathia (Visk) and evaluated between 2001 and 2003. In this recent study evaluation of resistance of old Hungarian genotypes to fire blight collected from the English National Fruit Collection (Brogdale, Faversham) is presented.
13 old Hungarian apple cultivars in 2002, and 38 genotypes in 2005 have been evaluated. We used `Idared' and 'Jonathan M41' as susceptible controls and 'Liberty' and 'Remo' as resistant ones. Shoots of two-year-old potted plants were inoculated with a mixture of virulent E. amylovora isolates (Ea2, Ea60, Ea67) at a concentration of 5 x 108 cells/ml. Resistance of apple cultivars was evaluated weekly, four times after inoculation by disease severity of symptoms. Numbers of bacterial colonies in 1 cm length shoot were determined in the fourth week after infection.
8 cultivars in 2002 and 9 cultivars in 2005 displayed notable resistance to fire blight based on one-year data. Based on the coincident data of both years, out of the cultivars collected also from Carpathia ‘Pónyik', and `Sikulai' were found to be resistant and gene sources additional old Hungarian valuable apple genotypes could be selected: `Szabadkai szercsika' and `Tordai piros !davit'. The cultivar `Szemes alma' originated from Visk has proved to be recurrently resistant.
The number of bacterial cells in shoots of the investigated cultivars correlated with the severity of symptoms. With this method, which was introduced by us earlier, we can screen cultivars displaying weak visible symptoms, which cannot be proposed as a source of resistance because of their latent infection.
Several morphological characters of the hypanthium (size and form of the surface, the shape of the hypanthium) and anatomical traits (number and pattern of stomata) of apple cultivars (Malus domestica L.) with different susceptibility to fire blight were studied. The size of hypantium surface was calculated by modelling the hypanthium...with a truncated cone. Three types of hypanthium surface form have been revealed: straight, convex and a complex "shouldered" one. The angle between the style and the wall of the hypanthium was narrow or wide. The stomata on the hypanthium surface can be arranged in a zone in the middle third of the hypanthium or dispersed more or less evenly. The number of stomata/flower substantially differed among the cultivars examined. The highest stomata number was detected in the flowers of the tolerant cultivar (Freedom)
No single characteristics of the hypanthium could convincingly be correlated with susceptibility to fire blight. We suggest, however, that combination of morphological properties that sustain moist environment in the hypanthium contribute to susceptibility.
Current infection risk prediction models utilize environmental parameters and field records, but do not take into account the estimated inoculum potential within the orchard. The object of this study was to survey the accuracy of three simple prediction methods under Hungarian climatic conditions, which could easily be used by the farmers. We a...lso tested whether the accuracy of infection risk predictions can be improved by taking into consideration the incidence and/or rate of flower colonization by Erwinia amylovora.
After preliminary investigations in 1999-2001, data concerning the weather-related infection risk were recorded in 5 apple and 1 pear orchards in 2002, and in 12 apple and I pear orchards in 2003. The weather data were processed by the easy-to-use risk assessment models of the mean temperature prediction line (MTL), Smith's Cougarblight 98C and Billing's integrated system (BIS), and by the MaryblytTM 4.3 computer-assisted model for reference. The population size of E. amylovora in the flower samples was estimated within an order of magnitude by PCR.
For all years and orchards tested, Maryblyt indicated 35 days on which there was an acute infection risk. The same days were indicated by all 3 methods in 23 cases (66%), 8 days were indicated by 2 methods (23%) and 4 days were indicated by 1 method only. A similarly good correlation was found for prediction of the date of the first massive infection risk: in 2003, for instance, there was a perfectly consistent prediction by all 4 models in 9 of the 13 participating orchards. A coincidental forecast was provided by 3 of the 4 models in the other 4 orchards.
The results indicate that any of the risk assessment models could provide an increased accuracy of the actual infection risk prediction if combined with an estimation of the incidence of Erwinia amylovora colonization in the open flowers. We found no convincing differences in the size of the epiphytic population in flowers of cultivars possessing high or low susceptibility to Erwinia amylovora.
We conclude that the easy-to-use methods tested could be used by the fanners to recognize weather-related risks, especially when coupled with an estimation of the proportion of the pathogen-infested flowers. This local prediction would provide rapid information (faster than the regional forecast systems) specifically for a given orchard.
Two apple cultivars that display enhanced resistance to fire blight (causal agent: Erwinia amylovora) were selected. The aim of the present study was to characterize the peroxidase (POD) enzyme activity of `Szemes alma' (a historical cultivar) and MR-03, (a Hungarian multiresistant hybrid of 'Prima') and compare them to susceptible 'Jo...nathan M 40' and resistant 'Remo' controls. Peroxides enzyme activity during E. amylovora infections was investigated in artificially infected apple shoots. Increases in enzyme activities were observed in a `Jonathan M40' and in 'Remo', MR-03, `Szemes alma' cultivars. There was a consistent relationship between total enzyme activity and fire blight disease severity. High activity of the peroxidase was positively correlated with the degree of resistance to fire blight. A general hypothesis that POD activity is related to fire blight susceptibility/resistance is supported by our results.
Floral activity was studied in two apple cultivars: an Erwinia-tolerant (Treedorn') and a sensitive one (`Sampion'). Since more types of protogyny occur in apples, the period of stigma activity is different. Papillae of the exposed stigma in flowers of 'Freedom' function longer (usually more than a week) than in the delayed homogamous `Sampion'.... Despite of this, cv. 'Freedom' is tolerant to Erwinia amylovora (Burr.) Winslow et al., suggesting no relationship between the floral biological type (including the exposure and longevity of stigma) and the infection by E. amylovora. According to SEM micrographs, nectary stomata in `Freedom' are already open in the flower bud, where nectar secretion starts and continues until the senescence of the stigma. However, the long period of nectar secretion does not create optimal conditions for bacterial growth, since nectar production is scant in the flowers of 'Freedom'. The surface of the nectary, its nectar-retaining capacity, and the amount and concentration of nectar may influence the susceptibility of apple cultivars. It is manifested well by the smooth nectary surface with nectary stomata rising slightly above the epidermis in flowers of cv. 'Freedom', contrasting the wrinkled, striate nectary surface with slightly sunken stomata in the flowers of 'Sampion'.
A useful method was improved to test and to evaluate the susceptibility of plants to fire blight and the virulence of E. amylovora strains. Six Hungarian strains from different host plants were tested on in vitro cultured apple rootstocks. Disease rating was used for the characterization of the process of disease development....The different strains had different capacity to cause disease, mainly in the first period of incubation. There were significant differences between the virulence of the strains.
Due to the lack of effective and non-phytotoxic materials for control of the blossom and shoot blight phase of fire blight in pome fruit trees, two novel control strategies have emerged: shoot growth retardation by bioregulators and applying resistance inducer compounds. Prohexadione calcium (ProCa) is the active ingredient of the bioregulator...Regalis® registered in several European countries. The reduction of shoot growth elongation is the most obvious effect of ProCa. Furthermore, it causes significant changes in the spectrum of flavonoids and their phenolic precursors, leading to the considerable reduction of susceptibility to fire blight.
In Poland, potted one-year-old apple trees of cvs. Gala Must grafted on M.26 and Sampion on M.9 (in 2001) as well as Gala Must on P.60 (in 2002) were treated with Regalis® at a range of concentration of 250, 150 or 150 + 100 ppm, respectively. The inoculation of shoots was made with the strain No.691 of E. amylovora (107 cfu/ml), on the 7th and 21st'day after treatments with Regalis. In Hungary, during the years of 2002 and 2003 one-year-old container grown apple trees of the cvs. Idared/M.9 and Freedom/M.9 were treated with the prohexadione-Ca, the active ingredient of Regalis® 100, 150 or 200 ppm, two weeks before inoculation with the Ea 1 strain of E. amylovora (107 cfu/m1).
In Poland, the suppression of fire blight in shoots reached up to 80%, depending on concentration and application time of Regalis®. In Hungary, the effect of prohexadione-Ca treatments, determined by the length of necrotic lesion developed, proved to be better than that of streptomycin used for comparison.
The aim of the first Hungarian apple breeding program for multiple resistance started in the beginning of the nineties is to widen Hungarian apple assortment by good quality, resistant apple cultivars with excellent productivity and ecological capability to the most important fruit growing areas in Hungary. In the first years of seedling produc...tion we made early selection for susceptibility to apple scab in greenhouse. Alter this, field observation of susceptibility to powdery mildew, scab and canker and a yearly negative selection was carried on. From 1997, fruit quality was evaluated as well, and from 2001 the resistance of shoots to Erwinia ainylovora (Burrill/Winslow et al.) was examined using inoculations in greenhouse conditions. From the progenies of crosses in 1992 and 1993, six candidates were announced to national recognition out of hybrids examined for more than a decade. Descriptions of these selections from 'Prima' progenies and the most important data of their resistance, growing habit, morphological characteristics and fruit quality are shown in this article.
The ecological farming and an increasing demand for healthy fruits free from chemical residuals necessitated the reevaluation of the existing cultivars and breeding suitable cultivars for the new requirements. We have gathered the old genotypes known in the Carpathian Basin, and we have selected the most suitable ones according to fruit quality... and resistance to diseases in the last fifteen years. The main aim was the selection of resistant genotypes against fire blight and powdery mildew as gene sources for our breeding program. On the other hand, we consider the best quality traditional cultivars as extensive landscape elements (wayside tree rows or extensive squash producing plantations).
During the pathological evaluation 'Szemes alma', 'Pónyik', 'Sikulai', 'Tordai piros 'cavil' and 'Szabadkai szercsika' cultivars proved to be good resistance sources (due to their outstanding resistance to fire blight). The latter four cultivars are not susceptible even to fungal diseases, and their quality is also acceptable so those are commendable for ecological farming or for renewing the sparse orchards. According to fruit quality and low susceptibility to diseases 'Batul' and 'Vilmos renet' can also be offered for extensive organic production.
We have made original or renewed pomological descriptions of the selected cultivars, and we also publish their characters according to the terms of UPOV TG/14/8 based on to our evaluations.