Progress 09/01/05 to 09/30/07
Outputs
OUTPUTS: Alternaria species occurring in onion foliage from Puerto Rico were characterized on the basis of morphology, pathogenicity and DNA analysis. A total of 280 isolates were obtained, from which 34 were selected as morphotypes belonging to Alternaria destruens, A. tenuissima, A. palandui and A. porri (A. allii) species group. Alternaria palandui and A. destruens are new records for Puerto Rico and the Caribbean. Sixty two percent of selected isolates were placed within a taxonomically undescribed Alternaria sp. group with a "semi-arborescent" sporulation pattern. Pathogenicity tests, showed that A. porri (A. allii), A. tenuissima and Alternaria sp. ("semi-arborescent") were pathogenic to onions, been A. porri (A. allii) the most virulent. DNA of the various Alternaria spp. was analyzed using RAPD-PCR, and PCR amplification and sequencing of the nuclear ITS region of rDNA gene. RAPD-PCR technique did not differentiate between species. Phylogenetic relationships based on rDNA ITS
sequences from Alternaria isolates and other Pleosporaceae were analyzed by neighbor-joining and by Kimura distance methods. Three clades were distinguished with strong bootstrap. A first clade with large filiform-beaked spores including A. porri (A. allii), was positioned as a monophyletic group, discrete from other members of the genus. A second clade, including a diverse group of small-spore Alternaria, and a third clade that includes Stemphyllium spp. Experiments in progress using AFLP's will provide additional evidence to discriminate among species. Bacterial diversity associated with onion foliage was also examined. A total of 39 bacterial strains were isolated. Of these, 92% were Gram negative, 49% hold plant pathogenic potential and 51% were putative endophytes. Pathogenicity tests showed that Acidovorax avenae subsp. citrulli, Burkholderia glumae, Pantoea agglomerans, P. dispersa, Pseudomonas sp., Xanthomonas sp. and Xanthomonas-like sp. caused symptoms to onion foliage under
field conditions. Seed-borne bacteria from three commercial onion cultivars were also studied. Fifteen different pathogenic, endophytic, plant growth promoters and biocontrol bacteria species were isolated and identified. Two field experiments were conducted to determine the effects of thrips populations in the severity of A. porri (A. allii) and A. avenae subsp. citrulli in onion foliage (cv. Mercedes). Disease severity and thrips damage were evaluated for both pathogens inoculated together and individually. Frankliniella fusca was the most abundant thrips species during both experiments. Also, F. occidentalis and F. shultzei were collected from onion tissues. A greater disease severity was observed either with A. porri (A. allii) inoculation by itself, controlling thrips populations, and with A. porri (A. allii) with natural thrips populations. Plants inoculated with A. avenae subsp. citrulli did not show symptoms. A suitable management of thrips populations in the field is
necessary to reduce the impact of Alternaria diseases.
PARTICIPANTS: Lydia I. Rivera, PI/Plant Pathologist, Dept.Crop Protection, UPR-Mayaguez; Irma Cabrera, Co-PI/Entomologist,Dept. Crop Protection, UPR-Mayaguez; Myrna Almeda, Collaborator/Microbiologist, Dept. Agronomy, UPR-Mayaguez; Juan Calle, Former Graduate Student, Dept. Agronomy, UPR-Mayaguez; Jessie Fernandez, Former Graduate Student,Dept.Crop Protection,UPR-Mayaguez; Merari Feliciano, Former Graduate Student,Dept.Crop Protection,UPR-Mayaguez; Lesly Colon, Graduate StudentDept.Crop Protection, UPR-Mayaguez; Mario Rodriguez, Undergraduate, Dept. Agronomy, UPR-Mayaguez; Robert McPhail, Undergraduate, Dept.Crop Protection, UPR-Mayaguez; Emory G. Simmons, Advisor, Alternaria Taxonomist, Indiana; David Straney, Advisor, Molecular Biology & Genetics, University of Maryland
TARGET AUDIENCES: Target audiences: Hispanic onion farmers will benefit from findings. Efforts: Transfer of science-based knowledge to undergraduate and graduate students through laboratory and field experiences, seminars on research findings and experiential learning opportunities (RAPD-PCR, AFLP's, data base management such as GenBank).
Impacts
We characterized various fungal and bacterial diseases and insect pests occurring in onion fields at the southern region of Puerto Rico. A better understanding of the relationships between diseases and insect pests in the tropics will allow us to develop effective control management practices in onion fields.
Publications
- Alameda, M. and L.I. Rivera-Vargas. 2007. Endophytic and pathogenic bacteria isolated from onions seeds in Puerto Rico. Proceedings of the Annual Meeting of the Puerto Rican Society of Agricultural Sciences (SOPCA), Catano, P.R. (Abstract)
- Feliciano Rivera, M. 2007. Efecto de los danos causados por tripidos (Thysanoptera: Tripidae) en la severidad de Alternaria allii y Acidovorax avenae subsp. citrulli en el cultivo de cebolla (Allium cepa L.) en Puerto Rico. M.S. Thesis, University of Puerto Rico, Mayaguez, P.R. 126 pp.
- Fernandez J. and L.I. Rivera-Vargas. 2007. Alternaria spp. occurring in onion foliage: Morphology, Pathogenicity and DNA analysis. Proceedings of the Annual Meeting of the Puerto Rican Society of Agricultural Sciences (SOPCA), Catano, P.R. (Abstract)
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Progress 09/01/05 to 08/31/06
Outputs
The following Alternaria species were identified associated to leaf spots in onions: Alternaria destruens, A. tenuissima, A. palandui, A. alli and Pleospora eturmiuna (Stemphylium eturmiunum). Sixty two percent of selected isolates were placed within a 'semi-arborescent' group, non-previously described taxonomically. Alternaria palandui, A. destruens and P. eturmiuna (Stemphylium eturmiunum) are new records for Puerto Rico and the Caribbean. Pathogenicity tests conducted in vitro and under field conditions with 60-day-old plants showed that Alternaria alli, A. tenuissima, Alternaria sp. ('semi-arborescent') and Pleospora eturmiuna were pathogenic to onion foliage, being A. alli the most virulent. Studies using AFLP's and minisatellites of Alternaria spp. are in progress to develop molecular markers to differentiate Alternaria species occurring in onions in Puerto Rico. Bacterial pathogens such as: Pseudomonas sp., Xanthomonas sp., Acidovorax avenae subs. citrulli and
Burkholderia glumae produced symptoms on foliage during pathogenicity tests conducted under field conditions with 52-day-old onion plants. In addition, a group of putative endophytic bacteria were isolated from onion foliage. To examine thrips species associated to onions (cv. 'Mercedes') two field experiments were conducted during Fall 2005 and Spring 2006 at southern Puerto Rico. Thrips were collected using two methods: 1) a direct method using a moist camel's-hair brush to collect specimens from leaves in vials and 2) an indirect method, collecting whole plants in plastic bags from the field. Thrips were clarified and fixed for identification using taxonomic traits such as: number of pairs of elongate pronotal setae, metanotal campiniform sensilla and tergite VIII posteromarginal comb of micotrichia. Three Frankliniella species were identified associated with young emerging leaves in onions: F. occidentalis, F. schultzei and F. fusca. This is the first report of Frankiniella spp.
affecting onions in Puerto Rico. Frankliniella fusca (41.1%) was the predominant species during the surveys. F. schultzei and F. occidentalis were uncommon during the Fall, comprising 6.6% and 7.6%, respectively. T. tabaci, the most important thrips of onions worldwide, was uncommon during Fall (3.0%). Others species under the genus Arorathrips were found associated to onions especially during the Fall. In addition, species of the Tubulifera suborder were observed. All F. occidentalis specimens collected were male. Population dynamics patterns of Frankliniella spp. were similar during the two seasons evaluated. Two field experiments were established to study the interactions between A. alli, thrips and Acidovorax avenae ss. citrulli. Data analysis is in progress.
Impacts
Three Frankliniella species were identified associated with young emerging leaves in onions: F. occidentalis, F. schultzei and F. fusca. This is the first report of Frankiniella spp. affecting onions in Puerto Rico. These findings are very important because of Frankiniella spp. potential to affect other vegetables cultivated in close proximity and their ability to transmit important plant viruses. Understanding the interaction of onion pathogens with insect pests in the tropics will allow us to develop better control management practices.
Publications
- Feliciano M. and L. I. Rivera. 2006. First report of powdery mildew of onion caused by Leveillula taurica (Lev.) Arn. in Puerto Rico. Proceedings of the Caribbean Food Crop Society. San Juan, P.R.
- Feliciano M. and L. I. Rivera. 2006. Frankliniella spp., new pest of onions in Puerto Rico. Proceedings of the Interamerican Society for Tropical Horticulture. San Juan. P.R.
- Fernandez, J. and L. I. Rivera. 2006. First report of leaf spot of onion caused by Pleospora eturmiuna in Puerto Rico. Phytopathology 96:S35.
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