Etiology and alternative control of potato rhizoctoniasis in South Africa

Rhizoctoniasis of potato occurs in all regions of the world where the crop is grown. The disease is caused by various anastomosis groups (AGs) of the fungus Rhizoctonia solani, of which AG-3 is the most common. Very little information is available on the AGs involved in potato rhizoctoniasis in South Africa. This study elucidated the etiology of the disease in the country and also provided guidelines regarding alternative control strategies. The vast majority (99.3 %) of R. solani isolates from symptomatic potato tubers collected at seven of the 14 potato production regions in South Africa belonged to AG-3, and 0.7 % to AG-5. Of the isolates from infected stems and roots, 82.1 % were AG-3, 12.8 % AG-4, and 5.1 % AG-5. Isolations from soil yielded 67.7 % AG-3, 22.0 % AG-4, 5.5 % AG-5, and 2.4 % of each of AG-7 and AG-8. In vitro screening indicated that AG-3 isolates were the most virulent. The progeny of seed tubers naturally infected with R. solani was rendered free of infection by dipping the tubers in hot water at 55 C for 8 minutes, 60 ºC for 6 minutes, or 65 ºC for 4 minutes. However, treatments more severe than 55 ºC for 8 minutes progressively increased tuber mortality. OA5 DP, an organic tin complex, proved to be the most effective of 20 disinfectants for killing sclerotia of the pathogen on seed tubers and rendering the progeny free of infection, but exhibited acute phytotoxicity towards the tubers. Significant control without any phytotoxicity was achieved with the didecyl ammonium chloride compound, Sporekill. Tolclofos-methyl was the only fungicide that provided total control of potato rhizoctoniasis, whereas seed tuber treatment with fludioxonil, kresoxim-methyl and metam-sodium significantly reduced disease in the progeny. Incorporation of of the biocontrol formulation TrykocideTM (Trichoderma harzianum) into soils artificially infested with R. solani AG-3 eradicated the pathogen. Significant reductions in pathogen populations were also evident in soils amended with azoxystrobin, kresoxim-methyl, MaxifloTM Azospirillum brasilense), AvogreenTM (Bacillus subtilis), cattle, chicken and sheep manure, citrus and mango waste compost, composted kraal manure, and shoot tissues of Brassica napus, B. oleracea var. capitata, Raphanus sativus, Sinapsis alba and Tagetes minuta. TrykocideTM provided total control of stem canker in artificially infested soil, whereas kresoxim-methyl, azoxystrobin, sheep manure, B. napus and B. oleracea var. capitata shoot tissue, mango waste compost, and the systemic resistance-inducing compound, acibenzolar-s-methyl, reduced the disease significantly.Dissertation (MSc (Plant Pathology))--University of Pretoria, 2006.Microbiology and Plant Pathologyunrestricte

Epidemiology of citrus black spot disease in South Africa and its impact on phytosanitary trade restrictions

Citrus black spot (CBS), caused by Guignardia citricarpa Kiely, occurs in various citrus producing regions of the world. Due to the potential phytosanitary risk associated with the export of fruit from CBS positive production areas to CBS-free countries, restrictive trade barriers have been introduced. This study aimed to further elucidate some epidemiological aspects of CBS that can be used to address critical questions identified in the pest risk assessment submitted by South Africa to the World Trade Organisation to address phytosanitary trade restrictions. Results indicated that Eureka lemon leaf litter exposed to viable pycnidiospores under controlled conditions or in the field in different production regions of South Africa, were not infected and colonised by G. citricarpa. Symptomatic CBS fruit or peel lying on the ground underneath citrus trees therefore can not lead to infection and colonisation of freshly detached leaves or leaf litter, or represent a source of inoculum in citrus orchards. Symptomatic fruit therefore pose no danger for the establishment of the pathogen in CBS-free orchards and are not considered to be a pathway for the pathogen. The period of leaf susceptibility to G. citricarpa was indicated to be maximum eight and ten months from development, for Valencia orange and Eureka lemon, respectively, in a greenhouse study. The susceptibility period of citrus leaves to infection by the black spot pathogen could be longer than previously perceived. Ascospores were captured, using the newly developed Kotzé Inoculum Monitor (KIM), from natural Valencia orange and Eureka lemon leaf litter during October to March with peak ascospore availability between December to February. The KIM is the first sampler designed to capture fungal spores directly from plant material in the laboratory without environmental influences and was effectively used to confirm that ascospores production is seasonal. The KIM in combination with environmental data can be used to improve control through more targeted fungicide applications. Techniques such as isolations and DNA amplification with species-specific primers to detect the pathogen directly from symptomless green leaves have a low success rate due to the restricted growth of the pathogen in latently infected tissue. Artificial leaf wilting enhanced the detection of G. citricarpa from symptomless leaves. Leaf wilting is a reliable, fast and effective method to detect the CBS pathogen and can be applied to monitor citrus nurseries and orchards throughout the year. It can also be applied to monitor pest-free orchards to maintain its CBS pest-free status. This study confirmed that sanitation practices, such as leaf litter removal and mulching of leaf litter with wheat straw can decrease the primary inoculum, ascospores, of CBS and contribute to better management of the disease in a commercial orchard. Regardless of the prevailing climatic conditions each year, control achieved through leaf litter management resulted in >95% clean fruit and are equal to the control achieved with industry standard fungicides. This approach provided improved integrated disease control and an alternative to chemical control.Thesis (PhD)--University of Pretoria, 2010.Microbiology and Plant Pathologyunrestricte

Fusarium species isolated from Pennisetum clandestinum collected during outbreaks of kikuyu poisoning in cattle in South Africa

Kikuyu poisoning occurs sporadically in South Africa. It is of major economic importance, as valuable dairy cows are often poisoned by it, and once affected, the mortality rate is high. Pennisetum clandestinum samples were collected during eight outbreaks of kikuyu poisoning in cattle in the Eastern Cape Province of South Africa from 2008 to 2010. The kikuyu grass samples were submitted specifically for the isolation and molecular identification of Fusarium species, as it was recently suggested that mycotoxins synthesised by Fusarium torulosum could be the cause of this intoxication. Ninety-four Fusarium isolates were retrieved from the grass samples, of which 72 were members of the Fusarium incarnatum/Fusarium equiseti species complex based on morphology and phylogenetic analyses of the translation elongation factor 1α sequence data. The South African isolates from kikuyu identified as members of the F. incarnatum/F. equiseti species complex grouped together in six separate clades. The other isolates were Fusarium culmorum (n = 3), Fusarium redolens (n = 4) and Fusarium oxysporum (n = 15). Although F. torulosum could not be isolated from P. clandestinum collected during kikuyu poisoning outbreaks in South Africa, the mycotoxicosis theory is still highly plausible.National Research Foundation (NRF)http://www.ojvr.orgtm201

Multitoxin analysis of <i>Aspergillus clavatus</i>-infected feed samples implicated in two outbreaks of neuromycotoxicosis in cattle in South Africa

Aspergillus clavatus intoxication is a highly fatal neuromycotoxicosis of ruminants, especially cattle. It is caused by the ingestion of infected sprouting grain and sorghum beer residue. Locomotor disturbances, tremors and paralysis are observed. Histologically, degeneration and necrosis of larger neurons in the medulla oblongata, the midbrain, the thalamus and the ventral horns of the spinal cord are observed. Although a range of mycotoxins such as patulin, cytochalasin E and pseurotin A have been isolated, there is limited information on which specific mycotoxin or group of mycotoxins are involved during outbreaks of intoxication in livestock. In the present study, two outbreaks of A. clavatus poisoning in cattle are briefly described. Feed samples were collected for fungal identification, and culture and multitoxin analysis. A range of fungal metabolites were detected, and the estimated concentrations (μg/kg) are provided. Both the sprouting barley and brewer’s grain were predominantly infected with A. clavatus and, to a lesser extent, Rhizopus arrhizus. The only common Aspergillus secondary metabolite present in all the samples was pseurotin A. Patulin and cytochalasin E were present in the sprouting barley samples, as well as the A. clavatus isolates cultured on malt extract agar for 2 weeks; however, neither of these mycotoxins could be detected in the brewer’s grain sample

Population genetic structure of Rhizoctonia solani AG 3-PT from potatoes in South Africa

Rhizoctonia solani AG 3-PT is an important potato pathogen causing significant yield and quality losses in potato production globally. However, little is known about the levels of genetic diversity and population structure of this pathogen in South Africa. A total of 114 R. solani AG 3-PT isolates collected from four geographic regions were analyzed for genetic diversity and structure using eight microsatellite loci. Microsatellite analysis found high intrapopulation genetic diversity, population differentiation and evidence of recombination. A total of 78 multilocus genotypes (MLGs) were identified with few MLGs shared among populations. Low levels of clonality (13-39 %) and high levels of population differentiation were observed among populations. Most of the loci were in Hardy-Weinberg equilibrium and all four field populations showed evidence of a mixed reproductive mode of both clonality and recombination. The PCoA clustering method revealed genetically distinct geographic populations of R. solani AG 3-PT in South Africa. This study showed that populations of R. solani AG 3-PT in South Africa are genetically differentiated and disease management strategies should therefore be applied accordingly. This is the first study of the population genetics of R. solani AG 3-PT in potatoes in South Africa and results may help to develop knowledge-based disease management strategies in South Africa and elsewhere.Potatoes South Africa and the Potato Pathology Programme at UP as well as the National Research Foundation.http://www.elsevier.com/locate/funbio2017-05-31hb2016Plant Scienc

Elephant hide and growth cracking on potato tubers caused by Rhizoctonia solani AG3-PT in South Africa

No Abstracthttp://apsjournals.apsnet.org/loi/pdishb201

First report of Rhizoctonia solani AG 4HG-III causing potato stem canker in South Africa

No abstract available.http://apsjournals.apsnet.org/loi/pdishb201

Multitoxin analysis of Aspergillus clavatus-infected feed samples implicated in two outbreaks of neuromycotoxicosis in cattle in South Africa

Aspergillus clavatus intoxication is a highly fatal neuromycotoxicosis of ruminants, especially cattle. It is caused by the ingestion of infected sprouting grain and sorghum beer residue. Locomotor disturbances, tremors and paralysis are observed. Histologically, degeneration and necrosis of larger neurons in the medulla oblongata, the midbrain, the thalamus and the ventral horns of the spinal cord are observed. Although a range of mycotoxins such as patulin, cytochalasin E and pseurotin A have been isolated, there is limited information on which specific mycotoxin or group of mycotoxins are involved during outbreaks of intoxication in livestock. In the present study, two outbreaks of A. clavatus poisoning in cattle are briefly described. Feed samples were collected for fungal identification, and culture and multitoxin analysis. A range of fungal metabolites were detected, and the estimated concentrations (μg/kg) are provided. Both the sprouting barley and brewer’s grain were predominantly infected with A. clavatus and, to a lesser extent, Rhizopus arrhizus. The only common Aspergillus secondary metabolite present in all the samples was pseurotin A. Patulin and cytochalasin E were present in the sprouting barley samples, as well as the A. clavatus isolates cultured on malt extract agar for 2 weeks; however, neither of these mycotoxins could be detected in the brewer’s grain sample.National Research Foundation (NRF)http://www.ojvr.orgtm201

<i>Fusarium</i> species isolated from <i>Pennisetum clandestinum</i> collected during outbreaks of kikuyu poisoning in cattle in South Africa

Kikuyu poisoning occurs sporadically in South Africa. It is of major economic importance, as valuable dairy cows are often poisoned by it, and once affected, the mortality rate is high. Pennisetum clandestinum samples were collected during eight outbreaks of kikuyu poisoning in cattle in the Eastern Cape Province of South Africa from 2008 to 2010. The kikuyu grass samples were submitted specifically for the isolation and molecular identification of Fusarium species, as it was recently suggested that mycotoxins synthesised by Fusarium torulosum could be the cause of this intoxication. Ninety-four Fusarium isolates were retrieved from the grass samples, of which 72 were members of the Fusarium incarnatum/Fusarium equiseti species complex based on morphology and phylogenetic analyses of the translation elongation factor 1α sequence data. The South African isolates from kikuyu identified as members of the F. incarnatum/F. equiseti species complex grouped together in six separate clades. The other isolates were Fusarium culmorum (n = 3), Fusarium redolens (n = 4) and Fusarium oxysporum (n = 15). Although F. torulosum could not be isolated from P. clandestinum collected during kikuyu poisoning outbreaks in South Africa, the mycotoxicosis theory is still highly plausible

Putative Aspergillus niger-induced oxalate nephrosis in sheep

A sheep farmer provided a maize-based brewer's grain (mieliemaroek) and bales of Eragrostis curvula hay to ewes and their lambs, kept on zero-grazing in pens. The 'mieliemaroek' was visibly mouldy. After 14 days in the feedlot, clinical signs, including generalised weakness, ataxia of the hind limbs, tremors and recumbency, were noticed. Six ewes died within a period of 7 days. A post mortem examination was performed on 1 ewe. The carcass appeared to be cachectic with mild effusions into the body cavities; mild lung congestion and pallor of the kidneys were observed. Microscopical evaluation revealed nephrosis and birefringent oxalate crystals in the renal tubules when viewed under polarised light. A provisional diagnosis of oxalate nephrosis with subsequent kidney failure was made. Amongst other fungi, Aspergillus niger was isolated from 'mieliemaroek' samples submitted for fungal culture and identification. As A. niger is known to synthesise oxalates, a qualitative screen to detect oxalic acid in the mieliemaroek and purified A. niger isolates was performed using high-performance liquid chromatography (HPLC). Oxalic acid was detected, which supported a diagnosis of soluble oxalate-induced nephropathy