Variation in virulence of Greek isolates ofPhytophthora citrophthora as measured by their ability to cause crown rot on three peach rootstocks

The virulence ofPhytophthora citrophthora isolated from various host-plants on three peach rootstocks (GF677, PR204, KID I) was examined. There was no significant difference among the rootstocks with respect to their susceptibility to testedP. citrophthora isolates. The most virulent isolate originated from sycamore (Acer pseudoplatanus); isolates from pistachio trees (Pistacia vera) also showed high virulence but were significantly less virulent than the sycamore isolate. Isolates originating from plum (Prunus domestica), almond (Prunus amygdalus) and lemon (Citrus limon) trees were moderately virulent on peach rootstocks; those from cyclamen (Cyclamen persicum) showed the lowest virulence of those tested. There was thus great variation in virulence among the testedP. citrophthora isolates. It is possible that the isolates ofP. citrophthora from sycamore, pistachio, plum, almond and lemon trees are a threat to peach trees, whereas the low virulence of the isolates from cyclamen hosts suggests that these pathogens are not a serious threat to peach trees. http://www.phytoparasitica.org posting Jan. 3, 2002.     

Dieback symptoms on olive trees caused by the fungus Eutypa lata1

The fungus Eutypa lata was isolated from olive trees for the first time in 1988 from two young 3-year-old trees. It was evident that the infection was initiated from the graft union. The trees were also infected by Armillaria mellea. In a second case in 1991, extensive cankers were observed along the branches which often were girdled and died. The infected area extended from the bark into the sapwood and even into the heartwood. Cultures made on potatodextrose agar from the margin of necrotic and healthy tissues resulted in typical colonies of the fungus E. lata, as well as of Verticillium dahliae. The colonies of E. lata produced the characteristic pycnidia of the anamorph Libertella blepharis 3–4 weeks later. Pathogenicity tests made on olive, apricot, almond and walnut trees resulted in the death of the inoculated twigs or in canker formation and extensive brown discoloration of the wood.     

Asymmetrasca decedensr: a new pest of almond in Spain 1

Asymmetrasca decedens was the most important of three leafhoppers found infesting almond trees in eastern Spain in late summer 1996, causing stunting of the shoots with small curled leaves. Only non-bearing trees were affected. Although the species has previously been recorded on peach in Spain, this is the first record on almond.     

Population structure of Valsa ceratosperma, causal fungus of Valsa canker, in apple and pear orchards

On the basis of mycelial compatibility, the population structure of Valsa ceratosperma, the causal fungus of Valsa canker on broadleaf trees, was investigated in apple and pear orchards in Japan. Field strains of V. ceratosperma from a single canker on trunks or scaffold limbs belonged to different mycelial compatibility groups. Thus, the population structure of this fungus was complex in most orchards. Because mycelia of strains originating from different conidia from the same pycnidium were compatible, infection by this fungus is thought to be ascospores.     

Incidence of Phomopsis amygdali,Botryosphaeria berengeriana and Valsa cincta diseases in almond under different control strategies 1

The fungi causing cankers and withering of stems and branches of almond under Spanish Mediterranean conditions are: Phomopsis amygdali, Botryosphaeria berengeriana (in the form of its Dothiorella anamorph) and Valsa cincta (anamorph Cytospora cincta). P. amygdali is the most important because of its high specificity. Its development causes serious damage, particularly in orchards near the sea. B. berengeriana is a polyphagous fungus with high virulence but, because its growth depends on high temperature and adequate humidity, it is only present in orchards located in the south of the Iberian peninsula. V. cincta is widespread and causes withering of twigs. Control strategies for these three fungi with systemic fungicides have been investigated in the laboratory and glasshouse by artificial inoculations using both twig segments and seedlings of almond cv. Marcona. Benomyl, thiophanate-methyl and imazalil applied both as foliar spray and by soil drenching, have shown low fungitoxic activity in the apical part of the stems and twigs of tested plants. P. amygdali and B. berengeriana were the most affected by the three fungicides, while V. cincta was less affected. Benomyl proved to give the best control, followed at a distance by thiophanate-methyl. No control was obtained with imazalil.     

Alternaria Brown Spot of Minneola in Greece; Evaluation of Citrus Species Susceptibility

During the last three years, a new disease was observed in northwestern Greece on Minneola trees, hybrid of mandarin and grapefruit. On May small brown necrotic leaf spots surrounded by yellow halo areas of various sizes appeared and covered a major portion of the leaves with extension of necrosis into the veins. On young fruits small, slightly depressed black spots were the first symptoms, which later became 2–7 mm in diameter. Brown spots were observed on the leaves and fruits in several orchards in the same area, causing leaves and fruits to drop. In some orchards over 50% of the fruits were affected. From the fruit and leaf spots the typical small-spore species Alternaria alternata was isolated. Pathogenicity tests were performed by artificially inoculating fruits of Minneola, common mandarin and Clementine. The symptoms of the disease were reproduced only on fruits of Minneola hybrids by the specific strain of the fungus Alternaria alternata pv. citri. Different citrus susceptibility tests indicated that mandarins Minneola, Nova and Page were very susceptible to tested isolates while Clementine SRA and Poros Clementine were not. All lemons and lime Seedless were not susceptible. Grapefruit New Hall was not susceptible, while the Star Ruby was. Orange Lane Late, Navel Late, Oval Poros, Olinda, Navel Athos were not susceptible and only Moro showed reaction being slightly susceptible only to one isolate.     

Agricultural factors affecting Verticillium wilt in olive orchards in Spain

In recent years, the spread of Verticillium wilt in olive orchards, caused by the soil-borne pathogen Verticillium dahliae, is often related to intensive modern farming of highly productive cultivars, planted at high densities, usually irrigated, and under a mechanised system. The effects of agricultural factors associated with olive orchards were investigated in an important olive-growing area in southern Spain, as tools in predicting outbreaks of the disease. A stratified double-sampling technique was designed to determine the number of olive orchards needed to survey. A sampling survey was conducted from 2002 to 2005 in 873 olive orchards randomly selected, the owners were interviewed for details of agronomic factors, and orchards were inspected for the presence or absence of the disease. Polymerase chain reaction assays were carried out for identifying V. dahliae pathotypes. Pathogen prevalence showed a significant linear correlation with the mean plant density (r ² = 0.93), associated predominantly with a less virulent non-defoliating pathotype (r ² = 0.96). Overall, irrigation × high density caused disease incidence to peak in super-high-density olive-tree-planting systems. Olive orchards that had V. dahliae, however, did not differ in pathogen prevalence regardless of the olive cultivars. Young olive orchards were significantly more affected by V. dahliae than were old ones, particularly orchards with trees 8 to 12 years old. Irrigation increased pathogen prevalence and disease incidence in very young orchards (<7 years old). The prevalence of the non-defoliating pathotype was statistically high in young orchards whereas the prevalence of a highly virulent defoliating pathotype was high in old orchards.     

Phytophthora crown rot of almonds in Australia 1

Phytophthora crown rot, caused primarily by Phytophthora cambivora, has caused considerable losses in almond orchards in the Willunga and northern plains areas approximately 50 km north and south of Adelaide (AU), respectively. Other Phytophthora species including P. citrophthora, P. cryptogea, P. megasperma and P. syringae have also been associated with diseased trees. Tree losses have been associated with P. cactorum isolated from trunk cankers on scaffold limbs on trees in the Riverland almond-growing area approximately 350 km north-east of Adelaide. The Al mating type of P. cambivora was the most pathogenic to almond seedlings. Cvs Mission and Chellaston, which are commonly used as rootstocks, were highly susceptible to crown rot whereas peach cv. Nemaguard was resistant. An excised twig assay has been developed to screen micropropagated shoots for resistance to P. cambivora. Metalaxyl applied at 5 g active substance per tree in spring, winter and autumn in a shallow trench close to the trunk prevented the development of trunk cankers. Foliar sprays of phosphonate (H₃PO₃) at 2 g active substance per litre in autumn and spring also inhibited canker development.     

Verticillium wilt of almond in California 1

Wilt of almond due to Verticillium dahliae, also known as‘black heart', is an occasionally serious disease problem in orchards in California (US). Verticillium wilt is most severe on trees 2–6-year-old, and symptoms lessen as trees mature. Usually only a few trees in a young orchard are affected, though sometimes many young trees die. Severe economic losses of 9000-11 000 USD per ha resulting from verticillium wilt in almond orchards have been recorded. Foliar symptoms may be difficult to discern after trees are 5–6-year-old. Unlike previous reports of new infections arising each year, recent research indicates that infections may perennate in vascular tissue of primary scaffolds and trunks. Some differences in tree damage are apparent among scion cultivars. Pre-plant fumigation with methyl bromide and/or chloropicrin is not reliably effective for prevention of verticillium wilt. Solarization has been effective when applied prior to or at planting. Post-plant therapeutic treatments are not consistently effective.     

Canker and wood rot pathogens present in young lemon orchards in south-west Arizona

Canker and wood rots are economically important preharvest diseases of lemons in south-western Arizona, where commercial lemon production is concentrated. However, the aetiology and epidemiology of canker and wood rots are not well understood. This study comprised a large survey of canker and wood rot incidence and severity in Arizona and the characterization of fungal species associated with the disease. A total of 5431 trees with ages ranging from 1 to 20 years old in 10 lemon orchards were surveyed from 2018 to 2020. Our survey results revealed that canker and wood rot occurred in all 10 lemon orchards studied. Canker and brown rots of twigs, branches, and trunks were the most prevalent symptoms of affected trees ranging from 1 to 20 years old. In contrast, canker and white rots of twigs and branches were observed mostly on 1- to 5-year-old trees. Disease incidence for both diseases was less than 2% on 1- and 2-year-old trees. Brown rot increased significantly in older trees, ranging from 62.9% to 100%. Fungi were isolated from canker and wood rot samples and identified based on morphological characters and DNA sequences. Fomitopsis meliae and Hypoxylon macrocarpum were the primary canker and wood rot pathogens isolated from the surveyed orchards, at frequencies of 89% and 11%, respectively. In pathogenicity tests, both fungi were capable of causing canker and wood rots on lemon cv. Lisbon branches and the necrotic length caused by F. meliae was twofold greater than that caused by H. macrocarpum.     

Characterization of fungi (Fusarium and Rhizoctonia) and oomycetes (Phytophthora and Pythium) associated with apple orchards in South Africa

Several species of fungi and oomycetes including Fusarium, Rhizoctonia, Phytophthora and Pythium have been reported as root pathogens of apple where they contribute to a phenomenon known as apple replant disease. In South Africa, little is known about specific species in these genera and their pathogenicity toward apple. Therefore, these aspects were investigated along with the development and optimization of qPCR tests for detection and quantification of the most virulent oomycete species. In eight investigated orchards, the oomycete Phythophthora cactorum was widely distributed, while nine Pythium species were differentially distributed among the orchards. Pythium irregulare was the most widely distributed and the most virulent species along with P. sylvaticum, P. vexans and Ph. cactorum. Seven binucleate Rhizoctonia anastomosis groups (AGs) were also differentially distributed among the orchards, with the majority appearing to be non-pathogenic while certain AG-I and AG-F isolates exhibited low virulence on apple. In the genus Fusarium, F. oxysporum was widely distributed, but isolates were non-pathogenic. Fusarium solani and F. avenaceum were less frequently encountered, with only some isolates having low virulence. qPCR data obtained from seedling roots inoculated with the most virulent Pythium species (P. irregulare, P. sylvaticum and P. vexans) and the genus Phytophthora were not always reproducible between trials, or isolates of the same species. In general, seedling growth inhibition was associated with the presence of a low amount of pathogen DNA (±40 fg μl⁻¹ to 2 pg μl⁻¹) in roots. Pythium irregulare, although having the lowest DNA concentrations in roots, was the only species for which a significant negative correlation was found between seedling weight and pathogen DNA concentration.     

Simultaneous detection and genetic variability of stone fruit viroids in the Czech Republic

In this paper, we report a large-scale survey for the incidence of Peach latent mosaic viroid (PLMVd) and Hop stunt viroid (HSVd) in stone fruit collections and commercial orchards in the Czech Republic. From the 645 samples analysed, PLMVd was detected in 80 (26.6%) of peaches and the HSVd in 3 (1.3%) of apricot and 1 (0.33%) of peach trees. Sixty-seven accession of peach (44.6%) from the Czech Clonal GeneBank were infected by PLMVd. In addition, we used naturally infected trees to standardise the simultaneous detection of PLMVd and HSVd plus host mRNA as the control by means of one-step multiplex RTC-PCR. Eleven PLMVd and two HSVd isolates were sequenced and analysed. All the PLMVd variants were highly homologous (97–100%) to previously reported PLMVd variants from Tunisian peach and almond trees, and clustered together in the previously reported phylogenetic group III. The HSVd variants obtained from apricot and peach trees were included in the previously proposed recombinant group PH/cit3.     

Molecular characterization of <Emphasis Type="Italic">Phoma tracheiphila</Emphasis>, causal agent of Mal secco disease of citrus,in Israel

Mal secco disease of citrus caused by Phoma tracheiphila is a devastating disease in the Mediterranean basin. Susceptible citrus species include lemon, citron, lime and others. Trees attacked by the fungus show characteristic symptoms; the smallest twigs die first, followed by the larger branches. Eventually, the whole tree is killed. The symptoms are clear in the orchards but by the time they are visible the disease is already well established. The need for a sensitive, reliable and rapid diagnostic method for the early identification of the fungus in trees and fruit exists. We have developed a PCR-based method for the identification of P. tracheiphila from plant tissues including fruit. Any such method must take into account the genetic variability in the pathogen population. Molecular methods were used to compare different isolates of P. tracheiphila. This study found no significant differences between different isolates from different citrus species from different parts of Israel.     

Molecular Phylogenetic Analysis of Ribosomal DNA Internal Transcribed Spacer Regions and Comparison of Fertility in <Emphasis Type="Italic">Phomopsis </Emphasis>Isolates from Fruit Trees

Sequences of the ribosomal DNA (rDNA) internal transcribed spacer (ITS) regions were examined to infer a molecular phylogeny of small-spored Phomopsis isolates, designated W-type (mainly white colony, weakly virulent, bearing both alpha and beta conidia at 25°C on PDA) and G-type (mainly gray colony, highly virulent, bearing only alpha conidia at 25°C on PDA), and P. amygdali from fruit trees. Phomopsis G-type and P. amygdali were a monophyletic group distinct from the W-type. The W-type isolates were divided into two monophyletic groups. Diaporthe citri, D. tanakae, P. asparagi, P. viticola, P. vitimegaspora and D. nomurai, which are morphologically distinguishable from W- and G-types, differed from the W- and G-types in molecular phylogenetic analyses. PCR-RFLP analysis of rDNA ITS regions was useful to distinguish each of the Phomopsis species and groups using three restriction enzymes. In mating tests, W-type isolates from fruit trees were heterothallic and inter-fertile even between isolates belonging to the different monophyletic groups. Isolates of the G-type and P. amygdali collected in Japan were cross-fertile. Some isolates from Lunaria annua, Ulmus glabra and Juglans regia belonged to one of the two monophyletic groups of the W-type and were cross-fertile with W-type isolates from Rosaceous fruit trees. Received 27 September 1999/ Accepted in revised form 27 January 2000     

First report of Almond witches’ broom phytoplasma1 (‘Candidatus Phytoplasma phoenicium’) causing a severe disease on nectarine and peach trees in Lebanon

Symptoms of shoot proliferation characteristic of phytoplasma diseases were observed on nectarine (Prunus persica var. nucipersica) and peach (P. persica) trees in the Sarada plain, south of Lebanon. The presence of phytoplasmas in the two orchards visited was confirmed by nested polymerase chain reaction using universal primers. The amplified DNA fragments were cloned and sequenced. Blast analysis of over 1000 nucleotides demonstrated the presence of ‘Candidatus Phytoplasma phoenicium’ which is considered to be the causal agent of Almond witches’ broom. This phytoplasma which belongs to the pigeon pea witches’ broom group (16SrIX) can be devastating since Almond witches’ broom has killed thousands of almond trees in Lebanon and Iran. Previous reports indicated that Almond witches’ broom may be transmitted by grafting to peach and nectarine under experimental conditions. This is the first report of a natural and epidemic spread of ‘Ca. Phytoplasma phoenicium’ in peach and nectarine. Farmers in the region were advised to eradicate the infected trees immediately. Further studies on the epidemiology of ‘Ca. Phytoplasma phoenicium’ and its vector(s) are recommended in order to develop successful eradication or disease management programmes.     

Testing variability in pathogenicity ofPhytophthora cactorum, P. citrophthora andP. syringae to apple, pear, peach, cherry and plum rootstocks

The relative virulence ofPhytophthora cactorum andP. syringae originating from almond trees, and ofP. citrophthora originating from citrus, to apple, pear, peach, cherry and plum rootstocks, was studiedin vivo andin vitro. Results of the different experiments were in good agreement. All testedPhytophthora isolates showed little virulence to pear rootstocks-causing only minor crown rot symptoms - and no virulence at all to apple rootstocks. In contrast, they were highly virulent to stone fruit rootstocks, causing crown rot disease. The non-pathogenicity of these isolates to pome rootstocks could be interpreted as strict host specificity.     

Bait twig method for soil detection of <Emphasis Type="Italic">Rosellinia necatrix</Emphasis>, causal agent of white root rot of Japanese pear and apple,at an early stage of tree infection

Mulberry twigs were inserted into the soil as bait to detect Rosellinia necatrix at an early stage of tree infection in the orchard. R. necatrix was frequently trapped on twigs near the trunk base at soil depths of 6–20 cm within 10–20 days in May–July, suggesting that the incubation period was dependent on soil temperature. Subsequently, we inserted twig in the soil around healthy-looking trees in naturally infested orchards. R. necatrix was trapped from 80.0% of Japanese pear and 75.0% of apple trees that later proved to be infected. This bait twig method facilitated quicker diagnosis of white root rot on Japanese pear and apple at early stages of infection and can be used to detect recurrence of the fungus after fungicide treatment.     

Bacteria from four phylogroups of the Pseudomonas syringae complex can cause bacterial canker of apricot

Pseudomonas syringae is described as a species complex, containing P. syringae-related species classified into 13 phylogroups and 23 clades. Pseudomonas syringae is one of the main pathogens of fruit trees, affecting nut trees, hazelnut and kiwi, pome and stone fruits. Bacterial canker of apricots is an important disease in regions of production with cold winters and conducive soils. This work characterizes the bacteria able to induce canker in apricots isolated in different French orchards. Bacteria from four phylogroups were able to induce canker. The pathogenicity to apricot was not linked to the pathogenicity to the three herbaceous species and cherry fruits tested, and was not always related to hypersensitive reaction on tobacco and ice nucleation activity. Bacteria pathogenic to apricot belong to phylogroups 01, 02, 03 and 07. The bacteria of phylogroups 01a and 07a (Pseudomonas viridiflava) characterized in this work have not previously been described as pathogenic to apricot.     

Transmission of ‘Candidatus Phytoplasma pyri’ by naturally infected Cacopsylla pyri to peach,an approach to the epidemiology of peach yellow leaf roll (PYLR) in Spain

Peach orchards in the northeast of Spain were severely affected in 2012 by a previously unreported disease in this area. The symptoms included early reddening, leaf curling, decline, abnormal fruits, and in some cases death of the peach trees. All the infected peach samples were positive for ‘Candidatus Phytoplasma pyri’, but none were infected by the ‘Ca. Phytoplasma prunorum’. In this work, potential vectors able to transmit ‘Ca. Phytoplasma pyri’ from pear to peach and between peach trees were studied and their infective potential was analysed at different times of the year. Transmission trials of the phytoplasma with potential vectors to an artificial feeding medium for insects and to healthy peach trees were conducted. Additionally, isolated phytoplasmas were genetically characterized to determine which isolates were able to infect peach trees. Results showed that the only insect species captured inside peach plots that was a carrier of the ‘Ca. Phytoplasma pyri’ phytoplasma was Cacopsylla pyri. Other insect species captured and known to be phytoplasma transmitters were present in very low numbers, and were not infected with ‘Ca. Phytoplasma pyri’ phytoplasma. A total of 1928 individuals of C. pyri were captured in the peach orchards, of which around 49% were phytoplasma carriers. All the peach trees exposed to C. pyri in 2014, and 65% in 2015, were infected by ‘Ca. Phytoplasma pyri’ 1 year after exposure, showing that this species is able to transmit the phytoplasma to peach. Molecular characterization showed that some genotypes are preferentially determined in peach.     

<Emphasis Type="BoldItalic">Didymella glomerata</Emphasis><Emphasis Type="Bold">causing leaf blight on pistachio</Emphasis>

During 2015 and 2016, we detected blighted leaves of pistachio (Pistacia vera) trees in different orchards in Arizona (USA). A Phoma-like species was isolated from pycnidia that appeared embedded in the leaf tissue. The pathogen was identified by means of morphological characteristics and DNA analysis (by sequencing of the ITS, BT, and EF regions) as Didymella glomerata. The optimum temperature for mycelial growth of the pathogen was around 25 °C. Inoculation tests were conducted on healthy and wounded pistachio leaves, fruits, and shoots of the cv. Kerman (female) and the cv. Peters (male). Overall, the pathogen was highly virulent on leaves of both pistachio cultivars and did not need injuries for infection. Conversely, the pathogen did not cause any macroscopic symptoms on the inoculated fruits and shoots but showed a certain endophytic behavior in the shoots. Also, data on the ability of different fungicides to inhibit the in vitro mycelial growth of the pathogen are presented.