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2007. Plant Management Network. This article is in the public domain.
Accepted for publication 25 September 2007. Published 15 November 2007.

First Report of Macrophomina phaseolina Causing Leaf and Stem Blight of Tropical Soda Apple in Florida

Fanny Iriarte, Erin Rosskopf, Mark Hilf, Greg McCollum, Joe Albano, and Scott Adkins, USDA-ARS, United States Horticultural Research Laboratory, 2001 South Rock Road, Fort Pierce, FL 34945

Corresponding author: Erin Rosskopf.

Iriarte, F., Rosskopf, E., Hilf, M., McCollum, G., Albano, J., and Adkins, S. 2007. First report of Macrophomina phaseolina causing leaf and stem blight of tropical soda apple in Florida. Online. Plant Health Progress doi:10.1094/PHP-2007-1115-01-BR.

Tropical soda apple, Solanum viarum Dunal (Solanaceae), is a perennial weed native to South America, and is considered one of the most invasive plant species in Florida (3). Macrophomina phaseolina (Tassi) Goid. causes charcoal rot disease on more than 500 plant species worldwide (8) and is an important pathogen on many crops in the United States. Disease incidence is often greatest when plants are stressed by drought and high temperature. Sclerotia produced in infected plant tissue function as long term survival structures in soil and as primary inoculum. M. phaseolina can be seed-borne, and pycnidia and conidia formed on certain hosts enable aerial transmission (5).

In August 2006 progressive leaf necrosis was observed in S. viarum plants adjacent to the USDA-ARS greenhouse complex in Fort Pierce, FL. Leaves of the 5-month-old plants presented progressive necrosis, then dried out and dropped. Necrosis progressed quickly from petioles through the stems (Fig.1A) and caused entire branches to die (Fig. 1B). A single fungus was isolated from pieces of symptomatic stem tissue and from pycnidia excised from infected stems (Fig. 2). The fungus was identified as Macrophomina based on dark, ostiolate pycnidia, bearing large, broadly ellipsoidal, hyaline conidia (6), which appeared after 10 to 14 days of incubation on quarter-strength potato dextrose agar (PDA) at 25C under constant fluorescent light.


Fig. 1. Stem lesions (A) and whole plant symptoms (B) on tropical soda apple plants infected with Macrophomina phaseolina originally observed adjacent to the USDA-ARS greenhouse complex in Fort Pierce, FL.



Fig. 2. Profuse production of Macrophomina phaseolina pycnidia on naturally infected stems of tropical soda apple plants (A, B) and fruit infection (A).

The identity of these isolates was confirmed by sequence analysis of the internal transcribed spacer (ITS) region. The ITS4 and ITS5 primer pair (7) was used for PCR amplification of a 600 bp fragment that was gel purified (Qiagen) and cloned into a pGEM-T plasmid (Promega). Clones were sequenced on an ABI3730XL automated sequencer. Sequences were edited and analyzed using Vector NTI (Informax) and GeneDoc (4) and were all of a single type. A "blastn" search of the consensus sequence deposited on NCBI (Accession No. EF545133), revealed 100% identity to M. phaseolina (GenBank Accession No. AM410954) isolated from strawberry in Spain.

Kochs postulates were completed on 4-month-old S. viarum plants grown in soilless potting medium in the greenhouse. Fungal isolates were grown on quarter-strength PDA at 25C for 5 days in the dark and 10-mm squares were transferred to 250-ml flasks containing quarter-strength PDB and allowed to grow for 4 days at 25C on a rotary shaker. Fungal mycelium was blended and diluted with sterile deionized water to make 1 liter of inoculum. S. viarum foliage was dipped into the mycelial suspension for 30 sec and then placed in a growth chamber at 29C and 90% relative humidity. Foliage of control plants was dipped into sterile water. Three days after inoculation, leaves developed marginal and veinal necrosis (Fig. 3) that quickly progressed to the petioles and stems by the fourth day. Symptoms on plants inoculated with M. phaseolina resembled those first observed in the field, whereas control plants remained healthy during the observation period. Pycnidia of M. phaseolina were present on infected tissues and the fungus was isolated from the symptomatic host leaves, prickles, and stems. To the best of our knowledge this is the first report of M. phaseolina as a pathogen of S. viarum. The high level of virulence of M. phaseolina observed in this study suggests that it has the potential to become a limiting factor for the persistence or spread of this weed in Florida. However, since M. phaseolina is a pathogen of many desirable crops, inoculum from tropical soda apple infestations could enhance the potential for severe disease in economically important crops growing nearby. This report provides further evidence of this noxious weed serving as a reservoir for potential pathogens of vegetable crops (1,2).


Fig. 3. Initial infection and necrosis spreading to leaf, petioles and stems of young tropical soda apple plants inoculated with Macrophomina phaseolina.


Literature Cited

1. Adkins, S., Kamenova, I., Rosskopf, E. N., and Lewandowski, D. J. 2007. Identification and characterization of a novel tobamovirus from tropical soda apple in Florida. Plant Dis. 91:287-293.

2. Church, G. T., and Rosskopf, E. N. 2005. First report of root-knot nematode Meloidogyne arenaria on tropical soda apple (Solanum viarum) in Florida. Plant Dis. 89:527.

3. FLEPPC. 2005. List of Florida's Invasive Species. Florida Exotic Pest Plant Council.

4. Nicholas, K. B., Nicholas, H. B., Jr., and Deerfield, D. W., II. 1997. GeneDoc: Analysis and visualization of genetic variation. EMBNEW.NEWS 4:14.

5. Pratt, R. G., McLaughlin, M. R., Pederson, G. A., and Rowe, D. E. 1998. Pathogenicity of Macrophomina phaseolina to mature plant tissues of alfalfa and white clover. Plant Dis. 82:1033-1038.

6. Sutton, B. C. 1980. The Coelomycetes. Commonwealth Mycological Institute, Kew, UK.

7. White, T. J., Bruns, T., Lee, S., and Taylor, J. W. 1990. Amplification and direct sequencing of fungal ribosomal RNA genes for phylogenetics. Pages 315-322 in: PCR Protocols: A Guide to Methods and Applications. M. A. Innis, D. H. Gelfand, J. J. Sninsky, and T. J. White, ed. Academic Press, Inc., New York, NY.

8. Wyllie, T. D. 1989. Charcoal rot. Pages 30-33 in: Compendium of Soybean Diseases, 3rd Ed. J. B. Sinclair and P. A. Backman, eds. The American Phytopathological Society, St. Paul, MN.