Browsing by Author "Moyo, Providence"

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    The effects of postharvest treatments and sunlight exposure on the reproductive capability and viability of Phyllosticta citricarpa in citrus black spot fruit lesions
    (MDPI, 2020-12-21) Moyo, Providence; Fourie, Paul H.; Masikane, Siyethemba L.; Fialho, Regis de Oliveira; Mamba, Lindokuhle C.; Du Plooy, Wilma; Hattingh, Vaughan
    Citrus black spot (CBS) is caused by Phyllosticta citricarpa, which is classified as a quarantine organism in certain countries whose concerns are that CBS-infected fruit may be a pathway for introduction of the pathogen. This study evaluated the reproductive capability and viability of P. citricarpa under simulated conditions in which the whole fruit, peel segments, or citrus pulp with CBS lesions were discarded. Naturally infected ‘Midknight’ Valencia orange and ‘Eureka’ lemon fruit, either treated using standard postharvest sanitation, fungicide, and wax coating treatments or untreated, were placed into cold storage for 5 weeks (oranges at 4 ◦C and lemons at 7 ◦C). Thereafter, treated and untreated fruit were incubated for a further 2 weeks at conditions conducive for CBS symptom expression and formation of pycnidia. The ability of pycnidia to secrete viable pycnidiospores after whole fruit and peel segments or peel pieces from citrus pulp were exposed to sunlight at warm temperatures (±28 ◦C) and ±75% relative humidity levels was then investigated. The combination of postharvest treatments and cold storage effectively controlled CBS latent infections (>83.6% control) and pycnidium formation (<1.4% of lesions formed pycnidia), and the wax coating completely inhibited pycnidiospore release in fruit and peel segments. Pycnidiospores were secreted only from lesions on untreated fruit and peel segments and at low levels (4.3–8.6%) from peel pieces from pulped treated fruit. However, spore release rapidly declined when exposed to sunlight conditions (1.4% and 0% after 2 and 3 days, respectively). The generally poor reproductive ability and viability of CBS fruit lesions on harvested fruit, particularly when exposed to sunlight conditions, supports the conclusion that citrus fruit without leaves is not an epidemiologically significant pathway for the entry, establishment, and spread of P. citricarpa.
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    Identification and characterisation of diatrypaceae species associeated with declining grapevines and alternative hosts in South Africa
    (Stellenbosch : Stellenbosch University, 2017-03) Moyo, Providence; Halleen, Francois; Lizel Mostert; Stellenbosch University. Faculty of AgriSciences. Dept. of Plant Pathology.
    ENGLISH ABSTRACT: Grapevine trunk diseases have devastating impacts on the sustainability of viticulture, worldwide. Eutypa dieback, in particular, has caused large economic losses and premature mortality of vines. This disease has, for many years, been associated with the Diatrypaceae fungus, Eutypa (E.) lata. Several species of Diatrypaceae were, however, recently discovered to be associated with Eutypa dieback-affected grapevines in different grape growing areas including Australia, Chile, Spain and United States of America. No extensive study has been conducted to identify and characterise the species of Diatrypaceae in South Africa. Surveys were conducted in vineyards located in different grape growing regions of the Western Cape and Diatrypaceae fungi were isolated from grapevines with dying spurs or wood with wedge-shaped necrosis in cross section, as well as from perithecia on dead wood. Isolates were studied using phylogenetic analyses of combined DNA sequences of the internal transcribed spacer regions (ITS1 and ITS2) and 5.8S rRNA gene as well as partial β-tubulin gene. Morphological characteristics of perithecia were also studied. Morphological and phylogenetic analyses revealed the presence of seven Diatrypaceae species to occur on grapevine in South Africa, namely Cryptovalsa (C.) ampelina, C. rabenhorstii, E. consobrina, E. lata, Eutypella (Eu.) citricola, Eu. microtheca and E. cremea, which was described as a new species. The most common species isolated from dying spurs, in order of abundance, were C. ampelina (46.4% of total number of isolates), Eu. citricola (26.8%), E. lata (20.1%), E. cremea (4.3%), Eu. microtheca (1.2%), E. consobrina (0.6%) and C. rabenhorstii (0.6%). On the other hand, from wedge-shaped necrosis, E. lata represented the most frequent species (89.2% of all isolates obtained) followed by Eu. citricola (8.5%), E. cremea (1.4%) and C. ampelina (0.9%). Five species namely, E. lata, C. ampelina, E. cremea, Eu. citricola and Eu. microtheca were found to produce perithecia on dead grapevine wood. These results suggest that Eutypa dieback in South Africa can be associated with several Diatrypaceae species. Different fruit and ornamental trees occurring near vineyards were investigated to determine whether they are colonised by Diatrypaceae species, which are associated with Eutypa dieback of grapevine. Isolates of Diatrypaceae were collected from these trees showing symptoms of dieback, cankers and perithecia. Isolates were analysed by morphological and phylogenetic analyses as described above. Fourteen species namely, C. ampelina, E. consobrina, E. lata, Eu. citricola, Eu. microtheca, E. cremea, Cryptosphaeria (Cr.) multicontinentalis, Cr. ligniota, Diatrypella sp., Eu. leprosa, Eu. australiensis and three undescribed Eutypella species were identified from 29 different fruit and ornamental trees, occurring in close proximity to vineyards. The five most prevalent species were E. lata, C. ampelina, E. cremea, Eu. citricola and Eu. microtheca, which were also the most prevalent on grapevine. These findings suggest that cross infections are possibly occurring between grapevine and other woody hosts growing near vineyards in South Africa. These five species were also the only Diatrypaceae species isolated from stone fruit trees. Pathogenicity of these five Diatrypaceae species on stone fruit trees (apricot and plum) was also determined. In these pathogenicity studies, all five species were pathogenic on both apricot and plum, producing brown-red discolouration, typical of Eutypa dieback of apricot. Finally, pathogenicity of Diatrypaceae species identified from grapevine and other woody hosts in South Africa was evaluated on grapevine, under field conditions. Artificial inoculations of these fungal species were conducted on fresh pruning wounds and lignified shoots of Cabernet Sauvignon as well as green shoots of Cabernet Sauvignon and Sauvignon blanc. After 10 months, all the species caused disease symptoms (brown discolouration) on pruning wounds and lignified shoots of Cabernet Sauvignon. Disease symptoms were also observed on green shoots of both cultivars. Pathogenicity results revealed that several species including C. ampelina, Eu. microtheca, Eu. leprosa, and Eu. citricola were equally virulent as the well-known pathogen, E. lata. Quantitative real-time PCR (qPCR) assays were also developed for the detection and quantification of E. lata and C. ampelina in grapevine wood. The qPCR assays were specific and successfully quantified target taxa in artificially inoculated wood samples. The present study provides knowledge on the identity of Diatrypaceae species associated with declining grapevines and other woody hosts occurring adjacent to vineyards in South Africa. This knowledge, together with qPCR assays can be useful in early diagnosis of infection caused by Diatrypaceae species in vineyards. Furthermore, pathogenicity studies have shown that many Diatrypaceae species, including those obtained from other woody hosts, are pathogenic to grapevine. As such, this study forms the platform for further studies aimed at managing Diatrypaceae species causing disease on grapevine in South Africa.
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    The role of arthropods in the dispersal of trunk disease pathogens associated with Petri disease and Esca
    (Stellenbosch : Stellenbosch University, 2013-03) Moyo, Providence; Mostert, Lizel; Halleen, Francois; Stellenbosch University. Faculty of AgriSciences. Dept. of Plant Pathology.
    ENGLISH ABSTRACT: Petri disease and esca are devastating grapevine trunk diseases and compromise the sustainability of viticulture world-wide. Despite being extensively studied, knowledge of inoculum sources and mechanisms of spread of the causal pathogens is limited. Arthropods have been suspected to play a role in the spread of Petri disease and esca pathogens. However, little information is known about the extent to which arthropods are associated with these pathogens. This study aimed to determine whether arthropods occurring within or on declining grapevines, are associated with trunk disease pathogens and to identify arthropods associated with pruning wounds. The potential of selected arthropods to act as vectors of trunk disease pathogens was also investigated. Two vineyards exhibiting grapevine trunk disease infections were sampled weekly for two years for collection of arthropods. Arthropods were collected using pruning wound traps, visual searches as well as trunk and cordon traps. Fungal spores from surfaces of arthropods were collected in water. Samples were subjected to nested PCR using primers Pm1/Pm2 and Pch1/Pch2 to verify the presence of Phaeoacremonium spp. and Phaeomoniella chlamydospora, respectively. Water samples were also cultured and grapevine trunk disease pathogens obtained were identified by sequencing the internal transcribed spacers 1 and 2 and the 5.8S rRNA gene or the partial beta-tubulin gene. A total of 10 875 arthropod individuals, belonging to more than 31 families, were collected from declining grapevines. The most abundant arthropods included millipedes, ants, spiders and beetles. Portuguese millipedes and cocktail ants were associated with fresh grapevine pruning wounds. Thirty-three percent of the 5677 water samples analysed, contained propagules of pathogens associated with Petri disease and esca. Of these, 37 % were recovered from millipedes, 22 % from cocktail ants, 15 % from spiders and 10 % from beetles. All the major groups of grapevine trunk diseases were detected on the arthropods. Phaeoacremonium species were detected in 1242 samples while Phaeomoniella chlamydospora was identified from 855 samples. Other fungi isolated included members of the Botryosphaeriaceae, Diatrypaceae and Diaporthales. The potential of grapevine sap as a food source for Portuguese millipedes and cocktail ants was investigated, in vitro. Millipede individuals were offered a choice between water and grapevine sap while ants in nests were presented with grapevine sap, tuna and water and monitored for ingestion of sap. Both taxa preferred grapevine sap over the other food items, indicating close association with pruning wounds. Subsequently, the ability of both taxa to transmit a DsRed-transformed Phaeomoniella chlamydospora isolate to fresh pruning wounds of canes in polystyrene strips, floating in water, and potted vines was tested. Arthropods were exposed to the fungus for 24 hours and transferred to the base of the plants and canes and were removed after three days. Isolations after a month revealed that millipedes and ants were capable of transmitting the fungus onto wounds and cause infection. Millipede faecal pellets were also evaluated as potential sources of inoculum. Millipedes were fed on Phaeomoniella chlamydospora for 24 hours, surface sterilised and allowed to defaecate in sterile Petri dishes overnight. Faecal material was collected, macerated in water and plated onto potato dextrose agar. Propagules of Phaeomoniella chlamydospora survived passage through the gut of millipedes and were passed out in a viable state to form colonies of Phaeomoniella chlamydospora. This study concludes that a wide variety of arthropods can be a source of inoculum of trunk diseases in vineyards. The results of the dissemination trial provides evidence that millipedes and ants are able to disseminate and infect vines with Phaeomoniella chlamydospora. It is therefore, highly likely that other grapevine trunk disease pathogens are transmitted in the same manner. This knowledge highlights the need for control of certain arthropods to be taken into consideration when managing grapevine trunk disease pathogens.