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2011 Plant Management Network.
Accepted for publication 20 September 2011. Published 18 October 2011.


Leaf Spot of Pinto Bean Caused by a Long-beaked Alternaria in Southeastern Arizona


Nicholas P. Garber and Mary W. Olsen, School of Plant Sciences, The University of Arizona, Tucson, AZ 85721


Corresponding author: Mary W. Olsen.  molsen@cals.arizona.edu


Garber N. P., and Olsen, M. W. 2011. Leaf spot of pinto bean caused by a long-beaked Alternaria in southeastern Arizona. Online. Plant Health Progress doi:10.1094/PHP-2011-1018-01-BR.


In 2007 to 2010, pinto beans (Phaseolus vulgaris cv. ‘Maverick’) in several fields in southeastern Arizona manifested distinct small circular brown leaf lesions from which a long-beaked Alternaria was consistently isolated on 2% V-8 agar (Fig. 1). No other diseases were observed. Severely affected plants had partial defoliation and stunted growth. Symptoms were not observed on pods. Affected bean fields were planted in late June and grown under center pivot irrigation. Disease developed in July after monsoonal weather patterns brought rains and dust storms to the area (Fig. 2). Disease was observed in both small loci that did not enlarge and entire fields in an estimated 300 acres of 6,000 acres of pinto bean harvested in 2010 in southeastern Arizona.


Fig. 1. Lesions on pinto bean leaves caused by Alternaria sp. in southeastern Arizona.

 

Fig. 2. Alternaria foliar disease on pinto bean after monsoonal rains and dust storms.


Plant pathogenic Alternaria taxa often are identified by large conidia with long filaments. Two such long-beaked species, A. zinniae and A. brasiliensis, cause leaf spot of bean in Europe (2) and Brazil (3), respectively. Pod spot caused by Alternaria taxa was reported in Arizona in 1984 (1), but disease incidence was not considered problematic.

Koch’s Postulates were carried out on 5- to 7-day-old P. vulgaris cv. ‘Maverick’ plants using a single spore culture isolated from a naturally infected field plant. The isolate was cultured on 2% V-8 agar under a 12-h light/dark cycle (40 watt, Phillips Plant and Aquarium, Andover, MA) for 7 days. Conidia were gently washed from the agar surface with tap water. The suspension was diluted to approximately 1600 conidia/ml and sprayed to run off on the first trifoliate leaves of each of 5 to 6 plants in each of three replicates. Control plants were sprayed with water alone. Plants were enclosed in plastic bags and grown in the laboratory under lights at room temperature. Foliar lesions identical to those on diseased field plants appeared on inoculated plants within 5 days. An isolate morphologically identical to that used for inoculations was recovered from lesions. Control plants had no lesions.

Molecular taxonomic identification was conducted using the same Alternaria used in pathogenicity studies. The isolate was grown for two days at room temperature in agitated potato dextrose broth, filtered aseptically, and DNA was isolated using the FastDNA SPIN Kit and FastPrep Instrument (MP Biomedicals, Santa Ana, CA). ITS 1 and 2 and 5.8S ribosomal DNA were PCR amplified using universal primers (4) and entered into NCBI GenBank as Accession Number JN119850. GenBank megablast showed 100% shared identity with the Alternaria from bean and A. protenta and 99% shared identity with A. macrospora, A. sesami, A. sesamicola, A. zinniae, A. carthami, A. cucumerina, A. iranica, A. tagetica, and six uncultured soil fungus clones from New Mexico (GenBank Accessions: EU480118, EU479797, EU480123, EU479794, EU479793, and EU479789). Alternaria recovered from bean leaf lesions in Arizona was morphologically similar to A. brasiliensis reported in association with disease of bean in Brazil (3). Conidia of the Arizona isolate averaged 57.1 µm long and 18 µm wide with beaks of 102 µm in length, compared to those of A. brasiliensis which measured 57.3 µm × 19.8 µm and 132.0 µm, respectively (3). Both taxa also displayed a small bulb-like swelling at the tip of the filament (Fig. 3). Morphological taxonomy can be misleading in Alternaria. In spite of the host specificity of many long-beaked Alternaria and the similarity of conidial morphology of the Alternaria isolates from Brazil and Arizona, they may be distinct species. Molecular comparison of A. brasiliensis was not possible as no ITS sequences of the Brazilian isolate are available in public databases. The Arizona isolate has been deposited in the USDA ARS Culture Collection, Peoria, IL (NRRL 54710).


 

Fig. 3. Conidia of a long beaked Alternaria sp. pathogenic on pinto bean.

 

This report identifies an Alternaria disease affecting pinto beans recently increasing in incidence in progressively larger areas in Arizona (author’s observation). The similarity of disease symptoms and morphology of the pathogen to a recently characterized Alternaria and its disease of bean in Brazil (3) causes concern about its potentially damaging effects on pinto bean production in Arizona.


Literature Cited

1. Cotty, P. J., and Misaghi, I. J. 1984. Zinniol production by Alternaria species. Phytopathology 74:785-788.

2. Heuvel, J. v. d. 1970. The influence of light and dark on attack of bean leaves by Alternaria zinniae. Eur. J. Plant Pathol. 76:192-195.

3. Queiroz, F. M., Muniz, M. d. F. S., and Menezes, M. 2000. Alternaria brasiliensis sp. nov., a leaf pathogen on Phaseolus vulgaris. Mycopathologia 150:61-65.

4. White, T. J., Bruns, T., Lee, S., and Taylor, J. W. 1990. Amplification and direct sequencing of fungal ribosomal DNA 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, eds. Academic Press, New York, NY.