So far, we've looked at both the enviro-mechanical reasons and the nutrient deficiency/toxicity reasons for rose leaves turning yellow. This week we willinvestigate 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)
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