Primrose (Primula)
Plant Health Problems
See Perennials for a detailed discussion of problems that may occur and are common to most herbaceous ornamentals.
Diseases caused by Fungi:
Leaf spot, Ramularia primulae.
Symptoms appear on the leaves as yellow blotches with ash-colored centers. When infection is heavy, entire leaves may brown.
Efforts to maximize plant vigor by fertilizing and watering are helpful. However, watering should be done early in the day to give the foliage a chance to dry before nighttime. It is also helpful to pick and remove symptomatic leaves as soon as they develop and to thoroughly remove plant debris in the fall. Although not usually necessary, applications of fungicides can be made when new growth emerges in the spring. Among the compounds registered for use in Connecticut is thiophanate-methyl. Consult the label for dosage rates and safety precautions.
Botrytis blight, Botrytis cinerea.
Flowers turn a papery brown and become covered with gray, fuzzy masses. Senescing flowers are particularly susceptible. Tan to brown spots with a target-like appearance can also develop on the leaves. These patches are often associated with flowers which have dropped onto the leaf surface. This disease is particularly troublesome during periods of extended cloudy, humid, wet weather.
Good sanitation practices including grooming the plants and removing spent or senescing flowers can minimize the potential for infection. These affected tissues should be carefully removed and discarded when they are dry. It is also important to avoid wetting the flowers when watering and crowding plants. Adequate spacing between the plants can promote good air circulation. Control can also be achieved with the use of fungicide sprays applied as soon as symptoms are visible. Among the compounds registered for use in Connecticut are copper sulphate pentahydrate, mancozeb, and thiophanate-methyl. Consult the label for dosage rates and safety precautions.
Diseases caused by Bacteria:
Bacterial leaf spot, Pseudomonas primulae.
Irregular to circular spots with distinct yellow halos develop on leaves. When spots are numerous, they coalesce and may kill entire leaves.
This disease can be minimized by improving air circulation by thinning the plants and by avoiding overhead irrigation since these bacteria are easily spread in splashing water. Picking and destroying infected leaves and cleaning up all plant debris in the fall are also very helpful. Any equipment or tools that come in contact with diseased plants should be disinfested with 10% household bleach, 70% alcohol, or one of the commercially available compounds.
Insect Problems:
Black vine weevil, Otiorhynchus sulcatus.
Primrose spp. Are highly preferred and susceptible to injury by this beetle. The larvae of this weevil often injure plants in nurseries and ornamental plantings by feeding on the roots. The tops of affected plants first turn yellow, then brown, and the severely injured plants die. Leaf notching by adults can also be unsightly. The 1/2" long adult weevil is black, with a beaded appearance to the thorax and scattered spots of yellow hairs on the wing covers. Only females are known. Adults are flightless and feed nocturnally. The legless grub is white with a brown head and is curved like grubs of other weevils. Adults and large larvae overwinter, emerging from May - July. The adults have to feed for 3-4 weeks before being able to lay eggs. Treating the soil with insect pathogenic nematodes may control the larvae and should be the first line of defense for landscape plantings. Acephate and fluvalinate are among the compounds registered for control of this pest in Connecticut, and may applied when there is adult feeding and before egg laying starts. The usual timing for these foliar sprays is during May, June and July at three week intervals. Insecticide resistance is very common; be aware that adults may appear to be dead following contact with fluvalinate, but may recover from poisoning within a few days. Consult the labels for dosage rates and safety precautions.
Flea beetles.
Three species, including the potato and strawberry flea beetles, feed on primrose. Among the compounds registered for control of this pest in Connecticut are carbaryl and acephate applied as foliar sprays. Consult the labels for dosage rates and safety precautions.
Greenhouse whitefly, Trialeurodes vaporariorum.
This insect infests primroses in greenhouses. The "pupal" stage of this whitefly has vertical sides, akin to a sardine can, with vertical ridges along the side, like the milled edges of a dime. The adult holds the wings tent-like above the body. Adults are white with a fine waxy covering on the body and wings. Whiteflies can be controlled with insecticidal soap or ultrafine horticultural oil, which are registered for control of this pest in Connecticut, sprayed on the undersides of the leaves. Biological control is possible with Encarsia formosa, a species of parasitic wasp. Season-long control can be achieved with imidacloprid applied as a systemic taken up by the roots. Consult the labels for dosage rates and safety precautions.
Slugs.
In the garden, slugs often eat primroses are often eaten by slugs, molluscs without a shell. Moist, shaded conditions are favored by both primroses and slugs. Slugs feed mostly at night, rasping holes in tender leaves or along leaf margins. Recent slug damage is easily recognized from the slimy, iridescent trail left by their crawling. At maturity they can be up to 4" long. They lay clusters of eggs, encased in slime, in the fall. Slugs can be controlled by lightly cultivating the ground in the spring to destroy dormant slugs and their eggs. A band of diatomaceous earth put around newly set plants will control slugs by rupturing their epidermis. During the day, slugs hide in dark and moist places, so they can be trapped by placing a small board in the flowerbed for them to hide. Scrape the slugs off the board and into soapy water to kill them. A bowl of beer, sunk into the ground with a roof of some sort, will act as a bait and the slugs will drown. Metaldehyde bait is registered for control of this pest in Connecticut. Consult the label for dosage rates, and safety precautions.
Twospotted spider mite, Tetranychus urticae.
This pest infests the undersides of the leaves, which become light yellow in color, and the plants have a generally unhealthy appearance. Sometimes the mites form webs, which more or less enclose the upper as well the lower leaf surface. Among the compounds registered for control of this pest in Connecticut are insecticidal soap and ultrafine horticultural oil. Spraying with insecticidal soap will give sufficient control if applied at least twice at 7-10 day intervals. The predatory mite, Neoseiulus fallacis, is most commonly found feeding where there are mite infestations. A single application of ultrafine horticultural oil (1/2 - 1% dilution) can be effective if predatory mites are present. Special care should be taken with soap or oil to obtain thorough spray coverage, because they only work on contact. Commercial growers may use hexythiazox or abamectin (a restricted use product) to control spider mites. Consult the labels for dosage rates and safety precautions. Avoid applying carbaryl or pyrethroids, which tend to be much more toxic to the predators than to the pest spider mites.