Yellow Leaves on My Roses - Part 3
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.
- 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
- 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.
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