Showing papers in "Australasian Plant Pathology in 2019"

Current status of the Botryosphaeriaceae in Australia

Abstract: The Botryosphaeriales, and in particular the Botryosphaeriaceae, are a well-studied group of fungi best known for the canker diseases they cause on woody hosts especially in stressed or damaged trees. Australian Plant Pathology herbaria contain many records for this group, but due to considerable taxonomic changes over the past decade, many of the species names have since been reclassified. In this article we used all published records with available sequence data of the Botryosphaeriaceae in Australia to examine the distribution and host range of these taxa. There are 24 genera encompassing 222 species in the Botryosphaeriaceae; 9 genera and 62 species have been recorded in Australia. Some genera such as Neoscytalidium are only found in warm, humid climates while Dothiorella species are more common in temperate climates. There were species, such as Lasiodiplodia theobromae, Neofusicoccum parvum and Botryosphaeria dothidea, which had a wide host range with many records. However, there were also several species found only in one location on a single host. While systematic data collection is still required, the information presented here provides a baseline of species present in Australia and will underpin future studies into this group of important pathogens.

A review of Botryosphaeriaceae species associated with grapevine trunk diseases in Australia and New Zealand

Abstract: Botryosphaeria dieback caused by species of Botryosphaeriaceae is an economically significant disease of grapevine worldwide including in Australia and New Zealand. Symptoms such as dieback of the shoots and cordons, cankers, stunted shoots, bunch rot and bud necrosis are observed, eventually leading to yield loss and reduced grapevine longevity. The disease was first recognised as an important issue in vineyards in Australia and New Zealand in the early 2000s and since then, considerable research has been conducted for better understanding of the causal organisms and their management. However, there is also a need to better understand the role of nursery planting material as a potential source of infection in new or replanted vineyards. This review reports on the current knowledge of Botryosphaeriaceae in grapevines in Australia and New Zealand including their identification, distribution across various wine grape growing regions, symptomatology, pathogenicity, epidemiology and management strategies. Future strategies to increase our knowledge and to better manage these economically important pathogens are discussed.

Botryosphaeriales associated with stem blight and dieback of blueberry ( Vaccinium spp.) in New South Wales and Western Australia

Abstract: Stem blight and dieback caused by species of the Botryosphaeriales are important diseases of blueberry worldwide. In recent years, stem blight and dieback symptoms have been increasingly observed affecting blueberry production in Australia. Thirty samples were collected from symptomatic plants in an orchard at Corindi NSW, a major blueberry growing region. In addition, samples from symptomatic blueberry plants were submitted by growers to the Plant Health Diagnostic Service, NSW Department of Primary Industries from eight orchards in New South Wales (NSW), and a single orchard in Western Australia (WA). Culture isolations, DNA sequencing and pathogenicity testing were undertaken to determine the species causing the disease. Fifty-two isolates were recovered in total, forty-eight from NSW, and four from WA. A multi-locus sequencing approach was used to assist species identification including the internal transcribed spacer region of rDNA including 5.8S (ITS), partial translation elongation factor 1-alpha (tef1-α), and DNA-directed RNA polymerase II second largest subunit (rpb2). Eight species from three genera were identified; the most common was Neofusicoccum parvum (n = 34), followed by N. kwambonambiense (n = 7), N. occulatum (n = 5), L. theobromae (n = 2), Botryosphaeria dothidea (n = 1), N. australe (n = 1), N. macroclavatum (n = 1) and Lasiodiplodia pseudotheobromae (n = 1). The pathogenicity testing showed all isolates produced lesions on blueberry stems. This study provides the first survey of Botryosphaeriales causing blueberry stem blight and dieback in Australia, and is a valuable resource for plant pathologists and growers trying to manage the disease.

Endangered species face an extra threat: susceptibility to the invasive pathogen Austropuccinia psidii (myrtle rust) in Australia

Abstract: Austropuccinia psidii (myrtle rust) is an invasive fungus native to South America that infects the young growing tissues of species in the Myrtaceae family, one of the dominant plant families in Australia. To date, 360 native species from 49 genera have been found to be susceptible in Australia, but the vast majority remain untested (81%). The aim of this study was to test a range of plant species whose susceptibility status remains unknown, including endangered species, species with a large distribution overlap with A. psidii and species from a genus that has not been previously tested. Different sub-species and provenances were also tested to assess for intra-specific differences. Of the 24 tested species/sub-species, 18 (including 12 endangered) were found susceptible to A. psidii to varying degrees (including the first species record within Triplarina being susceptible), while one presented a hypersensitive reaction and six were resistant. The most susceptible species were the critically endangered Melaleuca megalongensis, and the endangered Eucalyptus copulans, E. parvula, E. scoparia and Melaleuca irbyana. No significant differences in intraspecific susceptibility were found between sub-species or provenances. We suggest that the susceptible species we have identified will be vulnerable to infection in their native ranges in the future, if they have not already become infected. For highly susceptible species, A. psidii should be considered as a major additional threat and appropriate control measures incorporated into existing threatened species plans. Monitoring of susceptible species’ populations in the wild and seed collection for seed banking are vital steps for ensuring their conservation.

Silencing of an aquaporin gene diminishes bacterial blight disease in rice

Abstract: The translocation of type III effectors of plant-pathogenic bacteria is required both for the expression of their role in virulence, and for the triggering of plant defenses. The effector translocation needs molecular associations between bacterial translocators and their recognition compounds situated in plant plasma membranes. We show here that rice aquaporin PIP1;3, a member of the plasma membrane intrinsic protein family, is implicated in translocation of a transcription activator-like (TAL) effector from the bacterial blight pathogen into the cytosol of rice cells. The TAL protein PthXo1 targets rice disease-susceptible gene SWEET11 to determine virulence of the bacterial strain PXO99A on susceptible rice variety Nipponbare. In Nipponbare, post-transcriptional gene silencing of PIP1;3 results in highly alleviated susceptibility and concomitantly decreased expression of SWEET11 as compared to levels of the bacterial virulence and the plant SWEET11 expression in the wild-type plant with normal transcription of PIP1;3. In coincidence, the efficiency in PthXo1 translocation is substantially reduced in PIP1;3-silenced rice lines in contrast to the wild-type plant. In the case of PIP1;3 silencing, moreover, the bacterial type III translocator Hpa1 loses the ability to mediate PthXo1 translocation from the bacterial cells into the cytosol of rice cells. By contrast, PIP1;3 silencing does not affect the performance of isolated Hpa1 as a pattern molecule to induce immune responses in rice. Our data suggest that rice aquaporin PIP1;3 is a candidate of disease-susceptible factor with functional relevance to PthXo1 translocation as a prerequisite of the bacterial virulence on the susceptible rice variety.

The pandemic strain of Austropuccinia psidii causes myrtle rust in New Zealand and Singapore

Abstract: The myrtle rust pathogen, Austropuccinia psidii, was recently detected in New Zealand and Singapore. We used microsatellite markers to identify the strain of A. psidii that caused these incursions. Our results show that the pandemic strain of the pathogen caused outbreaks in both New Zealand and Singapore.

Xanthomonas perforans: a tomato and pepper pathogen associated with bacterial blight and dieback of Eucalyptus pellita seedlings in Indonesia

Abstract: Leaf and shoot blight, often accompanied by die-back symptoms, on Eucalyptus species, hybrids and clones have been reported from a number of countries. More than one bacterial species has been found to cause these symptoms. In this study, a leaf disease of E. pellita in Indonesia was investigated. The disease was found primarily on nursery plants and young trees that recovered within the first year of growth. Leaf samples were collected from symptomatic trees, and isolations consistently yielded a Xanthomonas sp. Sequencing of the 16S rRNA gene region and multilocus sequence analysis (MLSA) was performed on 19 of the 61 Xanthomonas isolates obtained. In the MLSA, four genes, namely, dnaK, fyuA, gyrB and rpoD, were sequenced and the isolates were identified as X. perforans. Four representative isolates, at a concentration of 106 CFU/ml, were leaf-infiltrated and spray-inoculated on to E. pellita, tomato and pepper seedlings. The type isolate of X. perforans was included in the pathogenicity trials as a positive control. All four isolates of X. perforans, inclusive of the type isolate, induced bacterial spot symptoms on tomato and pepper seedlings. They also caused water-soaked lesions on the leaves of E. pellita seedlings, characteristic of the symptoms observed in the field. This is the first report of X. perforans infecting leaves of a woody host.

Management of bacterial wilt in tomato using dried powder of Withania coagulan (L) Dunal

Abstract: The potential of finely ground dried powder of different parts (leaves, succulent shoot and stem) of the desert medicinal plant, Withania coagulans, (L) Dunal to control bacterial wilt (BW) of tomato was explored using four different doses (0 g, 10 g, 20 g, and 30 g Kg−1 soil) and three different (0 days before transplanting DBT, or 10 DBT and 20 DBT) application timings. Both, in-vitro and in-vivo experiments were conducted. In in-vitro studies, each of the three concentrations (5%, 10%, and 20% w/v) of aqueous extracts of leaves, succulent shoot and stems inhibited the growth of the BW pathogen, Ralstonia solanacearum. The aqueous extract (20% w/v) of dried powder of leaves produced the maximum zones of inhibition (ZI) (20.8 mm) followed by that of succulent shoot (19.2 mm) and stem (16 mm) while the minimum ZI (11.2 mm,) was produced by the aqueous extracts of (5% w/v) stem powder. Consistent with the in-vitro results, the effect of the ground powders of the medicinal plant was found to be dose- and plant-part-dependent in in-vivo studies as well. Dried powder of leaves performed better than those of succulent shoot and stems. Leaves powder used at 30 g kg−1 soil under in-vivo conditions, reduced area under disease progress curve (AUDPC) by 37.54%, pathogen population g−1 of the infested soil by 45.04%, enhanced shoot length by 37.45%, root length by 63.36%, and plant fresh biomass by 38.62% as compared to untreated inoculated control plants. Dried powder of succulent shoot (tender shoots plus leaves) used at 30 g/kg soil, ranked second in terms of controlling bacterial wilt. It reduced AUDPC by 32.33%, pathogen population g−1of soil by 32.66%, augmented shoot and root lengths by 35%, and 62.39%, respectively and plant fresh biomass by 38.41% as compared to control plants. Lower doses of dried powders of all parts of the medicinal plant gave inferior results. Similarly, the application time of 20 DBT was found to be better than 10 DBT and 0 DBT. It achieved a reduction of 32.91% in AUDPC, and an augmentation of 41.32%, 54.42%, 54.53% in shoot length, root length and plant fresh biomass, respectively in comparison to untreated inoculated plants. Therefore, it is concluded that dried powder of leaves or succulent shoots of W. coagulans applied at the rate of 30 g kg−1 soil, 20 DBT, can be included as an effective component of integrated disease management (IDM) against BW.

Australian cultures of Botryosphaeriaceae held in Queensland and Victoria plant pathology herbaria revisited

Abstract: The Botryosphaeriaceae is one of the most widespread and cosmopolitan endophytic group of fungi. However, the species of this group can cause severe disease when the hosts are under stressful conditions. The aim of this study was to identify living cultures from the Botryosphaeriaceae family preserved in the Queensland and Victorian Plant Pathology Herbaria using DNA sequence analyses. The 51 isolates were collected between 1971 and 2017, from 35 different host genera, with the dominant host genera being Mangifera (11 isolates), Acacia (10), and Persea (5). Multilocus sequence analyses resulted in the re-identification of 41 isolates to the genera Botryosphaeria (2 isolates), Diplodia (4), Dothiorella (1), Lasiodiplodia (19), and Neofusicoccum (15), as well as some that belonged to genera outside of the Botryosphaeriaceae (10). New records for Australia were Botryosphaeria sinensis, Diplodia alatafructa, Lasiodiplodia gonubiensis, Neofusicoccum cryptoaustrale, and N. mangroviorum. These were identified as a result of a workshop organised by the Subcommittee on Plant Health Diagnostics. The results of this study provide the fundamental information regarding the diversity of Botryosphaeriaceae species present in Australian.

Identification of resistance in tomato against root knot nematode (Meloidogyne incognita) and comparison of molecular markers for Mi gene

Abstract: Tomato production is limited by many biotic stresses of which root knot nematode (RKN, Meloidogyne incognita) is a major pest. The present study aimed to identify resistance sources in controlled conditions and compare molecular markers for efficient and rapid screening of M. incognita resistance. Among the ten genotypes evaluated, HAT-310 and HAT-311 were found immune to M. incognita infestation. Further, six crosses with these two resistant sources, (HAT-311 x Swarna Lalima, HAT-296 x HAT-311, EC-596747 x HAT-311, Swarna Lalima x HAT-310, EC-596743 x HAT-310 and Swarna Lalima x HAT-311), exhibited immune responses against M. incognita. Four molecular markers viz. JB-1, REX-1, PMi12 and Mi23 were employed in eighteen germplasm to characterise resistance and susceptibility of the genotypes against infestation by M. incognita. JB-1 yielded 420 bp in all the genotypes after digestion and hence could not be used to differentiate between nematode resistance and susceptibility. Marker PMi12 yielded additional DNA fragments in addition to the expected bands and did not give consistent results. REX-1 and Mi23 markers successfully differentiated between nematode resistant and susceptible genotypes. Moreover, Mi23 separated the homozygous and heterozygous resistance sources since the restriction enzyme analysis was not needed. The resistant genotypes identified from the present study may be used further in nematode resistance breeding programmes of tomato and the Mi23 marker can be used for rapid screening of the germplasm.

Management of Ceratocystis manginecans in plantations of Acacia through optimal pruning and site selection

Abstract: Wilt disease has become a threat to Acacia plantations in SE Asia. There are about 1.3 million ha of Acacia auriculiformis, A. mangium and Acacia hybrid plantations in Vietnam that require management of wilt disease caused by Ceratocystis manginecans. To identify options for management, we examined affect of pruning (tree age, pruning time and pruning technique) and site traits (altitude, slope and position) on disease expression from natural infections. Ceratocystis wilt disease and disease indices were more serious in younger than older pruned trees. Pruning in the rainy season resulted in heavier infection by Ceratocystis than pruning in the dry season. Decreasing bark tearing during pruning reduced the levels of infection and disease. Tip pruning of branches reduced disease compared to pruning close to the trunk. The interaction effects of terrain factors showed that A. auriculiformis and Acacia hybrid plantations were most severely affected when trees were planted in foot-hills, at altitudes below 300 m and on slopes below 15o. Results for the pruning time and pruning technique trials have application for reducing the impact of Ceratocystis wilt disease in Acacia plantations in Vietnam.

Ralstonia solanacearum, Ganoderma australe, and bacterial wetwood as predictors of ironwood tree ( Casuarina equisetifolia ) decline in Guam

Abstract: Ironwood (Casuarina equisetifolia subsp. equisetifolia) trees on Guam have been in decline since 2002. This study applied proportional odds logistic multiple regression modeling to a set of biological variables in order to find significant decline predictors as a first step towards identifying pathogenic contributors. Based on the analysis of a set of 77 medium and large trees, the bacterium Ralstonia solanacearum and the fungus Ganoderma australe species complex were found to be significant predictors, with p values of 0.087). When nine covariates were applied univariately to a data subset of 30 tree cross-sections, the significance of R. solanacearum was strongly upheld while that of G. australe was moderately reduced. Also significant were percent cross-sectional area with bacterial wetwood and the formation of ooze within 24 h. Wetwood bacteria of Klebsiella spp. and K. oxytoca were found across all levels of decline and were not significant predictors. Other enteric bacteria identified included Kosakonia, Enterobacter, Pantoea, Erwinia, and Citrobacter.

Screening disease resistance of Acacia auriculiformis clones against Ceratocystis manginecans by artificial and natural inoculation methods

Abstract: Tropical species of Acacia are grown in plantations for pulpwood in many countries of South-east Asia. A vascular wilt disease caused by Ceratocystis manginecans is compromising their productivity and continued use as a commercial species and has become an issue in Vietnam. In this study, we evaluated the potential resistance of 45 three-year-old Acacia auriculiformis clones to C. manginecans using different pathogenicity tests: natural infection in the field, natural infection in the field promoted by artificial wounding, and the inoculation of excised branch segments in a controlled environment. Ten of the 45 clones expressed disease symptoms in the field under natural infection conditions; mean percentage of infection varied from 3.5 to 21.7%. Although the screening protocols of artificial wounding in the field to promote natural infection and excised branch segment inoculations led to a greater number of clones being infected, respectively 22 and 38, there were significant correlations between the percentage of infected trees, disease index and lesion length (L) following natural infection and each of these protocols. There were no significant differences in L between the top 15 most tolerant clones following excised branch segment inoculations. Seven clones (AA78, AA83, AA89, AA92, AA93, AA95 and AA103) were resistant (L = 0.0) with MAIs of >20 m3/ha/y. This finding suggests that there may be opportunities to employ resistant clones in breeding programs and to increase disease tolerance to C. manginecans in A. auriculiformis and Acacia hybrid (A. mangium × A. auriculiformis) through genetic gain.

Botryosphaeriaceae causing branch dieback and tree death of macadamia in Australia

Abstract: Incidence of branch dieback of macadamia with characteristic symptoms including dark necrotic lesions on the wood and blackening of the vascular tissue is increasing in commercial macadamia orchards in the south-eastern production regions in Australia. In many cases, disease progresses from the branch to the main trunk resulting in total tree death and reduced orchard productivity. Previously, only Botryosphaeria ribis was associated with branch dieback of macadamia, however, recent observations suggest other species in the Botryosphaeriaceae may be involved. This study aimed to identify the causal agents of branch dieback of macadamia and examine their pathogenicity on a main macadamia cultivar (HAES 246). Thirty-four representative Botryosphaeriaceae isolates, obtained from over 150 samples of branch dieback symptoms on macadamia trees, were identified using the partial sequence of the internal transcribed spacer of the rDNA and partial sequences of β-tubulin and elongation factor gene regions. Six species in the Botryosphaeriaceae were identified. Lasiodiplodia pseudotheobromae (n = 18; 53%) was the most prevalent species, followed by Neofusiccocum parvum (n = 5; 14%), L. iraniensis (n = 4; 12%), N. luteum and L. theobromae (n = 3; 9% each) and N. australe (n = 1; 3%). Using an in planta assay, pathogenicity tests showed that all six species caused dieback and necrotic lesion symptoms on macadamia plants. Inoculated plants died within 4 weeks of inoculation, showing the characteristic symptoms (blackening of the wood tissue). Inoculation trials revealed differences in aggressiveness among the six species.

Phyllosticta capitalensis and P. paracapitalensis are endophytic fungi that show potential to inhibit pathogenic P. citricarpa on citrus.

Abstract: Citrus black spot (Phyllosticta citricarpa) is an economically important disease of citrus in Australia. A closely related endophyte, P. capitalensis, also occurs on citrus in Australia, and the two fungi are known to co-exist in orchards. The diversity of other species of Phyllosticta on Citrus in Australia is unknown. Citrus black spot is managed by fungicide treatment and options such as biological control may be better economic alternatives. We studied the diversity of Phyllosticta on Citrus in Australia with a phylogenetic species hypothesis. We report P. paracapitalensis for the first time and confirm the presence of P. capitalensis in Queensland. Many examined isolates of Phyllosticta previously identified as P. capitalensis were reidentified as P. paracapitalensis. We also provide evidence through Koch’s postulates that the two endophytic species are non-pathogenic on citrus fruit. In addition, disease incidence and severity of citrus black spot was significantly reduced when fruit were pre-inoculated with one of the endophytes 14 days prior to pathogen inoculation on the same fruit. Our results indicate these endophytes may have potential antagonistic effects against P. citricarpa under field conditions.

Eucalyptus rust climatic risk as affected by topography and ENSO phenomenon

Abstract: Brazil is among the most important Eucalyptus producers in the world, contributing with approximately 25% of the world′s Eucalyptus plantations area. Even under favorable climatic conditions, the Eucalyptus yields in Brazil are often limited by several factors, including fungal diseases. Among others, Eucalyptus rust, caused by Puccinia psidii, is the one of the most severe fungal diseases and can cause extensive losses in wood production. Considering the significant influence of climatic conditions on Eucalyptus rust, this study aimed to analyze the effect of climatic variability, El Nino Southern Oscillation phenomenon (ENSO), and topo-climatic conditions on the favorability zones for Eucalyptus rust in south-central Brazil. The assessment of the climatic favorability for Eucalyptus rust was carried out for the main Eucalyptus producing regions in Brazil. For that, information related to climate, seasonal and inter-annual climatic variability, and disease response to topography effect were considered. The ENSO phenomenon affects the favorable climatic zones for Eucalyptus rust occurrence in a different way for the studied regions. For SC and PR states higher rust occurrence was observed in El Nino years and reduced in La Nina years while for the other states no trend was observed. The effect of topoclimatic conditions established in this study indicated its influence on the areas of climatic favorability for Eucalyptus rust occurrence. The results indicated that within favorable areas to the disease, it is possible to identify areas of low risk (escape areas) depending on the face of the terrain. This finding highlights the importance to consider the topo-climatic effect and ENSO phenomenon on plant disease monitoring and management.

Molecular characterization of Mungbean yellow mosaic India virus infecting tomato ( Solanum lycopersicum L.)

Abstract: The severe mosaic and mild leaf curl symptoms were observed on tomato crops in the agricultural farm at Chitrakoot, Madhya Pradesh, India. Based on typical viral symptoms and high populations of whiteflies in the field, the association of begomovirus was suspected. The identification of a bipartite begomovirus associated with the disease was performed by sequence analyses of their cloned full-length genome. The complete genome sequences were submitted in GenBank under accession numbers MF683072 (DNA-A) and MF683073 (DNA-B). The sequence analyses of both genome of bipartite begomovirus revealed highest nucleotide sequence identity and close phylogenetic relationships with Mungbean yellow mosaic India virus (MYMIV) reported from India, therefore, following the ICTV guidelines, the begomovirus under study was identified as an isolate of MYMIV. Best of our knowledge, this is the first report of MYMIV infecting tomato.

First report of wilt disease in Dalbergia tonkinensis caused by Ceratocystis manginecans

Abstract: Dalbergia tonkinensis is a rare and valuable rosewood which is being domesticated in home gardens in north Vietnam. Concern for the viability of this new industry has arisen with growers reporting the recent death of trees. This study identified a new wilt disease causing large bole cankers, associated blue staining to the wood, wilted canopies and tree death in home gardens and plantations. From βT1a and βT1b sequence analysis, the Ceratocystis isolates causing wilt disease in D. tonkinensis belong to Ceratocystis manginecans and were shown to be pathogenic with D. tonkinensis seedlings in a nursery trial. They were also able to form stem cankers in Acacia and Eucalyptus. Cankers in 2–11 year-old trees were associated with wounding, primarily from pruning of branches to improve tree form and drilling of trunks in older trees to assess heartwood formation. To mitigate against further damage, it is recommended that pruning be restricted to the dry season and that the practice of drilling trunks be kept to a minimum.

Seed tuber incidence, identification and pathogenicity of Verticillium species infecting potatoes in South East Australia

Abstract: Verticillium wilt, caused by the soilborne fungi Verticillium dahliae and Verticillium albo-atrum, is a serious disease of potato as well as many other crops. Potato seed tuber surveys (2010 to 2012) from Victoria and Tasmania, Australia identified V. dahliae, V. albo-atrum and V. tricorpus infecting the stem end vascular tissue of seed tubers. The species were identified by traditional morphology and phylogenetic analysis of the ITS region. Isolation of V. dahliae within a seed lot varied greatly and ranged from 0 to 55%. Verticillium spp. were isolated from the stem-end vascular tissue of tubers from seed lots from Victoria and Tasmania with an overall percent infection of 27.7 (V. dahliae), 8.4 (V. albo-atrum) and 4.8 (V. tricorpus). Verticillium dahliae was isolated from 11% of tubers with discoloured stem-end vascular tissue and 3.3% of tubers without stem-end vascular decolourisation suggesting that tuber stem end vascular discolouration symptoms were not a reliable indication of Verticillium wilt infection. In glasshouse trials, V. dahliae isolates from different geographical locations varied in pathogenicity during infection of susceptible potato cv Shepody, moderately resistant cv Ranger Russet and eggplant cv Black Beauty. The majority of V. dahliae isolates were highly aggressive in potato and eggplant, especially the Tasmanian V. dahliae isolates. Infected plants of cv Shepody inoculated with V. albo-atrum and V. tricorpus showed typical wilt symptoms however, the severity of infection caused by V. tricorpus was substantially lower compared to highly aggressive isolates of V. dahliae. In eggplant, V. dahliae isolates also varied in pathogenicity in terms of disease severity but all isolates significantly (p ≤ 0.05) reduced eggplant growth.

Phytophthora aleatoria sp. nov., associated with root and collar damage on Pinus radiata from nurseries and plantations

Abstract: During routine surveys of Pinus radiata plantations in the Nelson region, New Zealand, a Phytophthora species was isolated in association with bleeding stem cankers and rhizosphere soil. This isolate grew more slowly than other Phytophthora species associated with P. radiata in New Zealand, and was morphologically similar to isolates of Phytophthora cactorum previously associated with horticulture production, and isolates that were morphologically identified as P. cactorum from P. radiata in Nelson since the 1970s. Phylogenetic analyses of the ITS, cox1, and s-tubulin _F1A and s-tubulin_F2A regions confirmed this to be a new species closely related to P. hedraiandra in Clade 1. The new species is described here as Phytophthora aleatoria sp. nov. It produces partially caducous, papillate, ovoid to rarely globose or limoniform sporangia, markedly aplerotic oogonia forming thin-walled oospores, and paragynous (mainly) or amphigynous antheridia on some cultures. To date, this species has been confirmed on P. radiata in New Zealand across a range of sites within the North Canterbury, Nelson, Hawkes Bay, Taupo and the Bay of Plenty regions.

Leaf wetness duration estimation and its influence on a soybean rust warning system

Abstract: A limitation for widespread implementation of Asian soybean rust (ASR) warning system is the scarcity of leaf wetness duration (LWD) data. The objective of this study was to assess the feasibility of using LWD estimated by different empirical methods as input data for an ASR-warning system. The study was carried out with data from field experiments conducted in Ponta Grossa, Parana State, Campo Verde and Pedra Preta, both in Mato Grosso State, Brazil, throughout the 2014–15 and 2015–16 seasons. The estimated LWD values were used as input in an ASR-warning system. More reliable estimations of LWD were obtained using the Number of Hours with Relative Humidity above 90% (NHRH≥90%). The use of estimated LWD resulted in disease overestimation in Ponta Grossa, underestimation in Pedra Preta and moments of under and overestimation in Campo Verde. Analyzing the ASR-warning system with the more conservative threshold, five sprays per season were recommended for all sites and the mean fractions of correct estimates (θ1) in Ponta Grossa were higher (θ1 = 0.876) than those obtained in Pedra Preta (θ1 = 0.632) and Campo Verde (θ1 = 0.662). Otherwise, for the less conservative spray threshold, a reduction of one spray was observed in Ponta Grossa (θ1 = 0.912) when compared to Pedra Preta (θ1 = 0.826) and Campo Verde (θ1 = 0.767), where five sprays were still recommended. Thus, LWD estimated by NHRH≥90% has enough accuracy and precision for being used as input in the ASR-warning system in the most traditional soybean regions of Brazil.

Abiotic conditions governing the myceliogenic germination of Sclerotinia sclerotiorum allowing the basal infection of Brassica napus

Abstract: Sclerotinia sclerotiorum is the causal pathogen of sclerotinia stem rot (SSR) in canola, causing significant yield losses in this crop globally, when conditions are favourable. The pathogen can cause disease symptoms on canola through ascospore infection from carpogenically germinated sclerotia or by basal infection from myceliogenically germinated sclerotia. While infection through carpogenic germination in canola is the main mode of infection and well-studied, little is known about myceliogenic germination of sclerotia and subsequent basal infection of canola. This review describes the lifecycle of S. sclerotiorum on canola and presents an overview of the current knowledge of the factors that influence myceliogenic germination. These include factors such as sclerotium maturity, rind melanisation, temperature and moisture or a combination of these factors. Subsequently, the most likely avenues of S. sclerotiorum-based basal infection in canola are discussed and compared to basal infection in other host crops. We conclude that myceliogenic germination can be promoted by incubation of sclerotia at extreme temperatures followed by exposure to moisture. Future research to determine the prevalence of myceliogenic germination and subsequent basal infection of canola in the field is required.

Evaluating Walnut ( Juglans spp .) for resistance to walnut blight and comparisons between artificial inoculation assays and field studies

Abstract: The host resistance of walnut (Juglans spp.) against Xanthomonas arboricola pv. juglandis (Xaj) was evaluated for 18 walnut genotypes in 2016 and 2017 at Chengdu, Sichuan, China, using three assays: one under natural conditions (field investigations) and two under controlled conditions (detached leaflet assay and whole plant assay). Field investigations were used as a benchmark for comparing the other two assays. Five genotypes, ‘JS 91’, ‘JS 92’, ‘JS 86’, ‘Qingxiang’ and ‘JS 71’, displayed partial resistance in all assays, suggesting that they may be good candidates for further evaluation. The rankings for resistance among the 18 genotypes showed significant correlations between field investigations and both the detached leaflet assay and whole plant assay. However, low determination coefficients indicated that assays under controlled conditions should be considered as complementary methods and not as a substitute for field analysis when evaluating resistance to walnut blight.

Non-host specificity of Botryosphaeriaceae on macadamia and blueberry

Abstract: The ability of species in the Botryosphaeriaceae to inhabit seemingly healthy plants, reside as endophytes yet result in serious disease under stressful environmental conditions is poorly understood, as is their ability to colonise many unrelated hosts often without causing disease. Although many species of Botryosphaeriaceae have a wide host range, there is dearth of information on pathogenic variation of isolates of the same species on different hosts. In this study we used 16 isolates belonging to two genera of Botryosphaeriaceae from seven host plant species, to test the hypothesis that there are significant differences in pathogenic variation, varietal preferences and cross-pathogenicity in Botryosphaeriaceae on two unrelated host crops (macadamia and blueberry). Using the detached leaf and stem inoculation assays, we observed significantly (P < 0.05) high levels of pathogenic variation among isolates of Neofusicoccum parvum and compared with three other Neofusicoccum spp. and the three Lasiodiplodia spp. Host specificity was not observed among the isolates, as all were pathogenic on both hosts, but the aggressiveness of the isolates varied on the hosts, regardless of the source of the isolates. Between the host plants, blueberry was significantly more susceptible, regardless of the isolates than macadamia. There were significant (P < 0.03) differences among the four macadamia cultivars, isolates and cultivar × isolate interaction. Similar results for pathogenicity, aggressiveness and pathogenic variations were obtained using both the detached leaf and stem inoculation assays. Implications of these findings for disease management in cropping systems are discussed.

Evaluating different approaches in the application of phosphonates for the control of apple root diseases

Abstract: Phosphonate fungicides are registered on various tree crops in South Africa for the management of oomycete root rot pathogens, but not on apple trees. The study investigated several phosphonate treatments previously evaluated independently by technical advisors in South Africa. A replicated orchard trial was conducted in non-bearing asymptomatic orchards; tree roots were infected by oomycetes but foliar symptoms were absent. Phosphonate foliar-, trunk paint- and soil drench treatments were equally effective, and consistently resulted in a significant reduction in Phytophthora cactorum and Pythium irregulare root DNA quantities relative to the non-treated control. The latter was not always true for phosphonate trunk spray treatments. Trunk paint applications applied at an annual dosage of 40 g phosphorous acid/tree yielded significantly higher root phosphite (breakdown product of phosphonates) concentrations than the soil drench and trunk spray applications; the latter were applied at lower annual dosages of 7.5 g a.i./tree and 20 g a.i./tree, respectively. Foliar sprays applied at a low annual dosage (1.8 to 3.0 g a.i./tree) often outperformed the soil drench and trunk spray treatments in root phosphite concentrations. No clear association was evident between root phosphite concentrations and pathogen suppression. Root phosphite typically peaked at 8-weeks post-treatment for winter applications, and between 2- to 4-weeks for summer applications. A rapid decline in root phosphite was evident over the 12-week summer period, but not for winter applications. Monitoring root growth in the untreated control plots showed that root growth was continuous but that it peaked in summer, with reduced growth in winter.

Detecting symptoms of Phytophthora cinnamomi infection in Australian native vegetation using reflectance spectrometry: complex effects of water stress and species susceptibility

Abstract: Diseases in natural and agricultural systems have been linked to species of the Oomycete genus Phytophthora, around the world. Direct detection of the pathogen requires sampling of soil or plant material, which can be expensive, difficult to obtain and error-prone. As an alternative, reflectance spectroscopy provides a potential indirect method for detecting symptoms of infection by P. cinnamomi. Here we evaluate the use of reflectance spectroscopy to detect physiological changes associated with infection in host plants using spectral indices designed to quantify changes in plant pigments (pigment indices), leaf water content (water indices) and fluorescence (fluorescence indices). Two grasses and two tree species with different susceptibilities to P. cinnamomi were inoculated and/or exposed to water stress in a glasshouse experiment. Inoculated plants were detected using pigment and fluorescence indices, which also had the capacity to separate inoculated plants from water stressed uninoculated plants. While inoculation may have caused an opposing spectral response to water stress in some indices, plants that were both water stressed and inoculated then demonstrated an intermediate response. Water stress was detected using the water indices in all four species, and spectroscopic changes associated with inoculation were often greater in the susceptible species. Our results indicate that reflectance spectroscopy at the leaf scale detects the effects of P. cinnamomi infection in native vegetation. Extending these results has the potential to improve early detection of disease in natural vegetation and avoiding manual sampling, thus improving management of the disease.

Siratro latent polerovirus (SLPV): a new polerovirus from Australia with a non-functional open reading frame 0

Abstract: Macroptilium atropurpureum (siratro) samples from Queensland, Australia were collected as part of virus surveys and screened using tissue blot immunoassays for the presence of luteoviridae viruses. Sequencing of a reverse transcription polymerase chain reaction (RT-PCR) product, generated with degenerate primers for poleroviruses, revealed the non-symptomatic virus in siratro was a novel polerovirus, given the proposed name siratro latent polerovirus (SLPV). SLPV-specific RT-PCR showed that SLPV was widely distributed in siratro throughout Queensland. SLPV was transmitted via Aphis craccivora (cowpea aphids) to siratro and Pisum sativum (field pea). The complete genome of one isolate of SLPV was found to be 6090 nucleotides in length and shared only 69% nucleotide identity with its closest match, phasey bean mild yellows virus. Poleroviruses contain six major open reading frames (ORFs) labelled ORF 0–5. The SLPV genome has a similar arrangement. However, the region that should encode for ORF 0 is missing an AUG start codon. Sequencing of this region, from eight SLPV isolates, from different geographic regions, showed all lacked the ORF 0 AUG site and some isolates contained point mutations and deletions in this region, thus inhibiting translation of ORF 0. These results suggest that SLPV does not have a functional ORF 0 which may affect viral pathogenicity and restrict host range. This is the first report, to date, of a polerovirus with an apparently non-functional ORF 0 region.

Development and validation of a user-friendly KASP marker for the Sw-5 locus in tomato

Abstract: Tomato spotted wilt virus (TSWV) is a serious pathogen of tomato. The most efficient strategy for control of TSWV is the breeding of varieties resistant to the virus. In tomatoes, the Sw-5 locus controls resistance to TSWV. Molecular markers based on single nucleotide polymorphisms (SNPs) are widely used for screening in plant breeding. In the present study, we developed primers specific to the Sw-5 locus for use in Kompetitive Allele-Specific PCR (KASP) which is a novel method based on SNPs. These primers were validated in a number of tomato genotypes with different genetic backgrounds, showing that this KASP assay can be used successfully for the selection of TSWV-resistant plants in tomato breeding.

Identification of novel Ascochyta lentis resistance in a global lentil collection using a focused identification of germplasm strategy (FIGS)

Abstract: The Australian lentil breeding program is historically genetically narrow and recent reports suggests a loss of resistance to Ascochyta lentis within resistant cultivars such as Northfield and Nipper. There is evidence that the pathogen population is becoming more virulent on other widely adopted cultivars, thus there is an urgent need to identify novel resistance sources that may be transferred into the cultigen. To reduce the substantial economic and time commitment in this search, a Focused Identification of Germplasm Strategy (FIGS) was applied. This entailed exploring potential association between collection site environment and climatic conditions favouring A. lentis development, to predict regions imposing favourable selection towards A. lentis resistance. Accordingly, a subset of 87 landraces (originating from 16 countries) with highest likelihood for A. lentis resistance was selected from 4576 accessions held by the International Centre for Agricultural Research in the Dry Areas (ICARDA), Morrocco collection. Significant variation for resistance was detected within the subset using completely randomized and replicated controlled climate bioassays with a highly virulent Australian A. lentis isolate, FT13037. Genotype IG 207 expressed the lowest percent area of symptomatic tissue and a further 12 genotypes demonstrated moderate resistance. Furthermore, IG 207 recorded lowest mean disease score against four other highly aggressive isolates and performed better than the currently used best resistance sources, ILL 7537 and Indianhead. In addition, delayed pre-penetration behaviour of isolate FT13038 on IG 207 leaflets indicated discovery through the FIGS technique of a novel and highly effective A. lentis resistance source.

Evaluation of fungicide efficacy against Neofusicoccum species causing dieback disease of blueberries in New Zealand

Abstract: Several Botryosphaeriaceae species have been reported to cause stem canker, twig blight and dieback of blueberries, with different species being reported in different parts of the world. Pruning wounds are regarded as primary infection sites for these pathogens. This research evaluated in vitro and in vivo efficacy of fungicides against the main Neofusicoccum species associated with blueberry dieback in New Zealand. In vitro evaluation showed that four out of the nine fungicides tested were effective at reducing mycelial growth and/or conidial germination and germ tube growth of three pathogenic isolates each of N. australe, N. luteum, N. parvum and N. ribis. In vivo evaluation carried out with fungicides on wounded and non-wounded plant tissues on potted and field blueberry plants showed that carbendazim and tebuconazole were the most effective for protecting blueberry plants from infection by Neofusicoccum species. This research showed the importance of protecting both wounded and non-wounded tissues, with more than one application of fungicides likely to be required to provide effective control of the disease under natural inoculum levels.