Diplodia mutila new reported causing seedling mortality of Iriartea deltoidea palm trees

The palm Iriartea deltoidea is one of the most abundant plant species in natural tropical ecosystems (Pitman et al., 1999). It is used by native people of the Amazonian region to construct dwellings. During the dry season in June 2006, 12 sick seedlings were sampled in an area of one hectare in lowland tropical forest in Manu National Park, Peru.

The affected leaves presented small circular necrotic spots with black pycnidia producing liquid masses of slowly maturing, non-striate, brown, 1-septate conidia measuring 26-28 × 15-20 mm. One month later, the seedlings were dead and the necrotic leaf spots had grown in size and produced abundant black pycnidia. Iriartea deltoidea fruits showing symptoms similar to those on leaves were collected in June and December 2006 in the same area. The disease was not found in nine other sampling sites along the Amazon. Single conidial isolates from leaves and fruits were grown on Potato Dextrose Agar (PDA) with chloramphenicol (100 mg/l). Conidial production was induced with a near-UV light 12-hour light/dark cycle. Based on growth characteristics and morphology of pycnidia and conidia the isolates were identified as Diplodia mutila (Sutton, 1980; van Niekerk et al., 2004). Fruit and leaf isolates were identical.

Two-month old I. deltoidea seedlings grown in a greenhouse were used for pathogenicity tests. Toothpicks were autoclaved and then infested with mycelia by placing them at the growing margins of 5-day old D. mutila cultures grown on oatmeal agar. The toothpicks were incubated at 26° C for 4 days (Silva & Juliatti, 2005). Leaves of six plants were inoculated by penetrating leaf blades with infested toothpicks. Control plants were treated with sterile toothpicks. Toothpicks simulate the mechanical damage that I. deltoidea seedlings present in natural conditions due to falling fronds and insects. The plants were covered with plastic bags and kept at 22° to 24°C and a 12-hour light/dark cycle for 16 days. Inoculated leaves showed necrotic symptoms identical to those seen in field populations. Infected seedlings died after 5 to 16 days, showing pycnidia with liquid masses of conidia. D. mutila was re-isolated from infected seedlings. The control seedlings remained healthy.

Diplodia mutila has been reported on numerous host plants worldwide (Sutton 1980). However, this is the first report of D. mutila causing Iriartea deltoidea seedling mortality in Peru or elsewhere.