TREE SCIENCE IN THE NEWS byRobin Romero
Why do evergreen needles turn brown? Evergreen needles go through a cycle of growth much like the hairs on a human head. Usually the oldest needles farthest from the growth tip turn yellow or brown first and fall off at predictable times. Pine needles last from three to five years, for example, while juniper needles can last for 10 years. Some research has shown that needles on some species of pine and spruce trees in more northern climates have even a longer life. Excessive browning of conifer needles may not be normal. It often occurs in the spring and may be caused by low soil moisture, dry winter winds, temperature fluctuations, damage from road salt, or air pollution. In our dry climate, don’t forget to water your trees to keep them healthy.
Huanglongbing (HLB), or citrus greening disease, is having a huge affect on the citrus industry in the US. Discovered in Brazil in 2004 and found in Florida in 2005, HLB has seriously affected citrus production in a number of countries in Asia, Africa, the Indian subcontinent and the Arabian Peninsula. Wherever the disease has appeared, citrus production has been compromised and millions of trees lost.
Early symptoms of yellowing may appear on a single shoot or branch, and the yellowing usually spreads throughout the tree over the period of a year. The tree’s productivity declines within a few years. Affected leaves show vein yellowing and an asymmetrical chlorosis referred to as “blotchy mottle” which is the most diagnostic symptom of the disease. HLB can be difficult to diagnose since it sometimes resembles mineral deficiencies of zinc, iron, and manganese or is confused with other diseases like Phytophthora. The root systems of infected trees are often poorly developed. The fruit produced are few, they are lopsided, and they fail to color properly. Many fruit drop prematurely from affected trees, contain aborted seeds, and have a salty bitter taste.
Some scientists at Washington State University predict that there may be few orchards left within five years. In order to find a cure for the disease, scientists must grow the bacterium for HLB in the lab, and so far they have been unable to do so. They hope that when they identify what causes the bacterium to die in the lab, they can create protection for citrus orchards. Other universities are involved in the project, but Washington State was chosen partly because, since citrus is not grown there, there would be no chance of accidentally infecting a crop.