Effect of temperature, leaf wetness, and rainfall on the production of Guignardia citricarpa ascospores and on back spot severity on sweet orange

A pinta preta dos frutos cítricos (Citrus sp.)é causada por Guignardia citricarpa e a produção de ascósporos depende da temperatura, molhamento foliar e chuva. O número de ascósporos foi monitorado usando uma armadilha de esporos e os fatores climáticos foram avaliados usando uma estação automotizada em pomares de laranja (Citrus sinensis) Natal e Valência, no período de novembro de 2000 a março de 2001 no município de Mogi-Guaçu/SP, Brasil. em ambos os pomares, os frutos foram ensacados para prevenir a infecção natural, e os sacos foram removidos semanalmente em diferentes estádios de desenvolvimento dos frutos durante 18 semanas. Ascósporos foram produzidos durante todo o período do experimento especialmente primeiramente depois da ocorrência de chuvas. em ambos os pomares o pico de produção de ascósporos ocorreu em janeiro e fevereiro. A produção de ascósporos foi correlacionada com o molhamento foliar apenas para o pomar de laranja Natal, mas não foi correlacionada com o total de chuva ou temperatura para as duas variedades. A doença foi mais severa em frutos expostos na 7ª, 8ª e 13ª semana, após o início do experimento para ambas variedades e, na 16ª semana para a variedade Natal. Verificou-se uma forte correlação entre severidade da doença e quantidade total de chuva para ambas variedades e uma fraca correlação entre temperatura e severidade para o pomar de laranja Natal. Não houve correlação entre severidade e molhamento foliar ou com o número de ascósporos.The black spot of citrus (Citrus sp.) is caused by Guignardia citricarpa with ascospore production depending on temperature, leaf wetness, and rainfall. The number of ascospores produced was monitored using a spore trap and climatic factors were recorded using an automated meteorological station of 'Natal' and 'Valencia' sweet orange (Citrus sinensis) orchards in Mogi Guaçu in the state of São Paulo, Brazil, from November 2000 to March 2001. The fruits were bagged to prevent infection and the bags removed from different sets of fruit for one week during each of the 18 weeks of the season in both orchards. Ascospores were produced during the entire experimental period, from spring through summer, primarily after rain events. In both orchards, ascospore production reached a peak in January and February. Ascospore production was related to leaf wetness only in the Natal orange orchard but was not related to total rainfall or temperature in either orchard. Disease was most severe on fruit exposed the 7th, 8th, and 13th weeks after beginning the experiment in both cultivars as well as after the 16th week for 'Natal'. There was a strong relationship between disease severity and total rainfall for both orchards and a weak correlation between temperature and severity in the 'Natal' block only. There was no relationship between severity and leaf wetness or ascospore numbers.Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP

Discord between morphological and phylogenetic species boundaries: incomplete lineage sorting and recombination results in fuzzy species boundaries in an asexual fungal pathogen

BACKGROUND:Traditional morphological and biological species concepts are difficult to apply to closely related, asexual taxa because of the lack of an active sexual phase and paucity of morphological characters. Phylogenetic species concepts such as genealogical concordance phylogenetic species recognition (GCPSR) have been extensively usedhowever, methods that incorporate gene tree uncertainty into species recognition may more accurately and objectively delineate species. Using a worldwide sample of Alternaria alternata sensu lato, causal agent of citrus brown spot, the evolutionary histories of four nuclear loci including an endo-polygalacturonase gene, two anonymous loci, and one microsatellite flanking region were estimated using the coalescent. Species boundaries were estimated using several approaches including those that incorporate uncertainty in gene genealogies when lineage sorting and non-reciprocal monophyly of gene trees is common.RESULTS:Coalescent analyses revealed three phylogenetic lineages strongly influenced by incomplete lineage sorting and recombination. Divergence of the citrus 2 lineage from the citrus 1 and citrus 3 lineages was supported at most loci. A consensus of species tree estimation methods supported two species of Alternaria causing citrus brown spot worldwide. Based on substitution rates at the endo-polygalacturonase locus, divergence of the citrus 2 and the 1 and 3 lineages was estimated to have occurred at least 5, 400 years before present, predating the human-mediated movement of citrus and associated pathogens out of SE Asia.CONCLUSIONS:The number of Alternaria species identified as causing brown spot of citrus worldwide using morphological criteria has been overestimated. Little support was found for most of these morphospecies using quantitative species recognition approaches. Correct species delimitation of plant-pathogenic fungi is critical for understanding the evolution of pathogenicity, introductions of pathogens to new areas, and for regulating the movement of pathogens to enforce quarantines. This research shows that multilocus phylogenetic methods that allow for recombination and incomplete lineage sorting can be useful for the quantitative delimitation of asexual species that are morphologically indistinguishable. Two phylogenetic species of Alternaria were identified as causing citrus brown spot worldwide. Further research is needed to determine how these species were introduced worldwide, how they differ phenotypically and how these species are maintained.This item is part of the UA Faculty Publications collection. For more information this item or other items in the UA Campus Repository, contact the University of Arizona Libraries at [email protected]

Crop loss, aetiology, and epidemiology of citrus black spot in Ghana

Citrus Black Spot (CBS), caused by Guignardia citricarpa, was detected for the first time in Ghana and in West Africa. The disease was first observed in the Eastern Region in 1999 with typical disease symptoms including hard spot, virulent spot and false melanose were observed on several citrus species. A survey revealed that the disease has reached epidemic levels in the citrus-producing areas of the Eastern and Ashanti regions and is spreading rapidly within these areas and to other regions of the country. Currently, CBS is the most important fruit disease of citrus in Ghana, causing about 22% crop loss. Although the disease does not cause postharvest decay and the internal quality of the fruit is not affected, significant amounts of blemished fruit are discarded at the markets. Disease incidence and severity was found to be higher on mature than on young citrus trees. Pycnidia were found on fruit with hard spot symptoms, and pycnidia and pseudothecia typical of Guignardia spp. were found on decomposing leaves. Two species, G. citricarpa and G. mangiferae, were isolated from 15% of the samples collected and identified using the Oatmeal Agar test and by PCR with species-specific DNA primers. Isolates of G. citricarpa produced CBS symptoms after 80 to 233 days on 75% of the artificially inoculated young fruit of Valencia Late sweet orange. The fungus was re-isolated from symptomatic, inoculated fruit completing Koch's postulates. Isolates of the endophyte G. mangiferae did not induce symptoms in the pathogenicity tests. In epidemiological studies, infections were detected from November to February for the minor cropping season and from May to November for the major season. Fruit of Valencia Late sweet orange were susceptible to G. citricarpa infection for up to 7 months after petal fall. Knowledge of the disease cycle in Ghana will improve methods for disease control