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by Nanette Londeree, Master Rosarian

We’re nearing the end of this year’s rose growing season and it may just be one of the best you’ve ever had. If you did experience problems in the garden, were you able to figure out what was wrong using some of the information we’ve been sharing over the past eight months?

Time to test your knowledge - put your detective / home doctor hat on and see if you can identify “What’s bugging” the roses below. Look carefully at the description of the rose, its growing conditions and the visual observations, then go through the summary of what we’ve covered and see what diagnosis you come up with:


  • Rose number one - a well-established climbing rose, ‘New Dawn’ growing in a Mill Valley garden with a little shade in the afternoon, hand watered, periodic applications of synthetic fertilizer

    What you see: scattered circular spots on leaves, some with the center missing
    Where you see it: many leaves throughout the plant
    When you see it: early November

  • Rose number two – a well established David Austin ‘Graham Thomas’ growing in full sun in Novato, on an automated drip irrigation system, little to no fertilizing

    What you see: tips of leaves scorched, some yellowing between the leaf veins
    Where you see it: old leaves
    When you see it: August

    Minute Pirate Bug

  • Rose number three – a three year old grandiflora, ‘Dream Come True’ growing in full sun in San Rafael, on automated irrigation with large-volume spray nozzles, heavy use of synthetic fertilizer

    What you see: tiny, oval shaped insects, purplish with white markings and a tiny, triangular head
    Where you see it: on open flowers
    When you see it: July

    Clear Dot

  • Rose number four - a five year old hybrid musk, ‘Penelope’ growing in full sun in Fairfax, automated irrigation with large-volume spray nozzles, moderate application of organic fertilizer

    What you see: stunted growth, very small leaves with few flowers
    Where you see it: entire plant
    When you see it: October

    Rose Slug Damage Clear Dot

  • Rose number five - a well-established floribunda, ‘Europeana’ growing near large trees in Corte Madera, hand watered, periodic applications of synthetic fertilizer

    What you see: skeletonized leaves
    Where you see it: random leaves throughout the plant
    When you see it: June

    Clear Dot Clear Dot

    A quick review of the basics:

    The “big picture” of plant problem diagnosis:

    ✓ Know what a healthy plant looks like
    ✓ Keep an open mind
    ✓ Know your enemies
    ✓ Take a real good look at the plant
    ✓ Think about the history of the plant and its surroundings
    ✓ Consider that it may be multiple problems
    ✓ Look for patterns
    ✓ Eliminate what it’s not
    ✓ Double-check the obvious
    ✓ Make a preliminary diagnosis

    Useful diagnostic terms

    Sign: direct evidence of the cause of the problem, either the presence of the actual pest, or some part of it.

    Symptom: the change in appearance of the plant part – e.g., spots on leaves, yellowing leaves, or deformed flower buds.

    Living factors (that create problems in roses): disease-producing organisms - fungi, bacteria, viruses, nematodes, and pests - insects, mites, mollusks, rodents, etc. Nonliving factors: mechanical (breakage, abrasions, etc), physical, environmental (temperature extremes, light, water) and chemical (nutritional deficiencies, toxicities, herbicides).

    Infectious disease: produced by living organisms (pathogens), most commonly bacteria, fungi, nematodes, viruses and protozoa.

    Biotic disease: Disease resulting from a living causal agent or pathogen.

    Abiotic disease or disorder: plant maladies caused by non-living factors; also called “physiological disorders.”

    Phenology: the timing of biological activities.

    Chlorosis: yellowing or bleaching of normally green plant tissue usually caused by the loss of chlorophyll.

    Necrosis: death of tissue accompanied by dark brown discoloration, usually occurring in a well-defined part of a plant, such as the portion of a leaf.

    Insect and arachnid pests and disease

    All mature insects have three pairs of jointed legs, one pair of antennae, compound eyes and up to two pairs of wings, while arachnids (spiders and mites) have two body parts, eight legs, no antennae, simple eyes and no wings. Methods of feeding can help in identifying pests - insects that damage roses fall into three general feeding categories:

  • piercing and sucking (poking a hole in tissue and drawing out the plant fluid), aphids being a prime example.
  • rasping – sucking mouthparts scrape away plant tissue rather than pierce it; thrips use this method
  • chewing mouthparts produce the most obvious damage where they bite, rip or tear plant tissue

    The majority of insects undergo some type of metamorphosis (a change in form) during their lifetime, either a simple metamorphosis – egg, nymph and adult, where nymphs, the young stage of the insect, look like an adult only they’re smaller and lack wings, or a complete change - egg, larvae, pupae and adult. It’s usually the larvae of these insects that cause damage (caterpillars, rose slugs, etc).

    Plant diseases are caused by living organisms called pathogens. For infectious disease to occur, three critical factors or conditions must be present – a susceptible host plant, a pathogen and the right environmental conditions. The relationship of these three elements is called the disease triangle. If any one of these elements is missing, disease will not occur.

    Putting it all together

    When diagnosing plant problems it’s important to gather as much information as you can before making any decisions, and the little “test” above is pretty basic with sketchy data. So if you got any of them right (answers below), you’re getting good at diagnostic sleuthing!

    In this series of articles we’ve looked at different elements of the plant problem diagnostic process; the basic things to evaluate and consider along with a whole slew of maladies as they affect each part of the plant – the canes, stems, leaves, buds and blooms and the roots. I hope you’ve been able to glean some new insights into identifying plant problems, and that you’ll be able to apply this knowledge to everything you grow.

    Quiz Answers: Rose number one - anthracnose; Rose number two - potassium deficiency; Rose number three - minute pirate bug (beneficial); Rose number four - crown gall; Rose number five - rose slugs

    Photos courtesy of Baldo Villegas’ website at

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    Last Modified: 08/06/2013