Spatial Analysis of Avocado Sunblotch Disease in an Avocado Germplasm Collection

Avocado sunblotch viroid (ASBVd) causes an important disease of avocado, Persea americana. Symptoms of avocado sunblotch were first observed in the avocado germplasm collection at the National Germplasm Repository in Miami in the early 1980s; however, the extent of infection was unknown. An ASBVd-specific reverse transcription polymerase chain reaction (RT-PCR) protocol was developed in 1996 and used to screen every tree in the collection. Surveys in 1996 and 2000 found that although 23 newly infected trees were detected, the proportion of ASBVd-positive accessions remained unchanged at 19%. However, in a 2009 survey, 50 newly infected trees were detected for an overall infection rate of 21%. Results of spatial analyses indicate that for the older plantings, the effective range of spread increased more than threefold during the 13 year span, while in the newer plantings, the pattern of infection indicates a reintroduction of the viroid rather than natural spread. Despite strict sanitization procedures in field and greenhouse operations, ASBVd infections have increased in the USDA collection. Although genetic diversity in the collection would be reduced, eliminating all ASBVd-positive plants may be necessary to ensure that other accessions in the collection do not become infected.