Sap blisters may be sign of sunburn or borers
: We have just harvested a huge crop of fruit from our seven-year-old apricot tree. However, I noticed sap blisters on some of the branches. The tree looks green and certainly healthy, but I’m worried that it may have some disease. I was told I might have borers, but I can’t see any. I bought something systemic to put around the tree and am wondering if that will be enough. It’s supposed to be good for getting rid of insects and borers.
A: These sap blisters, as you call them, are places where sap is oozing from the limbs, usually on their upper surfaces. Frequently these blisters are located on limbs exposed to long hours of direct sunlight. They can be caused by sunburn, or sun damage to the living wood, or by boring insects. These two often go hand in hand; borers are attracted to sun-damaged wood. Without cutting into the wood where the sap is coming from, it is really difficult to tell if it is from boring insects, but there is a likelihood that it is.
Some systemic chemicals that control borers are available, and, if you are going to apply this chemical, it is best to do that after harvesting the fruit. Then, you will have at least a whole year for the insecticide to get broken down or dissipate before next year’s harvest.
Read the label of insecticides carefully and follow their directions and precautions to the letter. Pesticides are dangerous and should only be used as a last resort, not as the preferred treatment.
At the University of Nevada Cooperative Extension Master Gardeners Orchard in North Las Vegas, we usually wait until winter and leaf drop to determine if the damage is from boring insects or not. If we see sawdust and sap coming from limbs and a lifting of the bark because the wood is dead in that area, then we either cut the bark away to expose the insects’ hiding places or cut off the limb if the damage is severe.
I also would recommend that you paint the upper surface of the limbs with a diluted white latex paint to help prevent sunburn. Do this during the winter when there are no leaves to get in the way.
Q: I increased my drip irrigation to six days a week due to the hot weather. Now, some of my shrubs look burnt. I wonder if the increased water is the problem. Could more water in the leaves cause the leaves to burn? I have two emitters per plant which give me 3 gallons per hour.
A: You told me the gallons per hour of your drip emitters but did not tell me how many minutes they are running during each irrigation. Both pieces of information are necessary to determine how much water a plant is receiving.
It is possible with frequent watering to give your plants too much water and cause some root dieback if the soil does not drain well. This root dieback and decrease in water uptake by the plant in turn can lead to leaf burning.
When plants are in the ground and they receive deep watering, they can usually go two or three days between irrigations in this climate and at this time of year unless the soil is extremely sandy. Plants in containers might require daily or sometimes twice a day watering, usually just before the heat of the day.
Woody plants in the ground seldom require daily irrigations. With annuals or smaller herbaceous perennials (plants that die back each winter to the ground and re-emerge in the spring), they most likely will need daily irrigations unless there is a thick layer of surface mulch to conserve water or they are true desert annuals and perennials.
Shallow, frequent irrigations applied to woody plants will give you problems unless you have very good drainage. If you have drip irrigation, the leaves of the plants should not get any water on them. However, if you do, these water droplets from potable water will not damage the plants. There is some belief that water droplets act like small magnifying glasses and can burn the leaves. This is not true.
If the majority of the brown leaves are on the hottest side of the tree and if this browning is at the leaf tips, then I am guessing they are not getting enough deep watering.
You can get an idea if the soil is getting too dry by purchasing a soil moisture meter for about $7 at a local nursery. Stick this meter halfway between emitters so that most of the probe is buried. After an irrigation the meter should read wet. Over a period of a day or two that needle on the meter should move toward the dry reading. When the needle is halfway between wet and dry, make sure it gets an irrigation.
Q: Should I prune a young pomegranate tree? In two years, it has grown from a small bush about 3 feet tall to over 10 feet and about as wide. It resembles something between a bush and a bramble. The tree had a lot of blossoms this spring but few of them seem to be making fruit. What do I do with this thing?
A: At this time of year you are better off not doing anything to the pomegranate tree for the remainder of the growing season. This winter, after the leaves have dropped, you must make a decision about whether your pomegranate tree will be a shrub or a tree with three to five major trunks coming from the ground.
Personally, I prefer a small multitrunked tree over a shrub for pomegranate. I think it is easier to manage. If you elect to go this direction, then this winter remove all stems coming from the ground except for three to five of the largest.
If you want to keep this plant low to the ground so it is easier to manage and pick fruit, then lower the height of these three to five stems to around 5 to 6 feet in height. When you cut these stems back, cut them at a major side branch in that 5 to 6 foot height vicinity.
Go through this small tree and remove branches that are crossing each other and generally thin out the canopy. You want to leave older wood in pomegranate because this is where new growth will emerge from next year to bear fruit. Be patient. It will produce fruit.
Bob Morris is an associate professor with the University of Nevada Cooperative Extension. Direct gardening questions to the master gardener hot line at 257-5555 or contact Morris by e-mail at morrisr@unce.unr.edu.