Leaf and stem spot of Alternanthera philoxeroides (alligatorweed) in Australia caused by Nimbya sp.

Abstract

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 eight body cells and a slender, septate apical beak of up to 2·5 times the conidium body length. Conidium body dimensions were 62–112 × 14–19 μ m (average 87 × 16 μ m). Overall length averaged 245 μ m and many beaks exhibited an apical swelling of 4–5 μ m diameter. Koch’s postulates were satisfied by inoculating alligatorweed with 5 × 10 4 conidia mL − 1 in the glasshouse. Purple/ red leaf lesions with a pale centre developed within 3 days, and elongate lesions on stems by day five. Some isolates caused chlorosis, leaf abscission and stem fragmentation. The Nimbya species was exclusively reisolated 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, conidial morphology (beak length, body length, width, shape, septation and ornamentation) of the NSW isolates does not fit the description of N. alternantherae (Simmons, 1995 and personal communication). 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.