A study on avocado sunblotch disease.

Abstract

Avocado sunblotch disease is a graft-transmissible disorder known for over 60 years and has now been recorded in at least eight countries around the world. Affected trees develop yellow, depressed streaks on young stems and fruit, marked rectangular cracking of the mature bark and a decumbent style of growth. Often a tree with symptoms produces completely symptomless shoots, termed 'recovery' growth, which are latently infected. There is a reported 95 to 100% transmission of sunblotch through the seed of such branches, and "the resultant seedlings are themselves symptomless. Indexing for sunblotch to ensure that scion and, in view of seed transmission, especially rootstock material is free of the disease is very important . The standard method used for many years has been to graft tissue onto healthy indicator seedlings and observe for symptom development for 18 months to two years. One aim of the study presented in this thesis was to develop more rapid methods for detecting the sunblotch agent. By conducting the standard indexing method in a glasshouse at controlled high temperatures of 30/28º C (day/ night) and by cutting back the indicator plants every three months, the time was reduced from two years to eight months. While this represents a considerable saving in time, the ideal must be to develop a laboratory diagnostic test that requires no more than a few days, at most, to complete. A comparative study was therefore initiated on the phenol metabolism of healthy and sunblotch-infected avocados to determine whether infection causes any major change that may reliably serve as a marker for diagnostic purposes. Significantly increased peroxidase (PO) and phenylalanine ammonia-lyase (PAL) activities, decreased indoleacetic acid (IAA) oxidase activity and higher sinapic acid levels were detected in bark tissue showing sunblotch symptoms, but not in symptomless 'recovery' growth. In contrast, increased polyphenoloxidase (PPO) activity and isoenzymes, total soluble protein levels, water soluble phenols and reduced ferulic acid levels were found in the bark of all infected trees tested, both with symptoms and symptomless. However, these latter changes have been associated with other plant-virus systems and are therefore not necessarily specific for sunblotch. Neither is any sufficiently large to be definitive as a diagnostic test. Two unidentified phenols were detected in infected, mature bark, but not in infected young bark and leaves. introduced the possibility of rapid disease detection by polyacrylamide gel electrophoresis (PAGE) of extracted RNA's as used for known viroids. In this study the presence of previously reported small molecular weight sunblotchassociated RNA's was confirmed using PAGE methods requiring two to four days to complete. This thesis presents as a further development a more rapid method of PAGE detection of RNA's enabling indexing for sunblotch to be completed in under six hours. Whilst the biochemical studies did not reveal diagnostically meaningful differences between healthy and infected avocados, there were tendencies towards differences between healthy and symptomless carrier tissues, further investigation of which may lead to a future understanding of symptom development and the symptomless condition. These include apparent higher PO and lower PAL activities in symptomless carrier tissue, as well as higher PO isoenzyme a[1] and lower IAA oxidase isoenzyme a[1] activities. General studies on sunblotch-infected avocados showed that fruit from symptomless 'recovery' growth branches are significantly larger and have a higher oil content than those from healthy or diseased branches, the latter finding possibly indicating a more advanced state of maturity of 'recovery' growth fruit due to earlier flowering. The avocado sunblotch agent was shown to have an in vivo thermal inactivation point of 55º C, a temperature higher than the avocado tissue can withstand thereby eliminating the possibility of thermotherapy of infected twigs. In a host range study four lauraceous plant species, Persea Schiedeana, Cinnamomum zeylanicum, C. camphora and Ocotea bullata, were successfully infected with sunblotch by grafting from infected avocado. This is the first demonstration of any host other than avocado. A phanerogametic member of the same family, Cassytha filiformis, was shown to be able to transmit the disease from avocado to avocado. No hosts from other families were found. During an electron microscope study of sunblotch-infected avocado leaf tissue, gross alterations of the chloroplasts in the yellow areas were observed. These changes included organelle swelling, loss of grana and stroma lamellae, rearrangement of remaining membranes and presence of vesicles. Also in the yellow areas paramural bodies were encountered in higher numbers and displaying altered structure than in healthy and symptomless infected leaf tissue. This study on avocado sunblotch disease was successful in both of its aims. Firstly with regard to quicker indexing techniques, the standard method using indicator plants was shortened from two years to eight months, while a rapid, six-hour test based on PAGE analysis, was developed. Secondly, more light has been shed on the biochemical and ultrastructural effects of sunblotch on its host, the avocado, as well as providing information regarding the thermal sensitivity and the host range of the agent.