Black Root Rot Research Articles

Soils from intercropped fields have a higher capacity to suppress black root rot in cassava, caused by Scytalidium lignicola

AbstractThe objective of the present study was to evaluate the natural suppressive capacity of soils from forest, and monocropping and intercropping systems, against root rot, caused by Scytalidium lignicola, in a greenhouse experiment. We used soils from a tropical dry forest (FOR) and two intercropping and two monoculture systems. The first intercrop was maize and beans (CORNCOWP), and the second intercrop was cassava, pigeon peas and beans (CASPIGPCOWP). The first monoculture was beans, and the second was passion fruit. The intercropping soils showed a higher capacity to suppress black root rot in cassava than the monoculture because such soils were able to reduce disease severity by about 50%. Bean soil in the monoculture showed less microbial biomass carbon than in the intercrop, with means of 10.05 and 38.2 mg/kg, respectively. The higher density of bacteria and fungal populations, microbial biomass, urease and arylsulphatase activities correlated with a decrease in disease severity. Soils from the intercrops produced changes in soil quality, primarily in the population and density of microorganisms, enzymatic activities, total organic carbon and nutrients, reducing disease severity in cassava plants. These effects were validated by multivariate principal component analysis and showed three distinct groups: one FOR, one intercropping and one monocropping. The majority of vectors were in the direction of FOR and intercropping soils. We have provided some of the first data related to the beneficial effects of intercropping on the suppression of black root rot in cassava, which is validated through different attributes.

First Report of Neoscytalidium dimidiatum Causing Root Rot in Sweet Potato in Brazil

In January 2018, a large number of tuberous roots of sweet potato were lost in a production field in Jaiba, State of Minas Gerais, Brazil. Immediately after harvest, the roots showed dark spots on the surface, extending to the center, causing an internal, brown to black necrosis. Two symptomatic roots (cultivar Canadense) from different parts of the field were subjected to fungus isolation by removal of fragments between symptomatic and healthy tissue, followed by surface sterilization in 70% ethanol and then in sodium hypochlorite (1%) for 1 and 3 min, respectively, rinsing in sterile water, and incubation in PDA plates for 25°C in the dark. A gray to black, fast-growing fungus with abundant arthroconidia in aerial mycelia was isolated from all samples after 4 days of incubation. Hyphal tips were excised and transferred to new PDA plates, and two pure cultures (isolates) were obtained. Based on these characteristics, the fungus was identified as Neoscytalidium. For morphological analysis, culture sporulation was induced as described by Machado et al. (2014). Microscopic examination of the fungal structures identified arthroconidia with one to two septa, released by hyphal fragmentation, 4.2 to 9.5 × 3.2 to 5.3 μm, and a pycnidial synanamorph with hyaline, ellipsoid to nearly fusiform conidia, 7.4 to 12.7 × 4.2 to 6.3 μm. The morphological characteristics were similar to Neoscytalidium dimidiatum (Phillips et al. 2013). To confirm species identification, genomic DNA was extracted, and sequences of the internal transcribed spacer of the rDNA and translation elongation factor 1-α (TEF1-α) were obtained (Machado et al. 2014) and deposited in GenBank (accession nos. MH636327, MH636328, MH349427, and MH349428, respectively). According to a BLAST search, the TEF1-α sequences showed 99% identity with N. dimidiatum (accession no. KF553902). A combined phylogenetic tree obtained by Bayesian inference using MrBayes version 3.1.1 (Ronquist and Huelsenbeck 2003) grouped the isolates within the N. dimidiatum clade, confirming the species identification. One representative isolate (URM 7888) was deposited in the culture collection “Micoteca URM Profa. Maria Auxiliadora Cavalcanti” at the Universidade Federal de Pernambuco (Recife, Brazil). The pathogenicity test was performed by inoculating asymptomatic roots (cultivar Canadense), previously sterilized on the surface with 0.5% sodium hypochlorite and unwounded or wounded with a sterile scalpel at two equidistant points, with conidia suspension. Arthroconidia suspension (20 μl, 2.4 × 10⁶ conidia/ml) obtained from 7-day-old colonies was pipetted onto the inoculation sites. Sterile distilled water was used for the control instead of inoculum. Five roots were used for each treatment. The inoculated roots were maintained in plastic boxes containing moistened paper towels at approximately 25°C for 15 days. After 7 days, it was possible to observe initial symptoms of internal brown to black rot, which increased in size after 15 days. The fungus was reisolated, and pathogenicity was confirmed. The control and unwounded roots remained asymptomatic. In Brazil, N. dimidiatum has been reported causing collar and root rot in physic nut (Machado et al. 2012), black root rot in cassava (Machado et. al., 2014), and dieback in mango (Marques et al. 2013). To our knowledge, this is the first report of N. dimidiatum causing root rot of sweet potato in Brazil.

Characterization and correlation of pathogenicity of Botryodiplodia theobromae isolates, the causal agent of black root rot of mulberry (Morus spp.)

The study reports the characterization of 10 isolates of mulberry black root rot causing fungus, Botryodiplodia theobromae obtained from the infected gardens of Karnataka, Andhra Pradesh and Tamil Nadu. The analysis based on cultural, morphological, pathogenicity and molecular markers (RAPD and SSRs) revealed significant variations among the isolates. Based on the disease reaction on susceptible V-1 variety, isolates were grouped as pathogenic (60%), moderate pathogenic (20%) and non-pathogenic (20%). Among all isolates, RAPDs revealed higher marker polymorphism however, based on Shannon’s Information Index (I) SSRs were more informative (0.781) compared to the former (0.444). Stepwise Multiple Regression Analysis (MRA) indicated a total of 5, 5 and 3 molecular markers were found to correlate with disease symptoms. Screening of germplasm using multiple strains of virulent isolates will enhance possibilities of locating diverse resistant genes. Pyramiding of these genes will aid in development of mulberry variety with durable resistance and sustainable sericulture.

Reactions of six cultivars of rice (Oryza sativa L.) to three fungal diseases

Fungal diseases are the most important threat for rice production in the world. Rice fields were visited and sampled from diseased bushes to identify fungal diseases in Kohgiluyeh and Boyer-Ahmad province, in the south-west of Iran. Fungi were isolated from diseased tissues by Agar-plate method, and purified by single-spore or hyphal-tip methods. They were identified by studying morphological characteristics. The virulence of isolated fungi was evaluated on six dominant rice cultivars, Champa of temperate region, Champa of subtropical region, Gerdeh, Shamim, Fajr and Tarom, under greenhouse conditions. The experiment was set up in a factorial experiment in completely randomized design with four replicates. The incidence of three fungal diseases: brown spot caused by Curvularia australiensis and Bipolaris cookei, foot rot caused by Fusarium globosum and Fusarium sambucinum, and black root rot caused by Exserohilum pedicellatum, was proven in this study. Although, all of the tested cultivars significantly showed different reaction to these fungi, but the cultivars Champa of subtropical region and Tarom were resistant to foot rot as well as the black root rot and the cultivar Gerdeh was resistant to brown spot disease.

Methodologies for selecting cassava with resistance to dry and black root rot under controlled conditions

ABSTRACT Cassava root rot diseases such as dry and black root rot are listed among the major threats, since its affect the main product (tuberous roots), causing a progressive decline in yield and affecting subsequent crop cycles, being the use of resistant varieties the most reliable control measure on field level. The objective of this study was to identify inoculation methods for the early evaluation of genotypes, considering the level of resistance to dry (DRR) and black (BRR) root rot diseases. Different methodologies and plant tissues were evaluated, based on the immersion of cassava tissues (roots and stem cuttings), soil infestation, and inoculation of detached tissues (leaves, tuberous roots and stem cuttings). The following parameters were evaluated for inoculations based on tissue immersion: disease index (ω); aerial part weight (g); fresh weight of the roots (g); and volume (cm3). For the inoculations on detached tissues, the percentage of lesioned area was determined. Immersion methods for roots and lesioned stems did not show typical symptoms of DRR and BRR during the two-month evaluation period. The soil infestation method did not differ from the stem immersion method as to the reduction of aerial part weight and the disease index, whereas both can be recommended for resistance selection trials. There was a positive correlation between the BRR and DRR data for the stem inoculation (r = 0.94, p = 0.001) and for DRR in the peel and root pulp (r = 0.73, p = 0.05). Therefore, the resistance within each tissue is apparently independent and should be compared with the behavior of the genotypes in the field.

Rhizoctonia fragariae causes black root rot on strawberry seedlings in Turkey

Samples of strawberry transplants were randomly collected from 47 growers in Aydin province of Turkey. A total of 10 Rhizoctonia isolates were identified by morphological characterization, analysis of rDNA-ITS sequences. Pathogenicity tests were carried on strawberry seedlings and detached stolon. Ten pathogenic binucleate Rhizoctonia isolates were defined as AG-A, AG-G and AG-K on the basis of ITS region sequence analysis. To our knowledge, this is the first report of the presence of Rhizoctonia fragariae in strawberry transplants in Turkey.

Modelling growth characteristics and aggressiveness of Neoscytalidium hyalinum and Fusarium solani associated with black and dry root rot diseases on cassava

In breeding for resistance, knowledge of the optimum conditions for both pathogen and disease development are essential. The aim of this study was to evaluate different conditions for mycelial growth, conidial production, and the severity of Fusarium spp. and Neoscytalidium hyalinum, which are the causal agents, respectively, of dry root rot (DRR) and black root rot (BRR) on cassava. Mycelia and conidia production were evaluated at five temperatures (10, 15, 25, 35 and 40 °C) and two culture media (PDA and SNA). The influence of temperature on the severity of the disease was also evaluated. For the DRR pathogen, the optimum temperature for mycelial growth was between 24 and 29 °C, while for sporulation it was 28 °C. For the BRR pathogen, the optimum range of temperature for mycelia growth and sporulation was between 30 and 36 °C. The PDA medium favoured mycelial growth and sporulation for both DRR and BRR pathogens. Highest BRR severity was observed at 32 to 36 °C and ranged from 25 to 32 °C for DRR. This is the first study to define optimal growth conditions for the DRR and BRR pathogens of cassava.

Development of user-friendly markers for disease resistance to black root rot of tobacco through genotyping by sequencing

Black root rot (BRR), a disease caused by the hemibiotrophic fungus Thielaviopsis basicola, seriously compromises yield and leaf quality in tobacco (Nicotiana tabacum). Full resistance to black root rot, conferred by the resistance to BRR 1 (RBRR1) locus from Nicotiana debneyi Domin, was transferred to a burley tobacco cultivar through interspecific hybridization. Some undesirable traits potentially caused by linkage drag restrict wider application of RBRR1 in flue-cured tobacco. Therefore, user-friendly molecular markers are needed to assist selection for resistance to black root rot and to break the unfavorable linkage. Genotyping by sequencing (GBS) is a rapid and robust approach for reduced representation sequencing of multiplexed genomic DNA samples that combines genome-wide molecular marker discovery with genotyping. In the present study, we used GBS to identify single-nucleotide polymorphisms (SNPs) linked to the RBRR1 locus, and PCR-based assays for detection of these SNPs were also developed. Sequence analysis of the SNP markers suggested that RBRR1 is located on chromosome 17, providing a basis for map-based cloning of this valuable gene. Co-dominant CAPS markers that co-segregate with the disease-resistant phenotype offer user-friendly tools for tobacco breeding and variety improvement. Furthermore, tested with diverse N. tabacum germplasm, SS192650 displayed 100% selection accuracy for resistance to BRR, suggesting that this marker can be used in diverse tobacco populations.

Phomopsis Black Root Rot of Cucumber in Washington State Caused by Diaporthe sclerotioides

Phomopsis black root rot of cucumber (Cucumis sativus L.), caused by Diaporthe sclerotioides (syn. Phomopsis sclerotioides), has been reported in Austria, Britain, Canada, Denmark, France, Germany, Italy, Japan, Malaysia, the Netherlands, and Norway (Shishido et al. 2016). In August 2017, wilted plants were observed in patches comprising 10% of a 5-ha organic cucumber crop in Skagit Co., WA, after fruit had formed. Crowns, tap roots, and secondary roots had pink to brown, dry, corky lesions, black pseudostromata, and rectangular pseudosclerotia within cortical cells. Sections of symptomatic roots and crowns (≤10 mm long) of eight plants selected randomly were surface-sterilized in 0.6% NaOCl for 3 to 5 min, triple-rinsed in sterilized water, dried on sterilized blotters, plated on water agar, and incubated at 24 ± 2°C with a natural day/night cycle. Hyphae were transferred to 1/2-strength PDA amended with chloramphenicol (100 ppm). After 2 weeks, each isolate covered the plate with gray-brown, dense mycelium. Flat, brown-black, irregular pseudostromata formed in the medium. Five isolates were grown in 1/2-strength potato dextrose broth on a shaker for 7 days, mycelia harvested, DNA extracted, and the internal transcribed spacer (ITS) 1-5.8S-ITS2 region of ribosomal DNA amplified using universal eukaryotic primers UN-UP18S42 and UN-LO28S576B (Schroeder et al. 2006). The sequences of isolates Dsl004, Dsl005, Dsl006, and Dsl008 (GenBank accession nos. MG434673.1, MG434674.1, MG434675.1, and MG434676.1, respectively) had 100% identity to that of a D. sclerotioides isolate (NR_111069.1); the sequence of Dsl002 (MG434672.1) had 99% identity because of a 7-nt insertion in the ITS1 region. All five isolates and DNA of D. sclerotioides ATCC18585 (ex-type culture) produced the expected 392-bp band of amplified DNA from the ITS rDNA using the highly specific primers CPs-1 and CPs-2 (Shishido et al. 2010). In addition, a 566-nt consensus sequence of the TEF 1-α gene of Dsl002, Dsl004 to Dsl006, and Dsl008 (MG868937) generated by PCR assay with primers EF-2F and EF-5AR (Taskin et al. 2010) was 100% identical to that of D. sclerotioides ATCC18585 (MG868936). Pathogenicity of the five isolates on cucumber was verified in a greenhouse using two methods, direct-seeding and transplanting. Two-week-old colonized PDA plates of each isolate were cut into 3 mm³ pieces and mixed in Sunshine Potting Mix #1 (Sungro Horticulture, Canada) (half a plate/1.0 liter potting mix/15-cm-diameter pot). A nontreated seed (cv. Turbo) was then planted in the medium in each of five replicate pots/isolate, and a 10-day-old Turbo seedling was transplanted into each of five replicate pots/isolate. For the control treatment, potting medium was amended with noncolonized PDA (five direct-seeded and five transplanted replicates). Plants were maintained at 18 to 26°C with 10 h light/day. Dsl004, Dsl005, Dsl006, and Dsl008 caused wilting and death of plants in 3 weeks with the direct-seeding method, and in 4 to 5 weeks for the transplanting method. Dsl002 caused stunting and minor wilting. Control plants remained healthy. The roots of inoculated plants had corky lesions with pseudostromata and pseudosclerotia. D. sclerotioides was reisolated from roots of inoculated plants but not control plants. The species was confirmed morphologically and by ITS rDNA and TEF 1-α sequencing. This is the first report of Phomopsis black root rot of cucumber caused by D. sclerotioides in Washington State or anywhere in the U.S.A. The disease was reported in greenhouse cucumber in neighboring British Columbia, Canada, in 1972 (Ormrod and Christie 1972). Research is needed to assess prevalence of the pathogen and management strategies to limit spread of the disease.

Screening of Faba Bean Lines against Black Root Rot Caused by Fusarium solan

Screening of Faba Bean Lines against Black Root Rot Caused by Fusarium solan

Carbon Source and Soil Origin Shape Soil Microbiomes and Tomato Soilborne Pathogen Populations During Anaerobic Soil Disinfestation

Soilborne disease complexes are an emerging constraint in protected culture tomato production systems in the Midwestern United States. Diseases in these complexes include Verticillium wilt (Verticillium dahliae), black dot root rot (Colletotrichum coccodes), corky root rot (Pyrenochaeta lycopersici), and root knot (Meloidogyne spp.). Anaerobic soil disinfestation (ASD) may be a viable, environmentally benign strategy for managing these complexes. Soils from two farms in Ohio were used to determine the impacts of ASD, using wheat bran, molasses, or ethanol as carbon sources, on soilborne diseases and soil bacterial communities. ASD with wheat bran or ethanol amendments led to significantly reduced tomato root rot severity, while nematode galling damage was significantly reduced following ASD with any carbon source compared with nontreated controls. When ethanol was used as a carbon source in ASD, the colonization of tomato roots by P. lycopersici and C. coccodes was observed less frequently than in control roots. A high throughput sequencing approach was used to characterize soil bacterial communities following ASD. Carbon source and soil origin influenced the composition of bacterial communities in soils treated with ASD. Bacterial community diversity decreased following ASD with wheat bran in all soils tested and following ASD with ethanol in soils from one farm. The abundance of bacteria in the phylum Firmicutes generally increased significantly following ASD, while the abundance of those in the phyla Acidobacteria, Actinobacteria, Chloroflexi, and Plantomycetes generally decreased following ASD. These findings provide insight into the impacts of ASD on microbial communities and soilborne diseases and will be used to optimize ASD as a tool for Midwestern vegetable growers.

Damage reality and Measure test of Soybean Black Root Rot in Fukushima Prefecture

Damage reality and Measure test of Soybean Black Root Rot in Fukushima Prefecture

Molecular identification and characterization of binucleate Rhizoctonia spp. associated with black root rot of strawberry in Serbia

Strawberry production is a popular, fast-growing agricultural business in Serbia. Its cultivar selection has been changing fast, following market demands. One of the limiting factors of strawberry production worldwide is black root rot, primarily caused by binucleate Rhizoctonia. Recently, outbreaks of black root rot of strawberry have occurred in Serbia and the estimated disease incidence was up to 30%. Isolates of binucleate Rhizoctonia AG-A were recovered from symptomatic strawberry plants, and characterized on the bases of morphological, molecular and pathogenic features. Despite their uniform morphological characteristics, the isolates demonstrated genetic variability within ITS rDNA, grouping into three different phylogenetic sub-clusters which comprise AG-A isolates originating from Italy, Israel, Japan and the USA. The binucleate Rhizoctonia AG-A from Serbia exhibited uniform virulence on strawberry after inoculation of daughter plants and detached leaf petioles, as well as on seedlings of bean, carrot and sunflower, while they were non-pathogenic to wheat, maize, tomato, pepper, tobacco, cucumber, lettuce, peas, cabbage, rapeseed and sugar beet.

A new genus and species for the globally important, multihost root pathogen Thielaviopsis basicola

The plant pathogenic asexual fungus Thielaviopsis basicola (Ascomycota) causes black root rot on many important agricultural and ornamental plant species. Since its first description in 1850, this species has had a tumultuous taxonomic history, being classified in many different genera. Thus far, DNA‐based techniques have not played a significant role in identification of T. basicola and have been used only to confirm its placement in the Microascales. This investigation reconsidered the phylogenetic placement of T. basicola, using DNA sequence data for six different gene regions. It included 41 isolates identified as T. basicola from 13 geographical locations worldwide. Phylogenetic analyses showed that these isolates grouped in a well‐supported lineage distinct from other genera in the Ceratocystidaceae, here described as Berkeleyomyces gen. nov. The data also provided robust evidence that isolates of T. basicola include a cryptic sister species. As a result, this report provides a new combination as B. basicola comb. nov. and introduces a new species as B. rouxiae sp. nov.

First Report of Root Rot Caused by Calonectria ilicicola on Blueberry in Yunnan Province, China

Blueberry (Vaccinium spp.) has been cultivated in more than 20 provinces in China as reported in 2015. In July 2016, a root rot disease of blueberry causing plant death was found in Qujing (24°49′N; 103°59′E), Yunnan Province. Approximately 5% of plants in the field showed symptoms characterized by leaf chlorosis, black rot on roots and stems, and finally the whole plant wilted in about 20 days. In order to identify the causal agent of this disease, typical disease samples were collected. Twenty infected root tissues were sterilized and put on potato dextrose agar (PDA) at 25°C (Fang 1998) and nine fungal isolates were obtained. The isolate YNQJ01Aa was chosen for further testing. Its colony on PDA was white to pale yellow. The conidia were collected from SNA colony after growing for 10 days. Conidia presented with one to three septa (mostly one septa). Fifty conidia from one isolate measured 59.37 to 71.33 × 5.93 to 7.91 μm (avg. 62.25 × 6.22 µm). Ascospores were produced in perithecia on PDA and were 30.16 to 38.89 × 5.78 to 7.76 µm (avg. 34.97 × 6.52 µm). By DNA sequencing of the internal transcribed spacer (ITS) rDNA region (GenBank accession no. MF785081), the identity of the isolate (YNQJ01Aa) was confirmed to be the genus Calonectria (99% homologous to several Calonectria spp. strains). The partial β-tubulin (TUB) gene sequence was also analyzed (MF785082) and the result of a BLAST search showed that it was 100% homologous to C. ilicicola strain (CBS 190.50). Further species identification was analyzed by phylogenic tree based on TUB (partial β-tubulin gene, T1/Bt2b), tef1 (translation elongation factor 1 alpha gene, EF1-728F/EF1-986R), and HIS3 (histone 3 gene, H3-1a/H3-1b) (Chen et al. 2011; Crous 2002). Sequences obtained were deposited in GenBank (TUB: MF785082, tef1: MF785083, HIS3: MF785084). Alignments were made using CLUSTAL X v.2.0, and a maximum likelihood phylogram was generated by MEGA v.7.0. Isolate YNQJ01Aa clustered together with C. ilicicola reference strain CBS190.50 (98% bootstrap). Three healthy 1-year-old ‘O’Neal’ blueberry plants were used to conduct pathogenicity tests by wound inoculation. Each plant was inoculated with 10 ml of conidial suspension (1 × 10⁵ conidia/ml) by the root irrigation method. Three plants were inoculated with the same amount of sterile water as control. All blueberry plants were incubated at 25°C. After 20 days, the test plants showed typical symptom of black root rot and finally the whole plant wilted and died, while the control plants remained healthy. The same colonial fungus was reisolated. This is the first report of C. ilicicola (anamorph Cylindrocladium parasiticum) infecting blueberry in China. Root rot disease can be difficult to detect early and therefore it is important to identify the etiology to develop appropriate disease management strategies. This fungus was previously reported on Medicago sativa in China (Pei et al. 2015).

Post-plant strategies for management of black root rot-related decline of perennial strawberry fields

Perennial strawberries affected with black root rot generally decline in runner production, yield, and overall plant vigor over time. Several plant pathogens have been implicated in black root rot, including plant-parasitic nematodes, Pythium and Rhizoctonia spp. Pre-plant treatments such as a 3- to 5-year crop rotation or chemical fumigation are the main means of disease management. During 2007 and 2008, five post-plant treatments were evaluated for mitigation of strawberry black root rot in a naturally declining site in Ottawa County in Michigan. Fungicides and nutritional amendments were applied as drenches or foliar sprays. Most treatments increased runner establishment, yield, and overall plant vigor, particularly azoxystrobin and azoxystrobin + potassium phosphite. Root necrosis and incidence of root infection by binucleate Rhizoctonia spp. were reduced. Results were more apparent and significant in 2008 than in 2007, indicating cumulative effects over the 2 years of the trial. During 2008 and 2009, a large-scale demonstration trial was conducted with these treatments in a strawberry field with a history of black root rot in Leelanau County, MI. Improvements in bed fill and average plant weight and reductions in root necrosis and Rhizoctonia isolations were noted. This research represents the first post-plant chemical management strategy capable of slowing or even reversing the decline due to black root rot in established strawberry fields.

Genome-wide association study for resistance to cassava root rot

SUMMARYCassava root rot (CRR) disease associated with a complex of soil pathogens has caused great yield losses in the crop. The objective of the current work was to obtain insights about the genetic architecture of CRR resistance caused byFusarium(dry root rot – DRR),Phytophthora(soft root rot – SRR) and Botryosphaeriaceae (black root rot – BRR) species, using genome-wide association studies (GWAS). Phenotyping data of 263 accessions (artificial inoculation) and 14 094 single-nucleotide polymorphisms (SNP) (missing data <0·10 and minor allele frequency >0·05) were used. The severity of CRR in peel and pulp was variable among accessions, but the pathogens that caused DRR were more aggressive. Broad-sense heritability ($h_g^2 $) was of medium magnitude for all groups of resistances for pathogens, with variation from 0·16 ± 0·019 (Fspp Pulp) to 0·31 ± 0·028 (Fspp Peel). The kinship matrix was used to correct for stratification as well as for clustering the accessions. Overall, this analysis showed that there was no relationship between agronomic traits and resistance to CRR and the four clusters obtained from kinship matrix. The GWAS identified 38 significant SNPs, of which eight and 22 are related to the severity of DRR in the pulp and peel, respectively. The other eight SNPs were associated with SRR-peel (1), SRR-pulp (1), BRR-peel (3) and BRR-pulp (3). Half of the SNPs associated with CRR resistance have functional annotations related to defence and response to pathogen attack as well as general cellular processes. The current study revealed that resistance to CRR is controlled by multiple loci with small effects, and significant SNPs can be used to identify putative genes that control these traits.

Effects of Silicon Amendment on Soilborne and Fruit Diseases of Avocado.

The effects of silicon (Si) amendment have been studied in several plant/pathogen interactions; however, studies in horticultural tree crops are limited. Effects of amendment with soluble potassium silicate (AgSil®32, approximately 30% available Si), or milled cement building board by-products (Mineral Mulch (MM) or Mineral Dust (MD), containing 5% available Si) were investigated in field and greenhouse trials with avocado. Orchard soil drench applications with potassium silicate improved yield and quality of fruit, but visual health of trees declining from Phytophthora root rot (PRR) was not affected. Orchard spray or trunk injection applications with potassium silicate were ineffective. Amendment of potting mix with MM and MD reduced root necrosis of avocado seedlings after inoculation with Calonectria ilicicola, an aggressive soilborne pathogen causing black root rot. Application of MM to mature orchard trees declining with PRR had a beneficial effect on visual tree health, and Si accumulation in leaves and fruit peel, after only 10 months. Products that deliver available Si consistently for uptake are likely to be most successful in perennial tree crops.

Pathogenicity of Nectriaceous Fungi on Avocado in Australia.

Black root rot is a severe disease of young avocado trees in Australia causing black necrotic roots, tree stunting, and leaf drop prior to tree death. Nectriaceous fungi (Nectriaceae, Hypocreales), are commonly isolated from symptomatic roots. This research tested the pathogenicity of 19 isolates from Calonectria, Cylindrocladiella, Dactylonectria, Gliocladiopsis, and Ilyonectria, spp. collected from young avocado trees and other hosts. Glasshouse pathogenicity tests with 'Reed' avocado (Persea americana) seedlings confirmed that Calonectria ilicicola is a severe pathogen of avocado, causing stunting, wilting, and seedling death within 5 weeks of inoculation. Isolates of C. ilicicola from peanut, papaya, and custard apple were also shown to be aggressive pathogens of avocado, demonstrating a broad host range. An isolate of a Calonectria sp. from blueberry and avocado isolates of Dactylonectria macrodidyma, D. novozelandica, D. pauciseptata, and D. anthuriicola caused significant root rot but not stunting within 5 to 9 weeks of inoculation. An isolate of an Ilyonectria sp. from grapevine closely related to Ilyonectria liriodendri, and avocado isolates of Cylindrocladiella pseudoinfestans, Gliocladiopsis peggii, and an Ilyonectria sp. were not pathogenic to avocado.

Dactylonectria torresensis as the Main Component of the Black Root Rot Complex of Strawberries and Raspberries in Northern Germany

In a long-term survey of black root rot of strawberries and raspberries in Northern Germany in 2007–2014, fungi with and without Cylindrocarpon-like anamorphs were isolated as potential pathogens. Dactylonectria torresensis was the most common species, being isolated from 18% of strawberry roots obtained from nursery plants and 37% of roots from production fields, as well as 21% and 29% (respectively) of raspberry roots. Less frequently isolated fungi with Cylindrocarpon-like anamorphs included Ilyonectria crassa, Ilyonectria sp. 2, I. pseudodestructans, I. robusta, C. obtusisporium and Ilyonectria sp. 1. Severe disease symptoms were reproduced by artificial inoculation of strawberries with D. torresensis, I. crassa and Ilyonectria sp. 2, milder symptoms with C. obtusisporium. A wide range of other root-pathogenic fungi such as Fusarium oxysporum, Verticillium dahliae, Ceratobasidium fragariae, Gnomoniopsis fructicola, Hainesia lythri, and species of Cadophora, Leptodontidium, Pythium, Phytophthora, Plectosporella, Pestalotiopsis and Truncatella were either isolated only sporadically or were not associated with black root rot symptoms, suggesting that they did not play any major role in this disease in Northern Germany. Visible disease symptoms and high frequencies of D. torresensis isolations in many batches of nursery plants indicated that these may comprise a major source of contamination of production fields. The previously unrecognised prominence of D. torresensis resolves a long-standing puzzle concerning the cause of the ongoing black root rot epidemic in Northern German strawberry and raspberry production.