Molecular Identification of Sphaceloma perseae (Avocado Scab) and its Absence in New Zealand

Abstract

Avocado scab was recorded as present in New Zealand in international databases on the basis of one isolate (ICMP 10613) identified by morphological features as Sphaceloma perseae. However, sequence analysis of the rDNA internal transcribed spacer (ITS) region showed that this isolate was dissimilar to the ITS region of other Sphaceloma species, and to S. perseae. By phylogenetic analysis, isolate ICMP 10613 was identified as a species of Phaeosphaeria. To identify S. perseae reliably and quickly, specific polymerase chain reaction (PCR) primers were developed and tested. These PCR primers detected the authentic strain and another strain available from international collections, but did not detect isolate ATCC 11190, or the New Zealand isolate ICMP 10613 which were deposited as S. perseae. No other fungi commonly present in New Zealand avocado orchards were amplified by these primers, nor were three other species of Elsinoe (E. ampelina, E. fawcettii and E. pyri). By phylogenetic analysis of ITS sequence, the atypical isolate ATCC 11190 was identified as Elsinoe araliae, whereas isolate ICMP 10613 was identified as Phaeoseptoria sp. (anamorphic Phaeosphaeria). Re-examination of the scar symptoms on New Zealand avocado fruit showed they were dissimilar to herbarium specimens of S. perseae from Florida and from Cuba. Leaf symptoms typical of this disease have not been found in New Zealand, and isolations from over 1000 scars on fruit onto selective media yielded no fungi identifiable as S. perseae. These results show that ICMP 10613 was mis-identified as S. perseae. The record of avocado scab in New Zealand was shown to be incorrect, and there is no evidence that the causal fungus occurs in New Zealand.