New Zealand has a very small citrus industry, with groves of trees grown primarily on the North Island. The extent of harmful plant diseases on citrus trees has been limited thanks to strict quarantine and agricultural laws and monitoring. While the more recent global citrus diseases of canker and huanglongbing are continuing to spread across North America and Asia, neither has entered New Zealand as of July 2010.
Citrus Brown Rot
Citrus brown rot is caused by fungi that proliferate when there is increased humidity and little air circulation. The fruit develops brown rot and prematurely drops from the tree.
Melanose causes lots of blemishes and pustules. Small dark, red-brown spots appear on infected citrus tree leaves and fruit, often merging. Fruit skin may crack. In general, melanose tends to be more of a problem on older, established trees during warm, humid weather conditions.
Leaves, twigs and fruits of citrus trees can manifest symptoms of the fungal problem known as verrucosis, or scab. It shows itself as irregular, gray, scabby, wartlike growth on fruit or stems. Although the fruits remain edible and juicy, their appearance is drastically changed, making them unsuitable for typical sale at market.
While few other worldwide citrus diseases occur in New Zealand, virus and viruslike diseases are among the most numerous problems these trees have in the island nation. While they tend to not kill citrus trees, severe types can greatly debilitate trees so that flowering or fruiting is diminished or vast parts of branches are harmed, weakening the entire tree over time. Three of the most common encountered in New Zealand are citrus tristeza virus, citrus vein-enation virus and citrus exocortis viroid. A recent "newcomer" is satsuma dwarf virus, according to the HortResearch Online resource.
Citrus Psorosis Complex
Also called "scaly bark," citrus psorosis complex is caused by a number of viruses that collectively harm an infected tree. The symptoms are variable, sometimes merely as tiny yellow flecks on leaves or fruit skins to entire branches becoming lined with scaly lesions and then dying. The sudden death of emerging young shoots on plants also is suggestive of this complex.