Pests & Disease

Growers of California natives are blessed that pest issues are such a modest issue in southern California. The pests that are a consistent nuisance to us are some of the same culprits throughout North America. For descriptions and control strategies for aphid, scale, mealybug, mildew, and other common garden problems, scroll below. For links to timely information for pests of specific concern for Central California including quarantine updates scroll to the bottom of this page.

Pest/Pathogen Control Strategies


Aphids occur in colonies of mixed ages, often on new, tender growth. Each is about the size of the head of a pin with bulbous, pear-shaped bodies. Aphids can be black, brown, green, yellow, even red. Like most sucking insects, they leave a sticky waste product called honeydew.


  • Wash insects off of plant at sink with water.
  • In the landscape, use a strong jet of water from a hose to dislodge them
  • Spray in place with an insecticidal soap
  • Mechanical control – squish insects in place with fingers
  • Repeat every 5-10 days 


Scale are often unevenly distributed on plant, but tend to concentrate in crannies, around new growth, and especially on the undersides of older leaves. Adults are disc shaped and smaller than the head of a tack, while magnification is necessary to see newly hatched scale. Though many species exist, the most common in the horticultural trade is soft brown scale, which is translucent beige. Once an adult attaches itself to the plant, it becomes immobile. Newly hatched crawlers can be easily killed with running water if they are noticed. Once adults have attached to the leaf they are very resistant to water or other chemical attacks. Scale leaves a honeydew residue.

  • Soap will kill but only if you've broken the shell
  • Some surface abrasion necessary (toothbrush, fingers, wooden skewer) to break shell
  • Young visible with a hand lens (every 7-14 days), young crawlers are vulnerable to soap and water alone


Mealybug, usually hiding in nooks and crannies near new, tender growth, is a soft woolly white and about the same size as an aphid. A layer of cottony growth insulates and protects this sucking insect. Water spray is a less effective control because of this, but alcohol and soap can penetrate. Also easily crushed by physical pressure (e.g. fingers, wooden skewer). Leaves a honeydew residue. Mealybug which attaches to plants below ground is a separate species – mealy will not move from leaves to roots or vice versa.

  • Above ground:
  • Safer soap or alcohol effective
  • Wooden skewer helps break liquid surface tension and reach deeper into nooks and crannies
  • Root Mealy:
  • Only feasible alternative to killing the plant, besides strong chemical application, is to wash roots under running water and re-pot with fresh soil


Thrips (both plural and singular) are more 'insect-like' in appearance than those mentioned above. Thrips have segmented, thin (this "-" big) bodies with rudimentary wings which make hopping and coasting on air currents possible. Not able to survive extremes in temperature or humidity, they are most active in spring and fall. Feeding damage is a silvery or pale yellow stippling effect on upper surfaces of leaves. Spots of sticky black (frass or waste) are also left on the feeding surface. Damage is more apparent than the pests themselves as they are very mobile and jump when disturbed. Thrips are weak-bodied and are damaged by rain or jets of water.

  • Strong jet of water on a regular schedule in their active months: spring and fall

Spider Mites  

Spider mites create subtle, dense webbing that coats leaves and provides protection from water or chemical spray. Sucking insects smaller than the head of a pin and difficult to see, magnification makes them easily visible as they crawl through the webbing. While many species exist, the most common in the horticultural trade is a pinkish color, usually translucent, with a pair of dark spots. Feeding damage creates a stippling effect on leaves of silvery gray or pale yellow. Webs of true spiders, generally beneficial in all respects, are much larger, cover greater distances, and are more three-dimensional. Mite webbing is smaller in scale, right on the leaf surface, or seen in the small spaces between stem and leaf. Infestations are an indication of a deeper cultural problem – try to identify the deeper problem the plant may be experiencing. Mites dislike moisture on leaves or high humidity.

  • Insecticidal soap ineffective 
  • Strong spray of water has only limited effect
  • Webbing must be damaged
  • Wipe leaf surface with paper towel soaked in alcohol
  • Be consistent in application until infestation is controlled
  • Consider deeper cultural issues of plant


Rust is a catch-all term for a large group of primitive fungi parasitic on plants. Species are often specifically adapted to a single plant species or a group of related species. Powdery pustules develop, usually on undersides of leaves. These can be fiery orange, yellow, or even gray or black. Once the affected area of leaf “blooms” with the rusts’ color, that section of the leaf dies and turns white, gray, or black. The size of the affected area varies. Microscopic spores spread on air currents or with water splash. Spores need some moisture to germinate on the leaf so avoid wetting leaves that are infected or watering in the late afternoon or evening. Late winter and spring are the most common seasons for infection. Some species require two separate plant hosts within a single year in order to complete their life cycle (e.g. the common cedar-apple rust, which jumps from junipers to toyons and back again in a single year).

  • Sulfur applied as a powder or sprayed on in solution will stop the spread of rust and can be used as a preventative
  • Defoliation (getting rid of infected leaves) slows infection rate
  • Find a way to break the reproductive cycle

Powdery Mildew  

Powdery mildew is also a fungus parasitizing plant, seen as a light dusting of silvery white 'powder' on leaves, flower buds, and stems of plants. It penetrates the surface and draws moisture and sugars from the leaf. Unlike most fungus, powdery mildew requires a dry leaf surface for spores to germinate -- water on the leaf is inhibiting to spores and to established infections.

  • Regular wetting of leaves has a negative effect on the fungus
  • Baking soda solution application very effective if used in three or four rotations, 5-10 days apart: 1g of water to 1 Tbs of baking soda and a dash of liquid soap to help it adhere to leaves
  • Improve environmental conditions by increasing air circulation and light penetration; drought stressed plants more susceptible

Other Fungi 

Grey Mold or Botrytis sp. is a soft, fuzzy, thread-like parasitic fungus, which looks very much like bread mold. It is more common in nursery environments as it is dependent on moist conditions.

  • Allow plants to get moderately dry between watering Allow free air circulation around plants
  • Usually only a problem in damp conditions

Damping off is a catch-all term for the way newly germinated seedlings will sometimes etiolate, or waste away, and is attributed to several fungi or bacteria.

  • Catch-all phrase for many pathogens
  • Sterile or near-sterile conditions always a good start for seedlings
  • Avoid over-watering
  • Good air circulation will keep water from sitting on leaves for long periods

Additional Pest and Disease Resources:

California oakworm/oak moth
Sudden oak death
Asian citrus psyllid and local quarantines
Bagrada bug for more photos, click here
Light brown apple moth and local quarantines

Goldspotted Oak Borer
Glassy winged sharpshooter/Pierce’s disease
Buy firewood locally! Moving firewood even a few miles endangers California trees, Learn More