Publication Type:Journal Article
Source:Plant Disease, Volume 100, Issue 11, p.2336 - 2336 (2016)
Black wattle (Acacia mearnsii), a tree species native to Australia, is considered the main source of bark for the tannin industry worldwide. It is the third most cultivated forest species in Brazil. Gummosis, caused by Phytophthora spp., is a major disease affecting black wattle plantations in that country, where the disease incidence can reach 43%. The most common disease symptoms are lesions on the trunk, which may or may not be accompanied by gum exudation. Severe infection can lead to plant death. Phytophthora nicotianiae and P. bohemeriae were reported as causative agents of black wattle gummosis in Brazil (Santos et al. 2006). In South Africa, besides these species, P. meadii was also recorded on black wattle (Roux and Wingfield 1997), and P. frigida on green wattle (A. decurrens) (Maseko et al. 2007). A survey in 6-year-old black wattle plantations located in the Piratini and Cristal counties in the state of Rio Grande do Sul in 2008 revealed the occurrence of a third Phytophthora species causing gummosis on black wattle in Brazil, P. frigida. Twenty-four isolates were obtained, and all were identified as P. frigida based on morphological characteristics and the sequence of portions of the ITS-5.8S rDNA, and cox I and cox II genes. Morphological characterization of colonies on carrot agar medium (CA) revealed colonies of all isolates with dense aerial mycelium, and five different colony patterns were observed: stellate, cottony, petaloid-to-cottony, slightly stellated, and slightly rosaceous. The colony growth rate was 12 mm/day at 24 to 30°C. All isolates produced sporangia abundantly in 10% nonsterile soil extract when grown under constant light. Most of the sporangia had prominent papilla. Most isolates had persistent sporangia formed singly or in a loose sympodium. The sporangial shape was predominantly ovoid, though there were other shapes such as globose, ellipsoid, and obpyriform found in some isolates, including some distorted shapes. The dimensions of 50 sporangia ranged from 29 to 71 × 20 to 53 µm (avg. 46 × 33 µm), with length-to-breadth ratios of 1.3 to 1.5 (avg. 1.4). The isolates produced globose chlamydospores, terminal or intercalary, and measured 21 to 55 µm diameter (avg. 32 µm). Oogonium diameter ranged from 22 to 37 µm (avg. 30 µm). Antheridia were amphigynous and oospores were globose, aplerotic, and 18 to 31 µm (avg. 24 µm) in diameter. Portions of the ITS-5.8 gene rDNA (730 bp) and the cox I (650 bp) and cox II (650 bp) genes were amplified by PCR. BLAST search of the GenBank database revealed that the fragments for ITS-5.8S gene rDNA (KU570067), and cox I (KU570065), and cox II (KU570066) sequence fragments from isolate P92 were 99 to 100% similar with the accessions of P. frigida (Robideau et al. 2011). To confirm pathogenicity, the 24 isolates of P. frigida was used to inoculate 10 one-year-old black wattle plants. For inoculation, an agar mycelial plug from a 1-week-old colony on CA was put on the stem wound done with a cork borer (6 mm diam.). Necroses of stems were observed 4 weeks after inoculation with the presence, or absence, of gum exudation, as observed initially in the field. P. frigida was reisolated from each infected stem. This is the first report of P. frigida occurring in A. mearnsii worldwide.