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Yellow Leaves on My Roses - Part 3

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Yellow Leaves on My Roses - Part 3

by Mark Whitelaw
(part 1 - part 2)

 

'Gerrit Greve'So far, we've looked at both the enviro-mechanical reasons and the nutrient deficiency/toxicity reasons for rose leaves turning yellow. Now we will investigate the pests and diseases which cause leaves to yellow.

Pests

Spider mite

Without a doubt, spider mites are the leading garden pest for causing yellowed leaves during cool wet springs and temperate falls. Mites are an arachnid, related to the spider. Some mites are pests, but most are beneficial. Of the most common pest mites, the species most likely to attack our roses is the Two-Spotted Spider Mite. When this mite attacks your rose, it pierces the epidermis and apparently injects some of its saliva in the process. After feeding, two minute chlorotic spots appear as the leaf tissue collapses. During severe infestations, the entire plant may turn yellow and die.

Other symptoms to look for:

  • Small stippling on leaves and stems
  • Small webs cluttered with debris connecting stems near the plant's interior
  • Shortened internodes and petioles
  • Failure to bloom
  • Twisted or distorted new growth.

Treatments:

  • Opening the interior of the plant through selective pruning
  • High-pressure watering wands especially designed for pest control
  • Natural predator mites and parasites to achieve a micro-environmental balance [see Note 1]; the release of Mite Midges (Feltiella acarisuga); or a combination of both
  • A variety of organic and synthetic pesticides [see Note 2].

    Note 1: I have had excellent success controlling pest mites on my roses using the beneficial predatory mite called Phytoseiulus persimilis. It enjoys warm, humid weather and reproduces twice as fast as the pest mites.
    Note 2: Research has shown that mites can quickly mutate to pesticide-resistant strains. If using synthetic pesticides, rotate between two or three different chemical types.

Whitefly

Whiteflies resemble a tiny, white 1/16 - 1/8 in. (1.5 mm) long moth. In fact, they are not a moth at all. They are a member of the Homoptera Order of critters and thus related to the aphids, scales and mealybugs. Although the adult doesn’t look much like a scale, the nymph (larva) and pupa do. Like their plant-sucking cousins, whiteflies pierce leaf tissues causing noticeable wilting, chlorosis, loss of leaves and/or stunted growth. In addition, like their cousins, they produce "honeydew" which attracts other pests and produces a medium for "sooty mold" fungus.

Other symptoms to look for:

  • A "cloud" of "dust" when brushing against the shrub actually thousands of whiteflies flying about, heading for cover)
  • Yellow stippling on the upper surface of the leaf

Treatments:

  • Insecticidal or hand held, battery operated vacuums
  • Barrier screening
    Traps, which include sticky yellow traps, non-baited traps, light traps or pheromone traps
  • Biological controls like Beauveria bassiana, Paecilomyces fumosoroseus, or Verticillium lecanii fungi (the latter for greenhouse use); parasitoids like Encarsia formosa and Eretmocerus californicus; and a host of other predators like lacewings, lady beetles, and predatory mites
  • Extracts of Nicotiana plants and a variety of other organic and synthetic pesticides including neem oil, horticultural oil, pyrethrin and garlic water. [see Note].

    Note: Research has shown that whiteflies can quickly mutate to pesticide-resistant strains. If using synthetic pesticides, rotate between two or three different chemical types.

Disease

Blackspot

Of all the reasons for yellowing leaves, this disease is one of the primary causes. Damage usually begins as a black or brown splotch. Shortly after, the leaf tissues surrounding this spot turn yellow. This is because fungi feed only on dead tissue. In this case, the fungus exudes a chemical called ethylene to kill the leaf tissue as it advances; the result of this is the destruction of chlorophyll in the cell living tissue and the creation of a golden "halo" or yellow surface area indicating the dead tissue.

Other symptoms to look for:

  • Black or brown spot.

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Treatments:

The pathogen is susceptible to fungicides within the first 90 hours but may not be noticed. For this reason, preventive fungicide applications are best made during susceptible periods of the year if these are your preferred control means. For other controls, see Rose Diseases and Their Control.

Downy mildew

This disease is most commonly confused with Blackspot. But unlike its "cousin," the damage is first noticed at the top, new-growth areas of the plant instead of the lower leaves as with Blackspot. Infected leaves develop purple-red, irregular splotches. Mature stages of the disease manifest as gray fuzz or "down" on the undersides of leaflets and possibly yellow leaf surfaces.

Other symptoms to look for:

  • Gray "down" on leaf undersides.
  • Black or purple spots on upper leaves

Treatments:

  • Same as for Blackspot.

Brown Root Rot (Cylindrocladium sp.)

Cylindrocladium is a fungal pathogen found rarely among amateur rosarians. This disease commonly attacks tropical plants, fruit and nut crops and some trees when soil conditions are too wet. For the rosarian, this may occur during greenhouse propagation of rose cuttings. Brown Root Rot first begins near or just below the soil's surface. Leaves turn yellow as the pathogen begins to kill the cutting.

Other symptoms to look for:

  • Brown decay near the stem, at or near the soil's surface.

Treatments:

  • Fungicidal soil drench.


Viral Diseases (Rose Mosaic)

The most common of the viral diseases, Rose Mosaic symptoms are yellow, staggered patterns on the leaf. These patterns frequently look like yellow electrical streaks or veins. Some authors suggest Rose Mosaic may not be fatal to the plant, and that most of the roses in the U.S. were infected with the disease and showed few detrimental effects as of 1970. Other authors, however, document Rose Mosaic as a cause for bloom distortion, reduced flower production and size, reduced plant vigor, early defoliation, and susceptibility to winter damage.

Other symptoms to look for:

  • Yellow streaks or "lightening" patterns on leaves.

Treatments:

  • None

Over the past several weeks we have looked at a variety of reasons for leaves turning yellow. Hopefully, the next time your rose's leaves turn yellow, you will be able to refer to these articles for assistance.

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