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In a field trial during 1995/96, epidemics of <I>Pyrenophora teres</I> and <I>Rhynchosporium secalis</I> were studied in winter barley with concurrent records of weather data to identify key environmental parameters that affect epidemics. Temperature was identified as a key influence in the onset of <I>P. teres</I> epidemics. Disease symptoms were observed to progress when daytime temperatures consistently reached 10°C and minimum nightime temperatures for the same period remained above 5°C. Short leaf wetness periods and longer photoperiods also correlated with increased disease levels during the <I>P. teres </I>epidemic. In <I>R. secalis,</I> relationships between disease onset and individual environmental parameters were not consistent, however, high rainfall events and prolonged leaf wetness periods were recorded prior to greatest disease increase. Hypotheses based on individual and combined weather criteria, based on the results of the 1995/96 field trials, were tested in controlled conditions. The effect of temperature on <I>P. teres</I> was confirmed, with small differences between ascospores and conidiospores. Latent period of both <I>P. teres</I> and <I>R. secalis</I> was influenced by cultivar resistance, inoculum concentration and plant growth stage. In a second field trial in 1996/97 reduced dose fungicide programmes, using hypotheses of epidemic development based on environmental criteria, were tested and compared favourably to a standard programme with greater fungicide doses. Environmental criteria were combined within a decision model for timed reduced-dose fungicide programmes for each pathogen, where risk scores were allotted for each set of criteria and fungicide treatment decision was based on the cumulative risk score. Both the <I>P. teres</I> and <I>R. secalis</I> decision models were tested in a final field trial in 1997/98. Lower disease levels and greater yield response, with lower fungicide input, was achieved from both models compared to a standard fungicide programme. The potential for using the <I>P. teres</I> and <I>R. secalis</I> decision models in a decision support system for cereals is discussed.

The use of agricultural fungicides exerts very strong selection pressures on plant pathogens. This can lead to the spread of fungicide resistance in the pathogen population, which leads to a reduction in efficacy of disease control and loss of yield. In this thesis, we use mathematical modelling to investigate how the spread of fungicide resistant pathogen strains can be slowed, using epidemiological models to understand how application strategies can be optimised. A range of different fungicide application strategies have been proposed as anti-resistance strategies. Two of the most often considered strategies rely on combining two fungicides with different modes of action. The first involves spraying the two fungicides at the same time (mixture) and the second spraying them alternately at different times (alternation). These strategies have been compared both experimentally and by mathematical modellers for decades, but no firm conclusion as to which is better has been reached, although mixtures have in general often been favoured. We use mathematical models of septoria leaf blotch (Zymoseptoria tritici) on winter wheat and powdery mildew on grapevine (Erysiphe necator) to investigate the relative performance of these two strategies. We show that depending on the exact way in which the strategies are compared and the exact case, either strategy can be the more effective. However, when aiming to optimise yield in the long-term, we show that mixtures are very likely to be the most effective strategy in any given case. The structure of mathematical models clearly impacts on the conclusions of those models. As well as investigating the sensitivity of our conclusions to the structure of the models, we use a range of nested models to isolate mechanisms driving the differential performance of fungicide mixtures and alternation. Although the fine detail of a model’s predictions depends on its exact structure, we find a number of conserved patterns. In particular we find no case in which mixtures do not produce the overall largest yield over the time for which the fungicide remains effective. We also investigate the effects of the timing of an individual fungicide spray on its contribution toward resistance development and disease control. A set of so-called “governing principles” to understand the performance of resistance-management strategies was recently introduced by van den Bosch et al., formalising concepts from earlier literature. These quantify selection rates by examining the difference between the growth rates of fungicide-sensitive and fungicide resistant pathogen strains. Throughout the thesis, we concentrate on the extent to which these governing principles can be used to explain the relative performance of the resistance-management strategies that are considered.

Made available in DSpace on 2014-06-11T19:33:39Z (GMT). No. of bitstreams: 0 Previous issue date: 2006-09-26Bitstream added on 2014-06-13T18:45:41Z : No. of bitstreams: 1 reis_rf_dr_jabo.pdf: 717177 bytes, checksum: a3ad4a94a8ee559ded22f5ec2741cd35 (MD5) / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) / Este trabalho teve como objetivos determinar o efeito da idade da lesão de mancha marrom de Alternária (MMA) na produção conidial em folhas, ramos e frutos, a produção conidial sob diferentes condições de umidade relativa, a influência da aplicação de fungicidas na supressão da produção de conídios, o efeito da idade de maturação de folhas na suscetibilidade ao patógeno e a influência do tamanho de frutos na suscetibilidade à Alternaria alternata. A reação de espécies cítricas à MMA também foi avaliada. A esporulação em folhas teve início cerca de 10 dias depois do aparecimento de lesões de mancha marrom de Alternária, sendo que a maior produção de conídios ocorreu a partir de 20 dias, estendendo-se até 40 dias de idade. A produção de conídios foi superior em folhas do que em ramos ou frutos. A esporulação por unidade de área de lesão em folhas foi maior nos tangelos Minneola e Orlando , em relação ao tangor Murcott . A produção conidial em lesões de folhas foi mais abundante a partir de 85, 92,5, 96 e 100% de umidade relativa. A aplicação de fungicidas do grupo das estrobilurinas e dos cúpricos suprimiu a esporulação em lesões nas folhas por cerca de 14 a 21 dias após a aplicação. Folhas de tangelo Minneola apresentaram maior quantidade e maiores tamanhos de lesões até 20 dias de idade. O híbrido Nova foi o mais resistente entre os materiais genéticos estudados, sendo que folhas a partir de 16 dias de idade... / The objectives of this study were to evaluate: i) the effect of age of Alternaria brown spot (ABS) lesions on conidial production on leaves, twigs and fruit, ii) conidial production under different relative humidities, iii) influence of fungicide application on conidial production and iv) effect of leaf age on susceptibility to the disease and the influence of fruit size on the severity of brown spot. The reaction of the different citrus species to ABS was also studied. Sporulation on leaves began about 10 days after symptoms developed, was abundant from 20 to 40 days, and declined thereafter. Conidial production was far greater on leaf than fruit or twig lesions. Spore production per unit area of leaf lesion was greater on the more susceptible hybrids, Minneola and Orlando tangelos than on the less susceptible Murcott tangor. Conidial production on leaf lesions was more abundant at 85, 92.5, 96 and 100% relative humidity (RH) and almost nil at lower RH. Application of pyraclostrobin, trifloxyztrobin, azoxystrobin and copper fungicides suppressed the sporulation on leaf lesions for about 14 to 21 days after application. Minneola tangelo leaves had the greatest number and the largest lesions by 20 days or later. The Nova and Murcott hybrids were the most resistant of the cultivars and leaves showed few lesions when they were 16-days-old or older at inoculation. Leaves of Dancy tangerine were more susceptible than on leaves of Orlando tangelo and the Sunburst mandarin. For disease severity, there was a significant... (Complete abstract click electronic access below)

Resumo: A mancha preta dos citros (MPC) doença causada pelo fungo Guignardia citricarpa Kiely [anamorfo: Phyllosticta citricarpa (McAlp.) Van der Aa], é a principal doença fúngica da cultura no Brasil. Todas as variedades de laranjeiras doces são suscetíveis ao patógeno, que deprecia comercialmente os frutos, além de provocar sua queda prematura e elevar substancialmente o custo de produção. Até o presente momento, informações acerca da etiologia da doença são escassas. Portanto, o presente trabalho teve como objetivos: determinar os tipos de sintomas expressos por G. citricarpa em frutos cítricos inoculados com suspensão de conídios; desenvolver uma metodologia de inoculação de G. citricarpa a qual seja eficiente e permita estabelecer relações qualitativa e quantitativa, tanto em termos de níveis de severidade e tipos de inóculo; determinar em casa de vegetação o período de suscetibilidade dos frutos de laranjeira 'Pêra-Rio' a G. citricarpa; verificar a influência da forma endofítica de G. mangifera na expressão de sintomas de G. citricarpa e; avaliar a relação da eficiência de controle químico da MPC versus o tempo de exposição dos frutos à descarga de conídios de G. citricarpa. Foi observado que G. citricarpa inoculada em suspensão conidial em frutos de laranjeira 'Pêra-Rio' produzem sintomas do tipo mancha dura, mancha sardenta, mancha virulenta e falsa melanose. Quando inoculada em frutos de tangor 'Murcott' produzem sintomas do tipo falsa melanose e mancha dura. Frutos de laranjeira 'Pêra-Rio' mostraram-se mais suscetíveis a G. citricarpa que o tangor 'Murcott'. A metodologia de inoculação de conídios de G. citricarpa, mostrou-se eficiente, prática e rápida, permitindo quantificar o inóculo, fato inovador. Em relação à suscetibilidade, os frutos de laranjeira 'Pêra-Rio' quando inoculados... (Resumo completo, clicar acesso eletrônico abaixo) / Abstract: Citrus black spot (CBS), caused by Guignardia citricarpa Kiely [anamorph: Phyllosticta citricarpa (McAlp.) Van der Aa], is of the most import citrus disease in Brazil. Practically, all varieties of sweet oranges are susceptible to pathogen, which depreciates commercially the fruits, causes drop prematurely and increase substantially the cost of production. Until now, information about the etiology of the disease is scarce. Therefore, this study aimed to determine the types of symptoms expressed by G. citricarpa in citrus inoculated with conidial suspension, to develop a method of inoculation with G. citricarpa which to be efficient and allow a qualitative and quantitative relations with levels of severity and inoculum types, to determine in greenhouse the period of susceptibility sweet orange fruits of 'Pêra-Rio' to G. citricarpa; the influence of endophytic form of G. mangiferae in the suppression of symptoms of G. citricarpa; and to evaluate the efficiency of chemical control of CBS vs exposure time of fruit to discharge of conidia of G. citricarpa. Conidia of G. citricarpa inoculated in sweet orange Pêra-Rio produced symptoms of the types: hard spot, freckled spot, virulent spot and false melanose. When conidia inoculated in 'Murcott' tangor fruits the symptoms were of the type false melanose hard spot. Sweet orange fruits showed more susceptible than 'Murcott' tangor. The methodology of inoculation of conidia used in this work showed very efficient and practice allowing discriminative studies related to the inoculum and, additional alternative to studies related to Citrus-G. citricarpa pathosystem. For the susceptibility, sweet orange fruits with 4.5 cm diameter, inoculated with G. citricarpa showed 56.94% of fruit with symptoms of MPC, 50% of fruits with 5.5 cm diameter and 27.75% when inoculated with ≥ 7 cm in diameter, is only... (Complete abstract click electronic access below) / Orientador: Antonio de Goes / Coorientador: Renato Ferrari dos Reis / Banca: Jaime Maia dos Santos / Banca: Edson Luiz Furtado / Banca: Rita de Cássia Panizzi / Banca: Marcel Bellato Spósito / Doutor

Hydroponic production was initially explored as an alternative to field production due to the ease of plant growth control and the hopes of preventing the majority of disease causing agents known to be present in general soil environments. Of primary concern in terms of pathogens are the water-borne and water-motile zoosporic fungi (especially Pythium spp.) which are able to spread easily throughout the system and cause root-rot and wilting. Few pesticides are currently registered for use in hydroponic systems due to the high costs of registration, while registered pesticides carry a high cost to the grower. Recent legislative moves by numerous countries are also resulting in a trend towards the re-use of hydroponic nutrient solution. As a result such hydroponic solutions require a greater level of disinfection to prevent disease outbreaks but without resulting in chemical buildup of phytotoxic and environmental concern. Sanitiser formulation has seen significant changes over the last few years resulting in sanitisers being used in many new areas and in a more environmentally friendly nature. Although sanitisers are not designed to have specific action against micro-organisms (as is the case with fungicides and anti-microbial agents such as antibiotics), most sanitisers are able to act on cell membranes due to the inherent surfactant properties. This study attempted to determine the suitability of various sanitisers and chemicals as alternate means of control of Pythium in recirculating gravel hydroponic systems by: 1). Exposing Pythium zoospores in a water suspension to the sanitisers Actsol®, Agral 90®, Fitosan®, Prasin®, Purogene®, TecsaClor®, Sporekill® and copper (as copper (I) sulphate) which all managed to eliminate 80% or more of the viable inoculum within a 10 minute exposure time at relatively low concentrations. 2). Testing the above sanitisers for phytotoxicity effects on cucumber plants in a static hydroculture system under laboratory conditions and lettuce plants in a gravel bed hydroponic system under greenhouse conditions. Purogene® and TecsaClor® exhibited a slight growth promotion effect at low concentrations, yet still caused negative phytotoxic effects when dosed at high concentrations. All other sanitisers exhibited some measure of phytotoxicity, observed as growth retardation and leaf discolouration, with phytotoxic effects increasing with increasing concentrations. Copper sulphate was found to be the most phytotoxic chemical tested. 3). Addition of the sanitisers to a small scale hydroponic system (greenhouse), as well as to a semi-commercial scale (field) gravel bed hydroponic system artificially infested with Pythium and cultivated with lettuce. The sanitisers were also compared to a commercially available fungicide, Phytex®. Only Phytex® and Purogene® managed to effectively reduce disease incidence and promote growth over an untreated, Pythium infested control. The results indicated that Purogene® was the most effective for application into a gravel bed hydroponic system cultivated with lettuce, while no sanitiser treatment was able to equal the improved growth and disease control recorded with treatment of the commercial fungicide Phytex®. Although all the sanitisers were able to reduce levels of Pythium inoculum in the hydroponic nutrient solution, this beneficial effect did not translate into increased yields, due to the growth retardation due to phytotoxic effects. / Dissertation (MSc (Plant Pathology))--University of Pretoria, 2008. / Microbiology and Plant Pathology / unrestricted

Early blight caused by Alternaria solani and brown spot caused by the small-spored Alternaria spp., Alternaria alternata, Alternaria arborescens, and Alternaria tenuissima are observed annually in midwestern potato production areas. The use of foliar fungicides remains a primary management strategy. However, Alternaria spp. have developed reduced-sensitivity and/or resistance to many single-site fungicides such as quinone outside inhibitor (QoI), succinate dehydrogenase inhibitor (SDHI), and anilinopyrimidines (AP) fungicides in recent years. Boscalid, fluopyram, solatenol, and adepidyn are EPA-registered SDHI fungicides applied commercially to a variety of crops including potato. High intrinsic activity was observed in fluopyram, solatenol and adepidyn to A. solani isolates. Adepidyn and solatenol reduced disease severity caused by A. solani in field evaluations. Molecular characterization of 2018 A. solani field isolates determined that the frequency of the D123E and H134R SDH mutations increased. In contrast, the H278R/Y and H133R SDH mutations were found at low frequency. Adepidyn demonstrated the highest intrinsic activity against the small-spored Alternaria spp. but high intrinsic activity was also observed with boscalid, fluopyram, and solatenol. In vivo experiments demonstrated that adepidyn, solatenol, and fluopyram were more effective at managing A. arborescens and A. tenuissima than boscalid. Under greenhouse conditions, adepidyn and solatenol reduced brown spot severity caused by A. alternata to a greater extent than did fluopyram and boscalid. Results of these studies determined that accurate pathogen identification of small-spored Alternaria spp. may be important for brown spot management. Fludioxonil and cyprodinil exhibited a higher efficacy against of A. solani isolates when compared to pyrimethanil in greenhouse assays. Fludioxonil and cyprodinil were also highly efficacious against the Alternaria spp. evaluated and appear to be a good addition into fungicide rotation programs for early blight and brown spot management. Anilinopyrimidine (AP) (pyrimethanil and cyprodinil) and phenylpyrrole (PP) (fludioxonil) fungicides have demonstrated high intrinsic activity against other pathogens. Determining the efficacy of these fungicides on Alternaria spp. is important to the potato industry.

This research addressed the identification and distribution of selected ERI fungi within two greens at Mississippi State University Golf Institute using novel molecular methods, spatial relationships of fungal distribution and turf health assessments, and fungal sensitivity to selected fungicides. A complex of ERI fungi was identified in both greens. Gaeumannomyces graminis, Gaeumannomyces sp., and Candidacolonium cynodontis were randomly distributed based on spatial autocorrelation in both green samplings, while Magnaporthiopsis cynodontis clustered in Green 2 for both samplings and in Green 12 of the 2018 sampling. Clustering of turf health values was variable by month in each green, and no spatial relationship was determined between turf health values and fungal presence. Five ERI fungal isolates were exposed to four fungicide classes with relative growth present at 100 ppm. The EC50 values for the selected fungi ranged from 2.33 to >100 ppm and suggests a lack of sensitivity to the selected fungicides.

Anthracnose, incited by Colletotrichum cereale, is a destructive disease of physiologically stressed creeping bentgrass putting greens in Mississippi and Alabama. Anthracnose severity and frequency of occurrence have increased over the past 15 years, and fungicide resistance may have had a role in the increase. In vitro bioassays were performed to evaluate thiophanate methyl and azoxystrobin against C. cereale isolates exposed to the fungicides and baseline isolates that had not been exposed to either fungicide. All isolates collected from creeping bentgrass were uninhibited by both fungicides at discriminatory doses. Partial nucleotide sequences of the â-tubulin 2 (thiophanate methyl) or cytochrome b (azoxystrobin) gene was compared to confirm fungicide resistance. Thiophanate methyl resistance was conferred by either a point mutation from glutamic acid to alanine at position 198, or phenylalanine to tyrosine at position 200. Azoxystrobin resistance was conferred by an amino acid point mutation from glycine to alanine at position 143.

The polyene macrolide antibiotic natamycin (Antibiotic A-5283) is commonly used to retard the growth of surface molds on various cheese varieties. Natamycin is commonly applied to the surface of cheese by dipping or spraying, using an aqueous dispersion containing 200 to 300 ppm of the additive. The large molecular weight of natamycin, 666 g/mol, and conjugated double bond structure causes it to be extremely insoluble in water and most food grade solvents. The inability to apply natamycin in true solution creates void non-treated areas on the food surface. These non-treated areas promote the growth of fungal organisms. A water soluble N-alkyl semisynthetic derivative of natamycin was synthesized by the Michael addition reaction of the parent with a N-substituted malemide. A comparative study investigating the effectiveness of the semisynthetic derivative of natamycin and the parent antibiotic in suppressing mold growth on one month aged shredded Cheddar cheese modified atmosphere packaged (MAP) was performed. A 20 ppm natamycin treatment effectively suppressed visible mold growth (<104 CFU/g) in MAP samples for up to 30 days after opening. The 20 ppm semisynthetic derivative performed similarly to the 10 ppm natamycin treatment in retarding mold growth. Visible mold growth did not occur for these treatments in MAP samples until 20 days after opening. Analysis of storage conditions revealed that an outgrowth of mold in shredded cheese occurred in MAP packages stored longer than 15 days. This bloom in mold growth was attributed to the degradation of natamycin and the semisynthetic derivative throughout storage. The stability and degradation of natamycin and the derivative were monitored throughout the study. Antibiotic concentration on the cheese surface was quantified by molecular absorption spectrometry. Results from this study showed, heavily contaminated samples caused the rate and loss of natamycin and the derivative to increase. Antibiotic concentration decreased at a similar rate in MAP and open package conditions. Natamycin and derivative were found to have similar degradation properties. / Master of Science