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

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 19·9% of samples taken from fruit mummies as positive. Bacterial abundance in fruit mummies during autumn, winter and spring was up to 10⁹ 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.     

Warning Systems for Fireblight1)

The ideal warning system should indicate conditions favouring inoculum production and dissemination, infection and subsequent symptom expression. It should give guidance on the best times for inspections to be made and for chemical control measures to be applied. The choice of system is governed by the crop, the climate and economic considerations. New systems must be developed to deal with new situations. Most previous systems have been concerned with the spring blossom periods of pear and apple, but systems for assessing fireblight activity at other times in the growing season are being developed. This is especially important in Europe where the blossom periods of susceptible ornamental hosts extend into July and where, in some countries, summer blossom on pear may be present throughout the season. Monitoring of inoculum levels on cankers, blossoms or shoots has proved valuable where economy in the use of sprays has justified its use; sprays are usually withheld until the pathogen is detected. Warning systems based on weather data are broadly in agreement but differ in detail. Sprays during bloom are often withheld until maximum temperatures exceed 18° C (or mean temperatures reach 14.5 —- 16.5° C) and there is precipitation or high humidity (> 60 % RH) at the same time. Some workers would spray immediately after moderate rain (e.g. 2.5 – 5.0 mm) when inoculum was known to be present. Spray applications immediately after damaging storms are the general rule. Proper evaluation of warning systems is difficult and is only possible in areas where fireblight is endemic and where adequate field records can be obtained.     

Eutypa Canker and Dieback of Apricots

Eutypa canker and dieback of apricot trees, caused by the ascomycetous fungus Eutypa armeniacae Hansf. et Carter, has been recorded in Europe, North America, Australia and South Africa. Samples of diseased sapwood yield the imperfect Cytosporina stage in culture, whereas perithecia of E. armeniacae, immersed in a stroma, develop 2 or more years after death of a branch in high-rainfall areas. The fungus is a wound parasite. Air-borne ascospores, disseminated during and after rain, infect xylem tissues which have been freshly exposed by pruning. Cankers develop around the infected wounds; leaves on that part of the branch distal to the canker subsequently wilt and die during summer. E. armeniacae has been detected on a diverse range of host species, hence elimination of inoculum is not feasible; both biological and chemical approaches to wound protection and ultimate disease control are under investigation.     

Official survey,in 2002, for detection of Erwinia amylovora in a proposed protected zone in Jihomoravsky region (Czech Republic)1

Inspectors of the Czech NPPO surveyed the occurrence of fireblight Erwinia amylovora in an area of Jihomoravsky region (South Moravia) proposed as an EU protected zone, including 16 designated buffer zones around nurseries. The disease was not detected in communes where fireblight hosts are grown (nurseries, variety testing stations, orchards) or in buffer zones around nurseries. In 902 communes where fireblight hosts are grown only in orchards or not at all, wild host plants were inspected at 2.629 observation points (2137 located by GARMIN GPS). In Vy?kov district, suspected fireblight was confirmed at one observation point on Crataegus, growing by a railway in Rousínov commune and, in the course of a delimiting survey outside observation points, in four other communes (Drnovice, Habrovany, Komo?any and Vy?kov).     

Experience with fireblight in the Czech Republic1

Erwinia amylovora has been spreading in the Czech Republic since 1986 regardless of emergency phytosanitary measures which have been taken. Its spread follows the predominant wind direction (west to east). The infested area now covers two thirds of the area of the country. Wild Crataegus spp. are the most important and widespread host plants. E. amylovora is still regulated as a quarantine pest, and areas free from fireblight are subject to special phytosanitary measures.     

First outbreaks of fireblight in Slovenia1

Slovenia was recognized as free from Erwinia amylovora until 2001. A low incidence of the disease was reported in 2001 and 2002 in the north‐west uplands of the Gorenjska region. In 2001, only 3 trees in extensive orchards were found positive at one location out of 791 monitoring locations all over the country. In 2002, the same location in Naklo and a second one in a 1‐km radius were found positive, whilst 875 other monitoring locations were fireblight‐free. Despite strict phytosanitary measures implemented after discovery of the first focus, the bacterium spread in 2003 to the entire Gorenjska region, with further spread towards eastern and southern Slovenia. The NPPO registered 184 new foci by the end of the season in Gorenjska. Twenty‐three foci are scattered in other regions. Phytosanitary measures were applied in every focus. Spread of the bacterium was probably enabled by favourable weather conditions, the presence of the inoculum and intensive bee‐hive movement, resulting in flower infections. Since eradication has not been successful, it is concluded that E. amylovora is now present at low prevalence in Slovenia.     

Ten years of experience with fireblight in Austria1

In 1993, fireblight (Erwinia amylovora) was detected for the first time in Austria, in the most westerly part of the country (Vorarlberg). During the following years, especially since 1998, a stepwise migration from west to east was observed. For control, mechanical measures are mostly used. As these measures require early recognition of symptoms, broad information campaigns for the public and extensive surveys of host plants have been set up. Suspicious plant samples are sent to the central laboratory for diagnosis. Regional plant protection services are assisted by trained volunteers. Eradication and pruning measures are performed by trained working teams. Infected plant material is burned. In all pome‐fruit production areas, the disease forecasting system Maryblyt has been established. Regulations concerning the movement of beehives have been adopted, together with prohibition of the planting of certain ornamental host plants (Crataegus, Cotoneaster). Chemical treatments are limited to one copper oxychloride product. In 2003, temporary application of Prohexadione‐Ca for the prevention of shoot infections was authorized. In some areas, the rigorous application of control measures has reduced the incidence of fireblight and possibly slowed down its progress. However these measures are not sufficient to protect against new, unexpected outbreaks.     

Fireblight in the Po valley (Italy): the monitoring programmes of Emilia‐Romagna and Veneto regions1

Erwinia amylovora, causing fireblight, is a very important quarantine pest for Italy. Since the beginning of the 1980s, import of host plants from countries where the disease occurs has been limited and subjected to laboratory analyses. Fireblight was found for the first time in Puglia (southern Italy) in 1990. Following this finding, a national monitoring network was set up in order to find new cases as rapidly as possible. In 1994, the first outbreaks of fireblight were found in Emilia‐Romagna. In 1997, a severe epidemic spread throughout this region and the first cases were reported in the bordering regions Veneto and Lombardia. To face this new situation, additional specific local monitoring was set up. This article describes the operational and legislative measures taken in Emilia‐Romagna and Veneto to contain the disease in orchards, to allow marketing of healthy plants for planting and to regulate the movement of beehives.     

Traditional and commercial apple and pear cultivars as sources of resistance to fireblight1

In Hungary, fireblight research programmes were initiated on pear in 1999 and on apple in 2000, with the aim of evaluating the susceptibility or resistance of commercial cultivars. Sources of resistance for future breeding were also sought among traditional apple cultivars collected from Ukraine and pear cultivars in the Hungarian gene bank (Szigetcsép). Experiments were done under secure conditions. Inocula were mixtures of characteristic Erwinia amylovora isolates from pear and apple in Hungary. Host responses (symptom development, disease severity and multiplication rate of bacterial cells in host tissues) were assessed on shoots, flowers and fruits. About 30 pear and 30 apple cultivars, and 35 apple hybrids, were tested and grouped into four categories for pear and three for apple. Of the pear cultivars tested, 50% were susceptible, 30% moderately susceptible and only 10% of low susceptibility. Different plant organs occasionally displayed different responses. Members of the last two groups might serve as useful candidates for growing under IPM conditions. Among the traditional Hungarian varieties tested, we found high resistance in ‘Sikulai’ and ‘Szemes alma’, which could be used as sources of fireblight resistance in breeding programmes and also grown in organic orchards. Furthermore, among the offspring of the apple ‘Prima’ (scab‐resistant), we have found highly resistant lines.     

Infection frequency of mature apple fruit with Erwinia amylovora deposited on pedicels and its survival in the fruit stored at low temperature

The infection frequency of mature apple fruit by Erwinia amylovora and the survival of E. amylovora in the fruit stored at low temperature were investigated. The fruit stems (pedicels) of 460 mature apple fruit were inoculated with 10⁵ or 10⁴ cfu of bioluminescent E. amylovora, tagged with lux genes. Nine days after inoculation, 43% and 27% of the fruit inoculated with 10⁵ and 10⁴ cfu, respectively, were infected. All infected fruit looked healthy. After 6 months of storage at 5°C, almost all of the 142 infected fruit had viable E. amylovora. Of the fruit containing E. amylovora internally, 19.5% had latent infections and the rest had blight symptoms. E. amylovora was not uniformly distributed in the fruit flesh, and internal brown lesions were observed where E. amylovora was densely distributed. These findings showed that mature apple fruit may be infected with E. amylovora, especially as latent infections, and act as a source for long-range dissemination.     

Fireblight in Békés County (Hungary) in 1996/20021

Fireblight (Erwinia amylovora) appeared in Hungary in 1996. Most damage occurred on apple, pear, quince and medlar, and also on the ornamentals Pyracantha, Sorbus, Cotoneaster and Crataegus. In 1996–2006, an official programme for elimination of infected parts of plants started in Békés county. This mainly concerned trees in towns and villages, since there are few pome‐fruit orchards in the county. Work teams under official direction pruned back or cut down trees. In total, some 13 000 trees were pruned back and nearly 11 000 were cut down. In addition, 21 villages were subjected to special phytosanitary measures. Infection decreased considerably between 1996 and 2002, but over 90% of the inhabited areas in the county remained subject to special measures, because of the very dispersed occurrence of fireblight.     

Effectiveness of the resistance inducer prohexadione‐Ca against fireblight in shoots of apple trees inoculated with Erwinia amylovora1

Due to the lack of effective, non‐phytotoxic and publicly acceptable materials for controlling fireblight in pome fruit trees, novel strategies against Erwinia amylovora are being sought. Resistance‐inducing compounds, such as prohexadione‐Ca, represent promising alternatives. Prohexadione‐Ca is the active substance of the bioregulator Regalis, currently being introduced in several European countries and overseas. Prohexadione‐Ca reduces shoot elongation due to inhibition of gibberellin biosynthesis. Furthermore, it leads to significant changes in the spectrum of flavonoids and their phenolic precursors in pome fruits, which causes reduced susceptibility to fireblight and other pathogens. In 2002 and 2003, container‐grown apple trees of the cultivars ‘Idared’ and ‘Freedom’ were treated with different dosages of prohexadione‐Ca two weeks before inoculation with E. amylovora. The effect of prohexadione‐Ca against shoot blight was determined by measuring the lengths of necrotic lesions and symptoms on vascular bundles caused by the pathogen. Treatments with prohexadione‐Ca turned out to be much superior to the ones with streptomycin, kasugamycin and a bacterial antagonist, which were used for comparison. Acibenzolar‐S‐methyl (Bion), another resistance‐inducing compound, was included in some of the experiments and gave intermediate results. The simultaneous control of excessive shoot growth and shoot infections by fireblight is seen as a major advantage of using prohexadione‐Ca in pome fruit trees.     

Fireblight situation in Bulgaria and measures undertaken by the NPPO1

Fireblight disease, caused by Erwinia amylovora was first detected in Bulgaria on quince in the region of Plovdiv in 1989. The disease was initially localized in that area but, during 1995/1997, due to favourable climate conditions, it became epidemic. Infected trees were grubbed out and destroyed. The main hosts are quince and pear (over 40% of affected trees), then apple, medlar and Cotoneaster. Containment measures undertaken by the Bulgarian NPPO are laid down in the Plant Protection Law, in Phytosanitary Regulation no. 1 for phytosanitary control, and in Phytosanitary Regulation no. 5 of 1996 (amended 1997) for containment of fireblight. Phytosanitary control is mainly focused on fruit‐tree nurseries and on the distribution of healthy plants for planting. In 2003, 41 protected zones and 29 protected sectors within infested areas have been established.     

Population structure of Phytophthora infestans collected on potato and tomato in Italy

Late blight caused by the oomycete Phytophthora infestans is a disease of potato and tomato of worldwide relevance and is widespread throughout Europe and the Mediterranean region. While pathogen populations in northern Europe have been sampled and characterized for many years, the genetic structure of populations from southern Europe, including Italy, has been less studied. Between 2018 and 2019, we collected 91 samples of P. infestans from potato and tomato crops in Italy, Algeria, and Tunisia on FTA cards and genotyped them using 12-plex microsatellites. These samples were compared to genotypes of P. infestans previously collected within the framework of the EuroBlight network and from published sources. Four clonal lineages were identified: 13_A2 (Blue 13), 2_A1, 23_A1, and 36_A2. Two other isolates collected could not be matched to any currently known clonal lineage. The 13_A2 and 36_A2 lineages were found exclusively in southern Italy and Algeria, while 2_A1 was only found in Algeria. This is the first report of the 36_A2 lineage in Italy. Two isolates from Solanum nigrum were 13_A2, suggesting this weed host could be a reservoir of inoculum. The 23_A1 lineage was found widely on infected tomato crops in Italy and is the same as the lineage US-23 that is widespread in North America. Differences in genotypes across the country suggests that there may be different sources of introduction into Italy, possibly via infected seed tubers from other countries in Europe, tubers for consumption from North Africa, or tomatoes.     

Recent Progress in Breeding for Fireblight Resistance in Apples and Pears in North America1

Resistance in apple is evaluated by needle–inoculation of succulent shoot tips with 10⁶–10⁷ cells of a virulent isolate of Erwinia amylovora (Burr.) Winslow et al. (the incitant of fireblight) and measurement of the resulting cortical lesion when extension is complete. Data are now available on practically all commercial cultivars, some of which have a useful level of resistance. Some newer cultivars, particularly those with resistance to scab (Venluria inaequalis [Cooke] Wint.) derived from Malus floribunda, have good resistance to E. amylovora. A very high level of resistance is present in Asiatic Malus species, including M. x robusta, M. x sublo–bata, M. x atrosanguinea, and M. prunifolia, and in the North American species M. fusca. This type of resistance appears to be inherited polygenically, and because of its detectability in young seedlings can be used conveniently in breeding. Objectives of pear breeding programs are aimed at developing superior fruit quality combined with resistance to fireblight, psylla, and Fabraea leaf spot. Many high quality cultivars of Pyrus communis are extremely susceptible to E. amylovora and sensitivity appears to be controlled by a dominant gene Se. A high level of resistance is present in P. ussuriensis but varies considerably between clonal selections of other Pyrus species. Pear seedlings from controlled pollinations are artificially inoculated in the glasshouse with a similar bacterial suspension as used for apples, and only the most resistant ones are selected and planted in the field for future evaluation. In Beltsville, heritability studies of crosses between non–sensitive parents have indicated that selection for resistance within progenies results in a high degree of genetic gain. Interspecific hybridization has an advantage over P. communis crosses mainly when insect or fungus resistance is required.     

Fireblight in Poland

Fireblight ( Erwinia amylovom ) was first detected in Poland in 1966 and then occurred irregularly, in isolated foci mainly along the northern coast of the country, until 1975. During this period, radical measures were taken for localization and elimination of the disease. Since 1976, the disease has become established in numerous areas across northern Poland, and since 1985 has spread towards the centre of the country. Pear, apple and wild hawthorn are affected. In recent years, damage in affected areas has been only at a moderate level. A Ministerial Decree makes fireblight control compulsory. Inspections are carried out throughout the country and any outbreaks must be notified. Complete elimination of infected and adjacent host plants is recommended in areas of low incidence. In heavily affected areas, it is recommended to prune away all infected parts. Severe restrictions apply to nurseries. General preventive measures are also recommended.     

Extended longevity of Erwinia amylovora vectored by honeybees under in vitro conditions and its capacity for dissemination

Fire blight outbreaks in Korea were first reported in 2015. Regular outbreaks have occurred since, indicating a continuous cycle of the fire blight pathogen in Korea. We determined the role of Apis mellifera (honeybee) as a vector of Erwinia amylovora by verifying the following: (a) E. amylovora longevity in/on honeybees; (b) the most common body parts that carry the bacteria; (c) the rate of bacterial spread to healthy host organs; and (d) the relationship between dispersal of viable but nonculturable (VBNC) and virulent bacterial cells. E. amylovora survived for 15 days on the exterior of honeybee bodies and was most abundant on the abdomen in comparison to other areas such as the labellum, wings, and hind legs. In the digestive system of honeybees, E. amylovora survived for 7 days, and bacteria were found in faeces for 3 days after exposure. The bacteria are likely to be VBNC on honeybees. Honeybees that were contaminated with bacteria transferred E. amylovora to healthy immature apple fruit, shoots, and flowers for 10 days after exposure. E. amylovora was also transferred from inoculated plant parts to uncontaminated honeybees. In addition, bacteria moved from inoculated plant tissues to unexposed honeybees and then from these honeybees to healthy plant tissues. Therefore, E. amylovora can survive in/on honeybees for extended amounts of time, which contradicts previous reports. The bacteria moved to host tissues via honeybees, suggesting that honeybees are the vectors of E. amylovora and play a role in the development of new outbreaks of fire blight disease in the central regions of Korea.     

Effect of crop growth and canopy filtration on the dynamics of plant disease epidemics spread by aerially dispersed spores

Most mathematical models of plant disease epidemics ignore the growth and phenology of the host crop. Unfortunately, reports of disease development are often not accompanied by a simultaneous and commensurate evaluation of crop development. However, the time scale for increases in the leaf area of field crops is comparable to the time scale of epidemics. This simultaneous development of host and pathogen has many ramifications on the resulting plant disease epidemic. First, there is a simple dilution effect resulting from the introduction of new healthy leaf area with time. Often, measurements of disease levels are made pro rata (per unit of host leaf area or total root length or mass). Thus, host growth will reduce the apparent infection rate. A second, related effect, has to do with the so-called "correction factor," which accounts for inoculum falling on already infected tissue. This factor accounts for multiple infection and is given by the fraction of the host tissue that is susceptible to disease. As an epidemic develops, less and less tissue is open to infection and the initial exponential growth slows. Crop growth delays the impact of this limiting effect and, therefore, tends to increase the rate of disease progress. A third and often neglected effect arises when an increase in the density of susceptible host tissue results in a corresponding increase in the basic reproduction ratio, R(0), defined as the ratio of the total number of daughter lesions produced to the number of original mother lesions. This occurs when the transport efficiency of inoculum from infected to susceptible host is strongly dependent on the spatial density of plant tissue. Thus, crop growth may have a major impact on the development of plant disease epidemics occurring during the vegetative phase of crop growth. The effects that these crop growth-related factors have on plant disease epidemics spread by airborne spores are evaluated using mathematical models and their importance is discussed. In particular, plant disease epidemics initiated by the introduction of inoculum during this stage of development are shown to be relatively insensitive to the time at which inoculum is introduced.     

Prohexadione‐Ca induces resistance to fireblight and other diseases1

Prohexadione‐Ca is primarily used for the control of shoot growth in pome and other fruit trees. During its development, treated apple and pear trees were found to be significantly less affected by fireblight (Erwinia amylovora) and other pathogens, although prohexadione‐Ca is inactive as a bactericide or fungicide. Prohexadione is a structural mimic of 2‐oxoglutaric acid, and the distinct dioxygenases involved in gibberellin biosynthesis that require this compound as a cosubstrate are blocked. As a result, less growth‐active gibberellins are formed and treated plants remain compact. 2‐Oxoglutaric acid‐dependent dioxygenases are also involved in flavonoid metabolism. In shoots of apples and pears, prohexadione‐Ca causes considerable changes in the formation of flavonoids and their phenolic precursors by inhibiting flavanone 3‐hydroxylase. Convincing evidence is available that prohexadione‐Ca triggers pathogen resistance primarily by inducing the formation of 3‐deoxyflavonoids, in particular luteoforol, with phytoalexin‐like properties. Morphoregulatory effects caused by prohexadione‐Ca are only of secondary relevance. The simultaneous control of excessive shoot growth and shoot infections by fireblight is seen as a major advantage of using prohexadione‐Ca in pome fruit trees.     

Using weather records and available models to predict the severity of fireblight should it enter and establish in Australia1

Australia is free from fireblight, a serious apple and pear disease. Published models and weather data from all major apple and pear-producing areas of Australia were used to predict the possible severity of the disease and losses in production should the disease enter and establish in Australia. The results suggest that fireblight could be severe in most areas in most seasons. In the worst case, with every area affected, this could result in up to 20% losses in apple production and up to 50% losses in pear production.