Submitted to: Journal of Phytopathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: June 28, 2011
Publication Date: September 15, 2011
Citation: Schnell Ii, R.J., Tondo, C.L., Kuhn, D.N., Winterstein, M.C., Ayala Silva, T. 2011. Spacial analysis of avocado sunblotch disease in an avocado germplasm collection. Journal of Phytopathology. DOI: 10.1111/j.1439-0434.2011.01838.x. Interpretive Summary: Avocado is infected by a virus-like organism called Avocado Sunblotch Viroid (ASBVd). Viroids are similar to viruses except they are not encapsulated within a protein coat. After infection the plants can remain symptomless for many years. When symptoms are expressed they include yellow or red depressed areas on the fruit surface, bleached areas on the leaf, and rectangular cracking patterns in the bark of older branches. Infection results in lower yields, inferior quality fruit, and a decline in the vigor and health of the tree. ASBVd was introduced into Florida in the 1930s and has been in the germplasm collection since 1980, when symptoms were first observed. In 1997, a new diagnostic technique was adapted to detect the viroid in symptomless plants. The technique uses a unique set of primers, reverse transcription and the polymerase chain reaction (RT-PCR) to selectively amplify the viroid. This technique has been used in three surveys to look at spread within the collection, in 1996, 2000 and 2009. In the 1996 survey 18.9% of the plants were infected with the viroid. A second survey was completed in 2000 and the proportion of plants found to be infected with ASBVd was 19.1%. The proportion of infected plants had not significantly changed during the four years between the two surveys. Based on this information and the genetic diversity among the infected accessions we decided not to remove ASBVd positive accessions from the collection. Instead physical isolation and strict sanitization was used to contain the disease. In 2009 the collection was surveyed again and fifty newly infected trees were detected that increased the infection rate to 21%. No pattern in direction of spread was discerned for non-adjacent new infections. The proportion of infected plants (historically or present) in the current collection is 24%. Despite strict sanitization procedures in field and greenhouse operations ASBVd infections continue to increase at a rate of 4.7% in the last nine years. The elimination of all ASBVd positive plants may be necessary despite the resulting loss of genetic diversity in the collection.
Technical Abstract: The first visual symptoms of Avocado Sunblotch Viroid (ASBVd) were observed in a few plants of avocado (Persea americana Mill.) in the germplasm collection at the National Germplasm Repository at Miami in the early 1980s. However, the extent of the infection was unknown because infected trees can remain symptomless for many years. 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 performed in 1996 and in 2000 found that the proportion of ASBVd positive accessions remained unchanged at 19%. A more recent survey was conducted in 2009. Over the nine years the germplasm collection increased by 57 accessions and 102 trees. Fifty newly infected trees were detected and the infection rate increased to 21%. Among the 50 newly infected trees, 24 were adjacent to previously infected trees, other newly infected trees, or plots from which infected trees had been removed. No pattern in direction of spread was discerned for the 26 non-adjacent new infections. With the exception of the oldest known infected trees the positive trees are symptomless and appear healthy. Fourteen plants previously found to be infected, were found to be negative in this survey. The proportion of infected plants (historically or present) in the current collection is 24%. Despite strict sanitization procedures in field and greenhouse operations ASBVd infections continue to increase. The elimination of all ASBVd positive plants may be necessary despite the resulting loss of genetic diversity in the collection.