If the lower limbs on your almond tree are turning brown, you have a problem.
While it is normal for the leaves on lower limbs to turn yellow because of being shaded by the limbs and leaves above, lower limb dieback (LLDB) goes much farther and can result in the death of your tree. LLDB first appeared in 2005. Scientists do not yet know what causes this condition, but learning how to avoid it can save your trees.
Symptoms of lower limb dieback
This disease normally appears in late April or May, on trees that are 7 or 8 years old. It starts out with the leaves on lower limbs yellowing, and then turning brown. Eventually, the entire branch becomes girdled by cankers and dies, right up to where it attaches to larger, scaffold branches, or the trunk. If you scrape the bark off of an affected limb, you will see brown spots in the wood. [Sorry, but I couldn't find an image I could use.]
Some almond varieties are more susceptible to lower limb dieback than others. Padre almonds are the most likely to get this disease, with Butte being a close second. Almond varieties that show some resistance to lower limb dieback include Aldrich, Carmel, Fritz, Mission, NePlus Ultra, Nonpareil, Sonora, and Wood Colony. If you are shopping for an almond bare root tree, you might consider one of these more resilient varieties.
Preventing lower limb dieback
While research is currently underway, it is believed that overly wet soil, low light levels, and root exposure to herbicides or excessive fertilizer may weaken trees, making them vulnerable to whatever it is that causes this problem. This is just the opposite of shade tree decline, in which severe drought slowly kills a mature tree, with the early symptoms being a lack of leaf cover in the upper canopy, or crown, of the tree.
Lower limb dieback occurs most often in years with a cool, wet spring, followed by high temperatures. Soil compaction and low infiltration rates are also believed to play a role in lower leaf dieback. Trees with hull rot also appear to be more likely to develop this condition. In this case, fumaric acid and other toxins are believed to accumulate in larger branches when multiple spurs are infected. In this weakened state, these trees are also more likely to be infected by fungal opportunists, such as Botryosphaeria dothidea and Phomopsis amygdali. Phomopsis amygdali causes the stoma to stay open, desiccating the tree. Botryosphaeria dothidea causes cankers on a wide variety of plants.
Keeping your trees healthy is the best way to prevent lower limb dieback. This means proper irrigation, reasonable applications of fertilizer (only after a soil test shows a need for it), and control of scale insects, which may play a role in the spread of this disease. Fixed copper, sulfur, and fungicide treatments have not been shown to be effective.
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