Some problems that plague plum trees (_Prunus spp._) reveal themselves when their naturally green leaves turn yellow. Young plum trees -- those that have not produced fruit yet -- are sometimes more vulnerable to fungal diseases and soil fertility problems than established trees because of transplant shock, which is the disturbance of their roots from being dug at a nursery or removed from a container. Trees must overcome the effects of transplant shock before their roots become -- literally -- grounded, allowing the trees to stand firmly on their own.
The leaves of young plum trees may also turn yellow because of insufficient water and bacterial and viral diseases.
The first three years after planting a plum tree -- typically before it bears fruit -- are the most critical for its healthy establishment.
Cultural and Environmental Problems
When a tree experiences drought stress -- not enough water for healthy growth -- its leaves scorch and turn yellow through a process called cladoptosis. This is a defense mechanism that allows the tree to conserve water by shutting off the water supply to its leaves. As the water supply dwindles, the leaves turn yellow and fall from the tree.
Heat stress -- usually during hot, dry summers -- causes similar symptoms to drought stress.
Unlike the guidelines for transplanting other trees, a plum tree shouldn't be fertilized when you plant it. A jolt of fertilizer at planting time is too harsh for a young, newly planted plum tree and can burn the leaves, causing them to turn yellow and then brown. Clemson Cooperative Extension experts suggest the following fertilization regimen: Wait until the following spring (or late winter) -- when the leaf buds begin to open -- before applying 1 cup of a granular 10-10-10 fertilizer in a 3-foot diameter around each tree. In mid-May and again in mid-July, add 1/2 cup of ammonium nitrate or calcium nitrate in a 2-foot diameter around each tree.
In subsequent years, make two fertilizer applications -- one in March and another in August. In March, apply 1 cup of 10-10-10 around each tree for each year of a tree's age (to a maximum of 12 cups). In August, apply 1 cup of ammonium nitrate or calcium nitrate around each tree for each year of a tree's age (to a maximum of 6 cups).
Chemicals used to kill plants -- typically weed killers -- can damage nontarget plants through chemical drift or volatilization. Yellowing foliage is the result.
Plum trees are susceptible to three primary fungal diseases:
A telltale symptom of verticillium wilt is yellowing leaves that retain some green color along the veins. Cut open a branch and look inside. If you see streaking just underneath the bark, the culprit is probably verticillium wilt. The fungal pathogen that causes this disease can lay dormant in the soil as long as 10 years to attack young or established trees. There is no cure for this disease.
Armillaria Root Rot
If a young tree has been planted in an area that has poor drainage, during a time of unusually high rainfall or if you've overwatered it, the roots can become infected with armillaria root rot. Like verticillium wilt, the fungal pathogen is long-lived in the soil, and there is no cure.
Peach Leaf Curl Fungus
Peach leaf curl, caused by the fungal pathogen _Taphrina deformans_, can cause infected leaves to turn yellow, pucker and curl. If you see these symptoms, it's too late to apply a fungicide. As a preventive treatment, you can apply a fungicide twice a year -- in fall/late winter as the tree enters dormancy and just before the leaf buds open in late winter/early spring.
Bacterial Leaf Spot
The bacterial pathogen Xanthomonas campestris pv. pruni can infect plum trees, causing the leaves to turn yellow. Bactericidal treatments are often ineffective, but preventive copper sprays may be applied just as leaves drop in autumn.
Peach Mosaic Virus
Plum trees infected by peach mosaic virus are usually slow to leaf out in spring. When the leaves do emerge, they are small, yellow and crinkled. There is no treatment for plant viruses; remove the infected tree immediately.