ABSTRACT
Two scab diseases are recognized currently on citrus: citrus scab, caused by Elsinoë fawcettii, and sweet orange scab, caused by E. australis. Because the two species cannot be reliably distinguished by morphological or cultural characteristics, host range and molecular methods must be used to identify isolates. Four pathotypes of E. fawcettii and two of E. australis have been described to date based on host range. The host specificity and genetic relationships among 76 isolates from Argentina, Australia, Brazil, Korea, New Zealand, and the United States were investigated. Based on pathogenicity tests on eight differential hosts, 61 isolates were identified as E. fawcettii and 15 as E. australis. Of 61 isolates of E. fawcettii, 24 isolates were identified as the Florida broad host range (FBHR) pathotype, 7 as the Florida narrow host range (FNHR) pathotype, 10 as the Tryon's pathotype, and 3 as the "Lemon" pathotype. Two new pathotypes, the "Jingeul" and the satsuma, rough lemon, grape-fruit, clementine (SRGC), are described, and four isolates did not fit into any of the known pathotypes of E. fawcettii. Of the 15 isolates of E. australis from Argentina and Brazil, 9 belonged to the sweet orange pathotype and 6 from Korea to the natsudaidai pathotype. E. fawcettii and E. australis were clearly distinguishable among groups by random amplified polymorphic DNA-polymerase chain reaction (RAPD-PCR) assays and the E. fawcettii group was divided into three subgroups, A-1, A-2, and A-3. The A-1 group was composed of the FBHR, FNHR, and SRGC pathotypes; some Lemon pathotypes; and the uncertain isolates. The A-2 subgroup included all of the Tryon's pathotype isolates and one of the three Lemon pathotype isolates and the A-3 group contained the Jingeul pathotype isolates. E. australis was differentiated into two groups: B-1, the natsudaidai pathotype isolates, and B-2, the sweet orange pathotype isolates. Isolates of E. fawcettii and E. australis were clearly distinguishable by sequence analysis of the internal transcribed spacer (ITS) region and the translation elongation factor 1 alpha (TEF) gene. There were also fixed nucleotide differences in the ITS and TEF genes that distinguished subgroups separated by RAPD-PCR within species. We confirmed two species of Elsinoë, two pathotypes of E. australis, and at least six pathotypes of E. fawcettii and described their distribution in the countries included in this study.
Subject(s)
Ascomycota/pathogenicity , Citrus/microbiology , Plant Diseases/microbiology , Argentina , Ascomycota/classification , Ascomycota/genetics , Australia , Base Sequence , Brazil , DNA, Fungal/genetics , DNA, Fungal/isolation & purification , Korea , New Zealand , Random Amplified Polymorphic DNA Technique , United StatesABSTRACT
Isolates of Colletotrichum acutatum were collected from anthracnose-affected strawberry, leatherleaf fern, and Key lime; ripe-rot-affected blueberry; and postbloom fruit drop (PFD)-affected sweet orange in Florida. Additional isolates from ripe-rot-affected blueberry were collected from Georgia and North Carolina and from anthracnose-affected leatherleaf fern in Costa Rica. Pathogenicity tests on blueberry and strawberry fruit; foliage of Key lime, leatherleaf fern, and strawberry; and citrus flowers showed that isolates were highly pathogenic to their host of origin. Isolates were not pathogenic on foliage of heterologous hosts; however, several nonhomologous isolates were mildly or moderately pathogenic to citrus flowers and blueberry isolates were pathogenic to strawberry fruit. Based on sequence data from the internal transcribed spacer (ITS)1-5.8S rRNA-ITS2 region of the rDNA repeat, the glutaraldehyde-3-phosphate dehydrogenase intron 2 (G3PD), and the glutamine synthase intron 2 (GS), isolates from the same host were identical or very similar to each other and distinct from those isolated from other hosts. Isolates from leatherleaf fern in Florida were the only exception. Among these isolates, there were two distinct G3PD and GS sequences that occurred in three of four possible combinations. Only one of these combinations occurred in Costa Rica. Although maximum parsimony trees constructed from genomic regions individually displayed little or no homoplasy, there was a lack of concordance among genealogies that was consistent with a history of recombination. This lack of concordance was particularly evident within a clade containing PFD, Key lime, and leatherleaf fern isolates. Overall, the data indicated that it is unlikely that a pathogenic strain from one of the hosts examined would move to another of these hosts and produce an epidemic.
Subject(s)
Colletotrichum/genetics , Colletotrichum/isolation & purification , Crops, Agricultural/microbiology , Ferns/microbiology , Fruit/microbiology , Host-Pathogen Interactions , Colletotrichum/enzymology , Colletotrichum/pathogenicity , Costa Rica , Florida , Fragaria/microbiology , Genes, Fungal , Glutamate-Ammonia Ligase/genetics , Glyceraldehyde-3-Phosphate Dehydrogenases/genetics , Introns/genetics , Molecular Sequence Data , Phylogeny , Plant Diseases/microbiology , Recombination, Genetic/genetics , Restriction Mapping , United StatesABSTRACT
Citrus black spot (CBS) is caused by Guignardia citricarpa, which incites lesions on citrus fruit and can induce fruit drop. Quiescent infections occur during the spring and summer, and symptoms appear at fruit maturity or after harvest. Thus, fruit from citrus areas affected by CBS represent a risk for introduction of this pathogen into new areas. The effects of preventive field fungicide programs, postharvest fungicide drenches, packinghouse fungicide applications, and storage temperatures on postharvest symptom development and viability of G. citricarpa in lesions were evaluated in five experiments on Murcott tangor, Valencia oranges, and lemons. Preventive field treatments and fruit storage at 8°C consistently reduced postharvest CBS development, whereas a postharvest fungicide drench or packinghouse treatment with fungicides had no effect on postharvest symptom development. In a separate experiment, postharvest appearance of symptoms was related to the percentage of fruit with symptoms at harvest. The preventive field fungicide program also consistently reduced the percentage of isolation of G. citricarpa from affected fruit, whereas storage temperature and packinghouse fungicide treatment gave variable results. The viability of the fungus declined with storage time of fruit after harvest, but G. citricarpa could still be readily isolated regardless of treatment. In another experiment, the viability of the fungus in detached fruit or peel was minimally affected by temperature or moisture during storage. The frequency of successful isolation declined with time, but G. citricarpa was still recovered frequently from symptomatic tissue at later times. The most effective means to reduce postharvest development of symptoms is through preventive application of fungicides during the fruit growing season and storage of harvested fruit at cold temperatures. None of the measures evaluated substantially reduced viability of G. citricarpa, and the pathogen would likely be introduced on symptomatic fruit from citrus areas with CBS.
ABSTRACT
Alternaria brown spot, caused by Alternaria alternata (Fr.) Keissler, causes leaf, twig, and fruit lesions and reduces yield and fruit quality of many tangerines (Citrus reticulata Blanco) and their hybrids (3). In 2003, characteristic symptoms of brown spot were observed on young leaves and fruit of 'Minneola' tangelo in the Satipo Province of Peru. In 2004, the disease was discovered in the provinces of Chanchamayo, Leoncio Prado, and La Convención in the Junin, Huanuco, and Cusco regions, respectively, as well as in the Apurimac and the Ene valleys. In 2005, it was confirmed in the province of Oxapampa in the Pasco Region. Brown-to-black lesions surrounded by yellow halos and veinal necrosis were observed on young leaves, often causing abscission of young shoots and twig dieback. Light brown, circular lesions were observed on fruit, and when severe, resulted in premature abscission. Isolations from infected leaves and twigs were made on potato dextrose agar (PDA) with 10 µg/ml of benomyl. Colonies that developed after 5 days at 27°C were olive brown-to-black and produced small, muriform, pigmented conidia typical of A. alternata. On PDA without benomyl, gray colonies with conidia typical of Colletotrichum gloeosporioides were recovered frequently. Inoculation of three detached young shoots of 'Minneola' by spraying with a suspension of 105 conidia/ml of A. alternata produced leaf and twig symptoms characteristic of the disease after 48 h and confirmed pathogenicity of three isolates. Symptoms were not observed on control leaves sprayed with water nor on an equal number of leaves inoculated with a suspension of 105 conidia/ml of C. gloeosporioides. Reisolation of A. alternata from diseased tissue fulfilled Koch's postulates. DNA was extracted from 17 isolates and a partial endopolygalacturonase gene was amplified and sequenced (2). Sequences of all 17 isolates were identical, and in BLAST searches of the NCBI database, the closest matches were A. alternata accession nos. AY295023.1, AY295022.1, and AY295021.1 with 100, 99.8, and 99.8% sequence similarity, respectively. Phylogenetic analyses revealed that all isolates from Peru clustered with brown spot isolates from Israel, Turkey, South Africa, and Australia (1). These results, along with morphological characterization and pathogenicity tests, confirm the identity of the fungus as the tangerine pathotype of A. alternata. The disease has significantly reduced yield and the commercial value of fruit and may be a limiting factor for the production of susceptible cultivars in those areas of Peru. References: (1) T. L. Peever et al. Phytopathology 92:794, 2002. (2) T. L. Peever et al. Mycologia 96:119, 2004, (3) L.W. Timmer et al. Pages 19-21 in: Compendium of Citrus Diseases. 2nd ed. L. W. Timmer et al eds. The American Phytopathological Society, St. Paul, MN, 2000.
ABSTRACT
Alternaria brown spot, caused by Alternaria alternata, causes yield losses and fruit blemishes on many tangerines and their hybrids in most citrus areas of the world where susceptible cultivars are grown. Although the conditions affecting infection and disease severity are known, little information is available on inoculum production on infected tissue. We found that 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 on 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. At 74% relative humidity, conidial production on leaf lesions was low, but it was abundant at 85, 92.5, 96, and 100%. Application of QoI or copper fungicides, but not ferbam, suppressed sporulation on leaf lesions for about 14 to 21 days after application. Additional applications did not appear to be more effective than a single spray in reducing inoculum production.
ABSTRACT
Postbloom fruit drop (PFD) of citrus, caused by Colletotrichum acutatum, infects petals of citrus flowers and produces orange-brown lesions that induce the abscission of young fruitlets and the retention of calyces. Proper timing of fungicide applications is essential for good disease control. Different systems for timing of fungicide applications for control of PFD in a major citrus-growing region in southern São Paulo state in Brazil were evaluated from 1999 to 2002. The following programs were compared to an unsprayed control using counts of diseased flowers, persistent calyces, or fruit: (i) a phenology-based program currently recommended in Brazil with one application at early and another at peak bloom; (ii) the Florida PFD model; (iii) the post-bloom fruit drop-fungicide application decision system (PFD-FAD), a new computer-assisted decision method; and (iv) grower's choice. In 1999, no disease developed, sprays applied with the phenology-based program had no effect, and the Florida PFD model saved two sprays compared with the phenology-based program. In 2000, PFD was moderate and the phenology-based and grower's choice treatments had a significantly lower number of persistent calyces and higher fruit numbers than the control, but no differences were found between those treatments and the PFD model. In 2001, PFD was severe with considerable yield loss. The PFD model, the phenology-based program, and the grower's choice reduced flower blight and the number of persistent calyces, and improved fruit yields with two to three applications, but the PFD-FAD achieved comparable yields with only one spray. In 2002, the disease was mild, with no yield loss, and the Florida PFD model and the PFD-FAD saved one spray compared with the other systems. The PFD model and the PFD-FAD were equally effective for timing fungicide applications to control PFD in Brazil. Scouting of trees is simpler with PFD-FAD; therefore, this system is recommended and should eliminate unnecessary sprays and reduce costs for growers.
ABSTRACT
Products that induce disease resistance in plants were evaluated on potted seedlings of rough lemon for citrus scab, caused by Elsinoe fawcettii; grapefruit for melanose, caused by Diaporthe citri; and Dancy tangerine for Alternaria brown spot caused by Alternaria alternata pv. citri. Plants were pruned to a single stem with mature leaves and treated at bud break or various times thereafter. New foliage was inoculated and subsequently evaluated for disease severity. Oxycom, Nutriphite, Messenger, Goemar H11, Serenade, ReZist, ProPhyt, Aliette, Actigard, and KeyPlex were evaluated and compared with benomyl or strobilurin fungicides as standards. Most products reduced disease severity compared with the untreated control, but were less effective than standard fungicides. The most generally effective products were ReZist and Actigard, those that contain or produce phosphorous acid (Aliette and Nutriphite), and a bacterial preparation (Serenade). Oxycom and Messenger controlled scab well in some tests. Products that induce host resistance may be useful for disease control in citrus in an integrated program with standard fungicides.
ABSTRACT
Citrus scab, caused by Elsinoe fawcettii, and melanose, caused by Diaporthe citri, produce external blemishes on citrus fruit, reducing acceptability of the fruit for the fresh market. In laboratory studies, rough lemon seedlings and grapefruit seedlings were inoculated with conidia of E. fawcettii and D. citri, respectively, and exposed to a range of temperatures and durations of leaf wetness. Scab was most severe at temperatures from 23.5 to 27°C and much less severe at 17, 20, 30, or 32°C. A leaf wetness duration of 4 h was sufficient for some infection, but 12 h of leaf wetness were needed for maximum infection with scab. Melanose was equally severe at 24 and 28°C, moderate at 20°C, and low at 32°C. Melanose infection was minimal with 4 h of leaf wetness, moderate with 8 to 16 h, and reached maximum levels at 24 h or more of leaf wetness. In field studies, grapefruit seedlings with new shoots were placed beneath trees weekly, and disease severity was evaluated in relation to environmental factors. Melanose severity increased sharply with an increase in total weekly rainfall, leaf wetness durations of greater than 80 h per week, and average temperatures above 22°C.
ABSTRACT
Alternaria alternata (Fr.) Keissler causes lesions on leaves, twigs, and fruit and reduces yield and fruit quality of many tangerines (Citrus reticulata) and their hybrids (2). Severe outbreaks of Alternaria brown spot were observed on 'Murcott' tangor (Citrus reticulata × Citrus sinensis) trees in southern São Paulo, southern Minas Gerais states in Brazil, and in Misiones and Corrientes provinces in Argentina. A single diseased 'Fortune' tangerine tree was observed in a grove in Misiones. On young leaves, brown-to-black lesions often expanded to cover large parts of the leaf, causing abscission of young shoots and dieback of twigs. Lesions were often surrounded by yellow halos. On fruit, dark specks from 0.2 to 0.5 cm were observed, and severe infection caused premature fruit abscission. Isolations were made on potato dextrose agar (PDA) after surface sterilization of leaf and fruit tissues in 1.5% sodium hypochlorite for 1 min. Plates were incubated at 27°C in the dark for 1 week. Isolated colonies were olive brown to black, and the fungus was stimulated to form conidia by scraping the surface of the mycelium. The pathogen was a typical small-spored Alternaria species, and the morphological characteristics of the conidia and conidiophores fit the description of A. alternata. Inoculation of three detached young shoots of 'Murcott' with a conidial suspension (105 conidia per ml) confirmed pathogenicity of three isolates obtained from widely separated groves in southern and eastern São Paulo State in Brazil and one from Misiones Province in Argentina. A control treatment with an equal number of shoots was sprayed with distilled water only. After 48 h, all isolates caused dark lesions on the leaves, characteristic of the disease. Symptoms were observed on inoculated, but not on control shoots. Koch's postulates were satisfied by reisolation of the fungus from symptomatic tissue in all cases. Although Alternaria brown spot was reported previously in neighboring Rio de Janeiro, Brazil on 'Dancy' and 'Ponkan' tangerines (1), serious disease problems now occur on 'Murcott', an important commercial variety in the major production area in Brazil. To our knowledge, this is the first report of Alternaria brown spot in Argentina. References: (1) A. de Goes et al. Fitopatologia Brasileira 26(Suppl.):386, 2001. (2) L. W. Timmer et al. Pages 19-21 in: Compendium of Citrus Diseases. 2nd ed. L. W. Timmer, S. M. Garnsey, and J. H. Graham, eds. The American Phytopathological Society, St. Paul, MN, 2000.