Question: I have several twelve year-old cherry and peach trees that have holes in the leaves. Also some of the cherries have a grayish spot forming on the fruit. I do not see any insects. What is causing the holes and what can I do about them?
Answer: It sounds as though your trees are affected by bacterial canker of stone fruit or Pseudomonas syringae. Other common names include gummosis, blossom blast, die back, spur blight, and twig blight. There are several different races of this bacteria. Symptoms may appear on some trees and not on others. These include canker development on twigs at the base of flower and leaf buds, and the base of spurs. Cankers normally spread upward, and the infected sunken areas are usually formed in late winter or early spring. Gum often exudes from the canker during the early part of the growing season. If the canker girdles a limb, it will die in short order. However, the root system stays healthy and may even produce sucker growth. Dormant leaf and flower buds may be infected and are often killed, but some invaded buds develop normally but will collapse in early summer. Leaves and fruit produced by these buds wilt and dry out. In contrast, leaves and flowers of other infected buds will remain symptomless. Leaf infections, especially on cherries, appear as water-soaked spots that later become brown and dry. At a later stage shot holes may be seen on leaves sporadically and not always symptomatic of the disease.
Control: Make sure that the trees are pruned, watered and fertilized properly. There is some indication that during mild wet winters bacteria populations can increase. Trees are particularly susceptible in sandy soils, water logged soils that drain poorly, and during prolonged drought periods. Careful watering is a must. Precautions should be taken while pruning not to spread the disease. Dipping pruners in 20% bleach solution after each pruning cut should help. Chemical control of bacterial canker is based on protective copper containing sprays applied in autumn and spring before flowering. In cherry and peach growing areas there are strains of bacteria canker that are resistant to copper sprays.
Source: Compendium of Stone Fruit Diseases. 1995. Edited by Joseph M. Ogawa, et al. The American Phytopathological Society, St. Paul, MN. Pages 48-50.
Question: My mesquite trees have round bumps on many of the limbs. In fact, some of the branches have ooze dripping from them. Is this scale?
Answer: Yes, the problem is soft brown scale in fact. Scale are a "super family" of over 1200 insects that feed on plant sap while females protect themselves with a soft or hard "shell" body covering. Males can be winged. Scale produce young by eggs or by bearing live young. The young, called crawlers, may crawl out from under mother's covering and move to another location, usually close by, and then set up "housekeeping." One to five generations will be produced each year depending on the species and environmental conditions. Scale are protected by the covering they make for themselves, and it is very hard to penetrate with pesticides. They may cause the tree to suffer some but do not kill healthy trees. I do not recommend controlling them. However, for those who want to give it a try here are a few suggestions.
Control: Physical removal by spraying a hard stream of water may work, however many times they are stuck on the plant very tightly. Rubbing off the scale with a stiff wire brush can be effective, but the neighbors may think you are nuts! Using systemic insecticides can help control scale but many times does not work very well. Suffocating or penetrating their "shell" are methods also used to kill this pest. Dormant oil sprays are used when plant leaves are no longer than a half inch in early spring. If used later leaf damage may occur. Rubbing alcohol applied to scale will penetrate their waxy shell covering and kill them. Use 70% isopropyl (rubbing) alcohol, mixing 1 to 2 cups of alcohol per quart of water. Since alcohol can damage some plants first test by spraying on a small area. Wait for a day or two to see if damage occurred. You can mix insecticidal soap up according to the label directions but substitute rubbing alcohol for half of the water. A recipe that has proven effective in the past for scale control and other insects is made by mixing one cup cooking oil plus 1 Tablespoon of dish detergent (non-citrus). Mix one to two teaspoons of this solution with one cup of water. Spray mixture on the infected plant until it drips off. It is best to spray a few leaves and then check for leaf burn the next day before spraying the entire plant. With many of these treatments the scale will not drop off of the plant but will remain attached even though they are dead. Pry some off several days after treatment to determine if the scale are dead. If not treat again.