Expert Contribution

Flowers and Pests

I love Spring in this area. From the end of this month through May there seems to be something new flowering every week. Not just the flowers but also the new leaves provide an ongoing delightful color display. However this is also when many pests become active. There are three very common pests that seem to be quite persistent. 

The best defense against pests is keeping the plants healthy. Stressed plants are more susceptible to attacks from pathogens and are slower to recover. My friend Dawn asked that I mention water. We’ve had no appreciative rain at the time of this writing. If we don’t get plenty soon consider watering small and medium sized trees and shrubs and even larger trees. Yes we are told “Don’t water the native Oaks”. Go ahead and water to offset drought conditions through the winter and early spring. Keeping a plant healthy is better than all of the fertilizers and poisons we can use.

Fire blight (Erwinia amylovora) is a bacterium that affects pome bearing plants in the Rose family. Pears, Apples, and Hawthorne are the trees most commonly affected in this area. There are 3 Pear species in the area: the one we grow for fruit and two ornamentals. Fire blight thrives and spreads in cool damp weather. Fog, light misty rain, and wind all contribute to the spread of Fire blight. It enters through buds as they open. Flowers and newly forming fruit will wither and die; leaves, shoots, and twigs will die and look as if they have been scorched. The bacteria will remain in the tissue and kill larger limbs if left unchecked.

Cut out the affected parts and when feasible sterilize your cutting tool between cuts. Injecting with a bactericide when they are in flower is the most effective method of control. 

Anthracnose is another cool weather pest. This fungus is most damaging to Sycamores and Ash in the area. These spores enter the leaf buds and will cause the new leaves to be small and distorted and die. Often most of the first leaves will die and shed followed by a second crop in warmer dry conditions. The fungus will migrate into the twigs and branches causing them to be distorted and die.

Inject with a fungicide just as the buds begin to swell and break to control new infection. Fall injections will be more effective, so it’s something for your autumn calendar. In severe cases cutting the tree below the infected areas can be helpful though it may cause Powdery Mildew on the new growth.

Aphids show up as the weather warms and new growth is abundant. Aphids suck juice from the leaves, use the protein, and secrete sugar. Fine sticky droplets on your car, sidewalk, or driveway are a sure indication of Aphids. Black material like soot on leaves is another sign. 

On small shrubs like Roses you can spray them with mild soapy water. Cutting back on the fertilizer on small plants can also be helpful. You will likely need to have a professional spray trees if they become too messy. A fall injection with an insecticide is the most effective control. 

Always feel free to question an arborist when there seems to be a problem with your tree. 


By James S. Duncan, ISA Certified Arborist


About The Author

Arboriculture
James S. Duncan
Atlas Tree Service Inc.
925-687-3631

James “Steve” Duncan is the manager for Atlas Tree Services and has been working for the company since 1989. He has been a Certified Arborist since 1997, specializing in helping homeowners and developers throughout the Bay Area with understanding the health and impact of the trees on their lands 

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