Geranium, Zonal or Ivy (Pelargonium)
Plant Health Problems
Diseases caused by Fungi:
Botrytis blight, Botrytis cinerea.
This is the most common disease of geranium. 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 chlorothalonil, mancozeb, copper sulphate pentahydrate, and thiophanate-methyl. Consult the label for dosage rates and safety precautions.
Root and stem rots, Thielaviopsis basicola, Pythium sp., Rhizoctonia solani.
The above-ground symptoms of root and stem rots are usually nonspecific and include a general wilting, decline, and collapse of the foliage and the entire plant. This general droopiness or flaccid appearance is often accompanied by browning and rotting of the roots and the crown. Darkened or discolored lesions can develop on the main stem near the soil line. Yellowing and death of the outer leaves follows, until finally the entire plant is dead.
Control can be difficult once plants are infected so prevention is important. It is helpful to avoid overwatering, especially in heavy soils, and to avoid watering directly into the crown area of the plant. Highly symptomatic plants can be rogued and removed since recovery is unlikely.
Diseases caused by Bacteria:
Bacterial blight, Xanthomonas campestris pv. pelargonii.
This disease can be very serious in garden geraniums. Symptoms include a leaf spot and a wilt or blight. Leaf spots vary from scattered, minute, tan to translucent dots to more characteristic brown, papery wedge-shaped areas. Leaves may wilt and drop and branches can turn black, collapse, and rot. This bacterium is very infectious, and plants may become infected from splashing water, from contaminated soil, or from grooming and general maintenance.
Control strategies are aimed at prevention. However, removing and roguing of diseased plants is critical. It is also important to avoid overhead irrigation since these bacteria are easily spread in splashing water. 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. Chemical control is not very effective once infection is widespread but it can supplement other methods for disease management. Among the compounds registered for use in Connecticut is copper sulphate pentahydrate. Consult the label for dosage rates and safety precautions.
Diseases caused by Nematodes:
Foliar nematodes, Aphelenchoides spp.
These plant-parasitic worms attack virtually all plant parts and may cause leaf lesions, yellowing, necrosis and leaf drop, and bud malformation. Water-soaked blotches appear on the underside of the upper surface and soon involve the entire leaf. The whole plant may become stunted. The nematodes live and move in water films.
Reducing leaf moisture and removal of infected tissues, debris, or plants is important.
Diseases caused by Physiological/Environmental Factors:
Oedema or corky spot, physiological.
Raised, scab-like swellings appear on the underside of leaves. These first appear as water-soaked blisters and may turn rusty-brown with age. This disease is often confused with scale infestations. This condition is often associated with inadequate light levels as well as overwatering, especially during periods of cloudy, cool weather. This problem is most commonly found on ivy geraniums.
This problem can be minimized by careful attention to soil moisture levels, especially during periods of cloudy, humid weather. Selection of resistant cultivars is helpful since cultivars can vary in their sensitivity to this problem.
Aphids, Myzus persicae and M. solani.
Certain species of aphids, like the green peach aphid, the foxglove aphid, Myzus solani, and perhaps other species infest geranium plants. A spray of insecticidal soap, ultrafine horticultural oil or malathion, which are among the compounds registered for use against this pest in Connecticut, will control aphids. Imidacloprid, applied as a systemic to be taken up by the roots, will also provide season-long control. Consult the labels for dosage rates and safety precautions.
Corn earworm, Helicoverpa zea.
This insect injures geranium plants in the fall. The moths, with a wingspread of about 1.5", are tan with darker markings. They arrive in Connecticut each season from more southern areas after which they lay eggs singly. Caterpillars reach a length of up to 2" and color varies from brown, tan, green, or pink with light and dark longitudinal stripes. The head is golden brown and the body has small bumps and spines, giving it a rough texture. There can be two or three generations in a year, depending on when the adults arrive on winds from the south. Spinosad and Bacillus thuringiensis var. kurstaki (Bt) is registered for control of this pest in Connecticut. Consult the label for dosage rates and safety precautions.
Cyclamen mite, Phytonemus pallidus.
This mite occasionally curls the leaves of geranium. This translucent, microscopic mite often infests the new leaf and blossom buds, and can proliferate on spent blossoms. Characteristic injury includes dwarfed, thickened and wrinkled leaves. On azalea, dahlia, and other plants with stems, the distance between internodes becomes drastically shortened. Infested plants do not bloom, or blossoms may be misshapen. Cyclamen mite is not easily controlled when mature plants are badly infested: effective miticides are not available to homeowners, so a commercial applicator may be required for effective miticide applications. Effective restricted use products are abamectin, dicofol, and endosulfan. Consult the label for dosage rates and safety precautions.
Garden slugs, Limax maximus.
Greenhouses, as well as moist, shaded outdoor gardens, are sometimes infested by garden slugs, which are molluscs. They feed mostly at night, eating notches in the margins or interior of tender leaves, leaving a slimy, iridescent trail wherever they crawl. During the day, they hide under rubbish.
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 planted geraniums will control slugs by rupturing their epidermis. Placing a small board in the flowerbed for the slugs to hide under during the day makes it easy to destroy many at one time. A bowl of beer, sunk into the ground, with a roof to protect from sun and keep large animals out, will act as a bait and the slugs will drown. If necessary, chemicals such as metaldehyde, which is among the compounds registered for use against this pest in Connecticut, will control slugs. Consult the label for dosage rates and safety precautions.
Greenhouse leaftier, Udea rubigalis.
This insect often injures geranium and other plants in greenhouses. Although this small greenish-white striped caterpillar may occasionally feed on leaves, it is not a serious pest and control is rarely needed.
Greenhouse whitefly, Trialeurodes vaporariorum.
The greenhouse whitefly commonly infests geranium under glass and is often carried into the landscape where they may persist. The tiny, white moth-like adult has a mealy appearance due to the small particles of wax that it secretes. It lays groups of eggs on the underside of leaves. The eggs hatch into small oval crawlers, which then settle down and become scale-like nymphs that suck sap from stationary locations on the leaves. These then spend about 4 days in an immobile pupal stage before becoming adults. About 5 weeks are required to complete the life cycle in the greenhouse. Biological controls can be effective against whiteflies, especially in a greenhouse environment. The predatory ladybeetle, Delphastus pusillus, specializes in whiteflies and feeds on all whitefly species. The parasitoid, Encarsia formosa, can control the greenhouse whitefly in the greenhouse. Another parasitoid, Eretmocerus californicus, attacks all species and can assist in controlling minor infestations in the greenhouse. Insecticidal soap or ultrafine horticultural oil, which are among the compounds registered for control of this pest in Connecticut, sprayed on the undersides of leaves, can be used against whiteflies in the greenhouse or landscape. When applied at a low, half-percent concentration, soap is selectively toxic to whiteflies rather than parasitic wasps. Azadiractin (neem) or fenoxycarb, also directed to the undersides of the leaves, act as insect growth regulators. Repeat applications will probably be needed because some stages in the life cycle are unaffected by insecticides. Imidacloprid, applied as a soil drench, will provide season-long systemic control. Consult the label for dosage rates and safety precautions. Chemical control using conventional insecticides is difficult because of widespread insecticide resistance.
Mexican mealybug, Phenacoccus gossypii.
Mealybugs often injure geranium and other plants in greenhouses. The Mexican mealybug is among several species that attack geraniums. It attacks all stages of growth, stunting plants by causing distortion of the leaves. Malathion, insecticidal soap or ultrafine horticultural oil, which are among the compounds registered for use against this pest in Connecticut, should give good control when applied according to label directions. Ants that tend mealybugs in order to harvest their honeydew, can be controlled by lightly dusting the soil surface with chlorpyrifos. Imidacloprid, applied as a systemic to be taken up by the roots, will also provide season-long control. Consult the labels for dosage rates and safety precautions.
This insect lives in colonies. The wingless immature insects are white and burrow in wood. The brown adults have wings and can be differentiated from ants in that they do not have three distinct body segments. The colony usually nests in the moist wood of old stumps, fence posts, or structural timbers of buildings. Both in the landscape and greenhouse, these insects have tunneled in the stems of geranium plants.
Manage this problem by using imidacloprid, which is among the compounds registered for control of this pest in Connecticut, as directed on the label.