New Disease Reports
New Disease Reports (2004) 10, 30.
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A leaf and stem spot of Alternanthera philoxeroides (alligatorweed) in Australia caused by Nimbya sp.

R.L. Gilbert 1*, B.A. Auld 1 and B.R. Hennecke 2

*ross.gilbert@agric.nsw.gov.au

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Accepted: 03 Dec 2004

A foliar and stem disease of Alternanthera philoxeroides (alligatorweed) occurred from Sydney to Bulahdelah in central-coastal New South Wales (NSW) in summer 2003. Foliar symptoms were purple/red circular lesions 1-3 mm in diameter and larger, often irregular, lesions up to 10 mm, with a brown centre and a purple/red border. Spots were usually discrete but sometimes grouped along the mid-rib or originating from the margin. Stem lesions were relatively rare and were elongate, often raised, purple/red with a small brown centre. There was occasional chlorosis around lesions and in some instances the whole leaf yellowed with premature abscission.

A Nimbya species was exclusively isolated from the lesions (DAR76189, DAR76190 and DAR76191). On potato-carrot-agar, conidia exhibited an even, minutely punctiform ornamentation with an average of 8 body cells and a slender, septate apical beak of up to 2.5 times the conidium body length. Conidium body dimensions were 62-112 x 14-19 µm (av. 87 x 16µm). Overall length averaged 245 µm and many beaks exhibited an apical swelling of 4-5 μm diameter (Fig. 1).

Koch's Postulates were satisfied by inoculating alligatorweed with 5x104 conidia per ml in the glasshouse. Purple/red leaf lesions with a pale centre (Fig. 2) developed within three days and elongate lesions on stems by day five. Some isolates caused chlorosis, leaf abscission and stem fragmentation. The Nimbya species was exclusively re-isolated from these lesions.

Nimbya alternantherae causes similar symptoms on alligatorweed in South America (Barreto & Torres, 1999), USA (Holcomb & Antonopoulos, 1976) and China (Xiang et al. 1998). However, conidia morphology (i.e. beak length, body length, width, shape, septation and ornamentation) of the NSW isolates does not fit the description of N. alternantherae (Simmons, 1995) (E.G. Simmons, Pers. Comm.). This is the first report of Nimbya associated with alligatorweed in Australia. Taxonomic studies and research into the use of Nimbya isolates for biological weed control are continuing.

Figure1+
Figure 1: Conidium of Nimbya sp. from alligatorweed in NSW. Filamentous beak often up to 2.5 times length of body.
Figure 1: Conidium of Nimbya sp. from alligatorweed in NSW. Filamentous beak often up to 2.5 times length of body.
Figure2+
Figure 2: Typical foliar symptoms of Nimbya sp. on alligatorweed. Artificial inoculation.
Figure 2: Typical foliar symptoms of Nimbya sp. on alligatorweed. Artificial inoculation.

Acknowledgements

We wish to thank Dr Emory G. Simmons for his comments and examination of representative isolates and Dr Gordon Murray for his editorial input.

References

  1. Barreto RW, Torres Andre NL, 1999. Nimbya alternantherae and Cercospora alternantherae: two new records of fungal pathogens on Alternanthera philoxeroides (alligatorweed) in Brazil. Australasian Plant Pathology 28, 103-107.
  2. Holcomb GE, Antonopoulos AA, 1976. Alternaria alternantherae: A new species found on alligatorweed. Mycologia 68, 1125-1129.
  3. Simmons EG, 1995 Alternaria themes and variations (112-144). Mycotaxon 55, 55-163.
  4. Xiang M, Liu R, Yeng Y, 1998. Nimbya alternantherae, a new record of the genus Nimbya from China. Mycosystema 17, 283.
This report was formally published in Plant Pathology

©2004 The Authors