Pests, Diseases And Disorders Of African Violets
Unfortunately African violets are not immune to attack by insect pests: some can be easily controlled, others with great difficulty, and a few not at all. The best method of beginning any control is by checking for symptoms of infestation every time a plant is handled in any way, for example when watering and grooming. Early recognition of pest attack and action to eradicate it will greatly lessen the problem.
There are two types of chemical control of pests, although it may be that both cannot be used against all insect pests. Contact insecticides act as the term implies, by the active agents coming into contact with the pest. This type is sprayed thoroughly onto all surfaces of a plant, and includes malathion, pyrethrins and resmethrin. Systemic insecticides are absorbed by a plant through the leaves and roots. They may be sprayed onto a plant to be taken in through the leaves, or watered onto the compost to be taken up by the roots. This type makes the sap of a plant poisonous to sucking insects and includes dimethoate. Additional caution should be taken with systemic insecticides as they appear to have a phyto-toxic effect on a plant when bright light falls on one which has been treated; therefore light intensity around such plants should be greatly reduced as a precaution for about one week after treatment. African violets under fluorescent lighting should be left without the lights, and plants on window-sills should be taken off and placed in a very shaded position.
Whenever using chemical insecticides always read the label and follow the directions closely. Make sure the chemical can be used on African violets safely, and never be tempted either to increase the recommended dosage or to decrease it. The former is most likely to kill the plant as well as the pest, and the latter will only improve the pest's resistance to the chemical, and if that happens the resistance is passed on to later generations of the pest, making it very difficult indeed to kill.
Biological control is now available for use against many pests, whereby a live predator or organism is used specifically to combat one pest. These controls are normally for use under glass but in certain circumstances, such as in a plant room, they may be used in the home. Bear in mind that predators starve and die once they have eradicated their specific pests, which means they have to be reintroduced should another attack occur. Bacillus organisms are so far not relevant to African violets.
The following pests are the ones which most commonly attack African violets, although you may be fortunate and never have to take control measures against some of them.
There are very many aphid species, but luckily only a few attack African violets -although these tend to be difficult to eradicate. The life-cycle of the aphid is unusual, to say the least. All individuals are unmated wingless females, and yet these are capable of giving birth to up to one hundred live young during their lifetime of about thirty days; development from egg embryo to adult takes five to six days. Even at birth these wingless females already have embryo aphids within their bodies, so that in theory it is possible for one female to produce forty-six generations in one year. When a food source becomes scarce and the wingless females become stressed as a result, a few winged females and winged males are born and fly off to mate and lay eggs. This usually happens in autumn and the eggs do not hatch until the following spring when the process of unmated wingless females begins again. If the eggs are laid in summer they hatch within a day or two, and more wingless females are born.
Aphids are sap-sucking insects, piercing leaves with their needle-like mouthparts and causing visible damage of speckling and pitting of leaf surfaces. The species which attacks African violets most frequently is the green one, although at times a black species may also enjoy a feed; both these are fat and juicy-looking. However, there is yet another tiny black species which looks like a spattering of sooty dust on the leaves.
Symptoms Of Attack
Control of Aphids - A chemical insecticide containing pirimicarb is an aphid specific, killing aphids but without harming beneficial insects. It is a contact insecticide, that is, it has to wet the aphid; it also has a slight vapor action spreading around the plant. Other contact insecticides containing rotenone or pyrethrins may also be used against aphids. Be- sides these there are biological controls for aphids, namely larvae that either parasitize them or kill and feed on them; however, the aphid population needs to be small, and a minimum temperature of 50°F (10°C) is required for the control to be effective. Large aphid populations should be reduced before biological control is attempted, by spraying with either a soft soap solution or a contact insecticide. If the latter is used, introduction of the predator should be delayed for seven days.
This mite, Polyphagotarsonemus latus, attacks the older, outer leaves of African violets by feeding on both the upper and lower surfaces, when they inject toxic chemicals into the leaves. Broad mite cannot be seen with the naked eye. They are colorless when young, turning to amber then to dark green as adults. Female mites can lay up to five eggs a day, but they have a short life-span.
Symptoms Of Attack
Control of Broad Mite - As a suggestion worth trying, control could be attempted by removing all the outer layers of leaves, thus reducing the plant to a centre crown. A more difficult control method, and one that could also be a drastic treatment, is to submerge the entire plant, compost and pot in a container of water held at precisely 110°F (43°C) for fifteen minutes. This temperature must be very closely controlled and not allowed to go above or below by even a fraction of a degree.
Steneotarsonemus pallidus is the scourge of all African violet growers. It cannot be seen with the naked eye as it is barely one hundredth of an inch long, and by the time the damage it causes is noticeable the plant is lost moreover, not only that plant but the whole collection would be infested. Under a microscope, adults can be seen to be oval in shape, amber to tan in color and they glisten as if oily. They live for about one month, and females will lay about one hundred eggs in that time. These mites also inject toxic chemicals as they feed on the young leaves of the centre crown.
Symptoms Of Attack
Control of Cyclamen Mite - It is drastic and heart-breaking procedure: every African violet is destroyed by incineration, whether mature or plantlet or leaf; the compost also goes into the incinerator. All pots and trays are soaked thoroughly in a very hot, strong bleach solution and left overnight, then thoroughly rinsed in hot water. All shelves where plants have been standing must be wiped down with a strong solution of bleach, then washed with hot water. And as a final precaution, no African violet should be brought into the home for at least three to four months to ensure that there is no re-infestation. It is quite possible that cyclamen mite is often brought into the home on cut flowers from the garden or florist.
FOLIAR MEALY BUGS
Several species of foliar mealy bug can attack African violets, but the most likely is Planococcus citri. However, all are about 0.25in (0.5cm) long, soft bodied, and covered with a white, waxy webbing that protects them from drying out. Their life-cycle is six to eight weeks, and the wingless females are mated by winged males that die immediately afterwards. The females may bear either live young, or lay two hundred to four hundred eggs in a mass; these usually hatch in five to ten days depending upon temperature. The young feed for four to five weeks, moving around a plant looking for suitable sites, and pupate on the plants, the adults emerging in one to two weeks. They excrete a large volume of honeydew.
Symptoms Of Attack
Control of Foliar Mealy Bug - A small infestation may be cleaned off plants using a cotton wool swab soaked in methylated spirit, ensuring that the larva inside each waxy woolly patch is removed. Heavy infestations may be treated by spraying the foliage thoroughly or drenching the compost with a systemic insecticide containing dimethoate, or spraying the foliage thoroughly with either a malathion or resmethrin solution. After treatment, frequent inspection of plants should be made to ensure the control is effective. Biological control of a small infestation is possible using the predator Cryptolaemus, a large type of ladybird. It lays its eggs in the cluster of mealy bug eggs, and the two larvae are similar in appearance although the predator larva is larger and moves more quickly. Both the predator and its larvae feed on the mealy bug, taking several weeks to eradicate it. With heavy infestations the populations should be reduced somewhat, using either methylated spirit or soft soap, prior to the release of the predator. Once all mealy bugs have been consumed, the waxy webbing should be washed from the plant with a soft soap solution.
SOIL MEALY BUGS
The most common species attacking African violets is the Pritchard mealy bug, Rhizoecus pritchard. Its infestation is very difficult to diagnose because it feeds on fine roots and is hidden in the compost. It is minute in size, being about half the size of a foliar mealy bug, and white in color due to the powdery wax exuded from its body, so it can look like a fragment of perlite in the compost. In fact it can go undetected until a plant is potted on or repotted. The glassy eggs are laid ten to twenty at a time and are surrounded by waxy webbing; they hatch in one to two days and begin to feed immediately. Their life-cycle lasts two to four months, and so all stages are present in an infested plant's root ball.
Symptoms Of Attack
Control of Pritchard Mealy Bug - The root ball should be thoroughly drenched with a solution of either a systemic insecticide containing dimethoate, or a contact insecticide containing rnalathion, made up at the recommended strength. After seven to ten days all compost should be washed away from the roots with hot water, the roots washed thoroughly and the plant potted up in a clean pot with fresh compost. It should be stood on a separate saucer away from other plants and given extra attention for the next three or four weeks whilst the roots are recovering. There are reports of the successful use of diatomaceous earth mixed in with the compost to control soil mealy bug. This material is the skeletal remains of diatoms and the fragments have very sharp edges; it appears that these cut the soil mealy bugs so severely that they die.
RED SPIDER MITE
This pest should be a rarity on African violets because of the fairly high relative humidity around the plants. However, Tetranychus urticae can transfer from other nearby houseplants or from cut flowers from a florist by the thread that all spiders produce. Red spider mite is found on the underside of leaves or around young growth, and it is a prolific breeder in dry conditions. It can be reddish, amber or green in color. In heavy infestations it can easily be seen moving fairly rapidly through its webbing.
Symptoms Of Attack
Control of Red Spider Mite - The easiest method of control is to maintain the relative humidity levels around the plants. Chemical sprays containing perimiphos-methyl and/or pyrethrins may be used, the former having a translaminer activity so it reaches the under-surface of leaves where this pest prefers to be. There is also a predator mite, Phytoseiulus persimilis, which will control red spider mite, although the temperature must exceed 50°F (10°C) for it to be effective. It is a voracious feeder and will devour the pests fairly quickly. However, do not use a systemic insecticide before introducing this predator.
There are two species of scale insect that are likely to attack African violets: the fern scale Pinnaspis aspidistrae, and the brown soft-scale Coccus hesperidum. The scale is wax, or resin, secreted by the body of the insect which lives under it. Fern scale is termed 'armored' because it has a hard covering; the females are light-colored and pear-shaped, and lose their legs and antennae soon after hatching so that for their remaining lifetime they are immobile. Fern scale males are long and narrow in shape and winged. Brown soft-scale adult females are brown in color, spherical in shape, and wingless; they reproduce by egg laying or live birth, and retain their legs. Brown soft-scale males are most likely to be winged, white in colour, and long and narrow in shape resembling small gnats. The hatchlings are called 'crawlers' and are very active, moving through a plant looking for a feeding site to settle and secrete their scale. All scale insects excrete honeydew.
Symptoms Of Attack
Control of sale Insects - Very light infestations may be dealt with by using a cotton wool swab or cocktail stick, removing the scale and the insect physically. A swab may be soaked in methylated spirit to help in this operation. With this method, frequent and close inspection of plants is a necessity. Chemical control is by spraying with an insecticide containing dimethoate, malathion or permethrin. A repeat dosage is recommended seven to ten days later in order to catch any surviving hatchlings or nymphs.
More commonly known as the 'fungus gnat', this is really considered to be a nuisance pest; it is very frequently present in composts that have been stored outdoors and left open to the elements, for example in garden centers. Sciarid flies are the tiny black flies often seen around plants. They breed in moist compost containing a high percentage of organic matter, the females laying minute white eggs in batches of about thirty at a time; these hatch in four to six days, and the larvae are very mobile, choosing fungi or decaying organic matter for their food. At five to fourteen days old the larvae pupate in or on the compost, the adults emerging five to six days later. The adult flies feed on nectar. Sciarid flies very rarely damage African violets; only when there is a very large population of larvae in a compost with very little organic matter will they feed on plant roots.
Symptoms Of Attack
Control of Sciarid Flies - Normally very good control is achieved by placing yellow-colored sticky traps among the plants: these traps are in fact no more than small strips of yellow plastic, coated with a non-toxic glue, yellow being an attractive color to flying insects. In the event of a heavy infestation, a drench of malathion may be used. Over-watering of plants tends to encourage sciarid flies. One other means of control is an insectivorous plant such as a pinguicula; these have sticky leaves specifically designed to catch the tiny flies, which they then ingest.
This is another nuisance pest, often to be found in the saucers of plants with very wet compost. However, even when present in very large numbers springtails do not damage African violets; it is more that they are unsightly and do not please judges of African violets if they are seen in the saucers. Springtails are about 0.125in (0.3cm) long and whitish in color, and they have a noticeable jumping action due to the spring-like structure which is normally folded beneath their bodies; this is not used when just walking about, but when the insect is disturbed it flicks it along. Adults deposit large batches of eggs in compost; the hatchlings resemble the adults, and growth is rapid, the insects reaching maturity in two or three weeks. They feed on very damp, decaying organic material.
Control of Springtails - Plants should not be over-watered, although springtails are very often taken as an indication that African violets are growing in compost of the right moisture with good drainage, and that they are well oxygenated at the roots.
There are many species of this insect pest but the most significant are the western flower thrips Frankliniella occidentalis, and the onion thrips Thrips tabaci. The adults are tiny, less than 0.1in (0.2cm) long, torpedo-shaped with narrow, fringed and feathered wings, and yellow in colour. The females lay their eggs either in crevices and cracks in the tissue of the plant, or into the tissue itself. The eggs hatch into translucent larvae and feed for about fifteen days before pupating. They feed by scraping and rasping at the leaves to suck sap, and they also feed on pollen grains. Pupae often drop down onto the compost and emerge as adults after three to nine days, depending upon the temperature; for example at 68°F (20°C) the life-cycle from egg to adult is twenty-two days. Because adult thrips are so small and are able to fly, they very readily come into the home through open doors and windows, on cut flowers brought in from florist or garden, on clothes, hands and pet animals.
Symptoms Of Attack
Control of Thrips - Rapid chemical control in the home is impossible because nicotine is not available for this purpose, as being excessively dangerous. Therefore the amateur must use other means. At the first sign of infestation, all flowers and buds must be removed from all the plants in the collection whether they show symptoms or not, because pollen is the main food source. No buds should be allowed to grow on the plants for at least one month. After all the flowers and buds have been taken off: the plants should be sprayed thoroughly and the compost drenched with an insecticide containing perimiphos-methyl -and remember to reduce the light intensity for the plants, as this agent has systemic action. After one month, buds can be allowed to grow; however when flowering commences, checks for thrips must be repeated and if any are found, once again flowers and buds must be taken off and insecticidal treatment repeated. Close inspection for reinfestation must continue.
Otiorhynchus sulcatus is a very rare visitor to plants growing in the home, but it is often found in the garden and so could come inside. The adult beetle is flightless, nocturnal and easily recognizable by its prominent snout. If seen, it is best to kill it before it can lay eggs. Nearly all the adults are female, so reproduction is parthenogenic; from spring to early summer the female will lay up to one thousand eggs, although thankfully not all hatch. The larva emerges after about two weeks and is easily identified by its 0.25in (0.5cm) long, fat, curled, creamy-colored body and very noticeable black or dark brown head. It will feed for around three months on the roots of plants, and then pupate in the compost, the adults emerging the following spring.
Symptoms Of Attack
Control of Vine Weevil - Dispose of the compost safely -do not put it out into the garden. The crown of the plant may be re-rooted after thorough washing of the damaged stem with hot water. Mix the powder gamma-BHC into the compost being used for re-rooting; inspect frequently for any subsequent root damage. When handling any compost containing the insecticide it is highly advisable to wear rubber gloves. Biological control can be carried out by the parasitic nematode Heterorhabolitis megidis. A culture of the nematode is added to water and poured onto the compost which should have been watered the day before. The compost temperature must be in excess of 50°F (10°C) for the nematode to become active against the vine weevil larvae, which will die in about three to five weeks -the warmer the temperature the quicker the action. Biological control should not be attempted until more than six weeks have elapsed since treatment with gamma-BHC.
Very few of the whitefly species attack African violets, but those that do can produce a heavy infestation quite quickly. The adult is less than 0.1in (0.2cm) long and entirely covered with a milky-white waxy powder. It has two pairs of wings, and both adults and young have mouthparts that enable them to suck sap from the leaves of plants. The females lay eggs in batches of about forty at a time on the backs of leaves, and the eggs hatch in four to twelve days; a hatchling takes the form of a crawling nymph that is nearly transparent. Once settled on a feeding site, the nymph moults and becomes legless; it is covered with a waxy scale. It will continue to feed for twenty-eight to thirty days until it emerges as an adult fly; the adults live for about forty days. Both scale nymph and adult secrete quantities of honeydew.
Symptoms Of Attack
Control of Whitefly - Control of this pest can be difficult. Thorough spraying, especially of the undersides of leaves, with an insecticide containing pyrethrins, permethrin or perimiphos-methyl will only control the adult population, therefore the treatment must be repeated every four days until adults cease to emerge. Some control of adults may be achieved by the use of sticky glue traps hanging amongst the plants; the adults are attracted to these by their color.
Biological control of whitefly is by the tiny parasitic wasp Encarsia formosa, which to complete its own life-cycle is entirely dependent on the pest because it lays its eggs in the nymph scales on the backs of leaves. A scale will be seen to turn black as the wasp's larva consumes the nymph, so that an Encarsia wasp finally emerges from the scale instead of a whitefly. Temperatures in excess of 50°F (10°C) are needed for Encarsia to begin to be effective, and the best temperature range is 65° to 85°F (18° to 30°C). When control by Encarsia is proposed, insecticides containing pyrethrins should not be used for seven days before their introduction, and permethrin and perimiphos-methyl should not be used for at least three weeks. Sticky glue traps should be removed from the plants so that Encarsia are not attracted to them.
African violets are comparatively tough plants that are not prone to attack by fungal diseases; if infection does strike it is usually as a result of bad cultivation on the part of the grower. In many instances control of the infection does not entail the use of chemical fungicides, though when necessary those with a contact or systemic action may be used. Fungicides are supplied in the form of liquid concentrates for dilution with water, or as powders to be applied either dry or to be mixed with water. The following diseases are the ones most likely to be met by the grower of African violets.
This grey mould fungus is always a secondary infection as it attacks plant tissues already damaged. It appears as grey fluffy needles on stems and leaves and spreads very rapidly, so that when seen it is really too late to use a fungicide treatment to save the plant. Therefore the plant should be incinerated, and its pot and the area where it was positioned thoroughly cleaned. Botrytis rarely transfers from plant to plant unless it is by handling another plant after the infected one.
CROWN, STEM AND ROOT ROT
These infections usually appear when cold conditions prevail, and when African violets are over-potted in wet compost.
Crown rot is the result of allowing water to remain within the young leaves of the crown. The leaves become brown, soft and mushy and the infection will spread to the surrounding leaves; if left, it will continue to spread into the stem. Root rot causes a plant to wilt and collapse because there are no healthy roots to sustain it. Usually the first thought when a plant is seen to wilt is to water it, which in this case only increases the problem because the compost is already much too wet.
The fungal infection of mildew -more often spoken of as powdery mildew -appears on the buds, flowers and leaves of African violets as a white powdery deposit. It is usually seen when plants are crowded together either in cold, wet conditions or when it is hot, still and excessively humid. The infection spreads rapidly through a collection and requires quick action.
Provided the compost is well pasteurized, this fungal disease should not affect African violets. However, it is a devastating infection. It is a soil- or compost borne fungus and is fatal to plants. Its spores are able to survive for a long time before they come into contact with a host, and then they grow hyphae which penetrate the roots or stems of the plant. Pythium also produces motile spores which will infect other plants standing on the same capillary matting or tray.
Symptoms of the disease are as follows: - The centre crown of an infected plant shrivels, its leaves begin to look whitish with brown edges, and soon afterwards the whole plant turns black and dies. Fungicides appear to have little or no effect on the problem, although it might be worth trying them.
Control - In real terms, the sole control is that the remains of the plant and any capillary matting should be incinerated, and the pots, trays and shelves cleaned very thoroughly with a strong solution of bleach. As an added precaution to prevent re-infection, no African violets should be grown in the area previously infected for several weeks. When restocking with leaves or plants, these should be watered carefully with a commercial preparation of copper sulphate and ammonium carbonate, the solution made up at the recommended strength.
This fungus grows on the honeydew excreted by pests. It should not be a problem because action should have been taken against the pests before the honeydew became infected. If it is present, however, the affected leaves should be taken off the plants, or if it is only just showing, the leaves can be washed with warm water containing a few drops of a liquid detergent.
WHITE MOULD ON COMPOST
Inexperienced growers are often worried when they see a white fluffy mould, or mycelium, on the compost surface. In fact this mould is encouraged by a compost that is too wet, and the situation is easily remedied by scraping the mycelium away and then either watering the plant with a solution of copper sulphate and ammonium carbonate, or dusting the surface of the compost with sulphur dust. The latter may appear a little messy but at least it does not make the compost any wetter than it is already.
These disorders are usually caused by inefficient culture of plants or unsuitable growing conditions, both of which can be remedied. However, not all the results of the disorders can be removed by changing either culture or conditions; generally the damage has to grow out.
WHITE NETTING ON LEAVES
At times a small patch of what appears to be white netting may appear on African violet leaves. It is caused by very cold water which if taken up by the roots and fed through to the leaf surface, will kill a small area of minute capillary veins. The patch does not increase in size, but neither does it disappear with time because the damage is permanent. Thus to prevent such patches of netting disfiguring leaves, always use water either at room temperature or tepid.
BROWN SPOTS ON LEAVES
If water droplets are left on leaves after spraying or accidentally after watering, the effect of very bright light falling on them is to convert them into magnifiers, and burned areas or brown spots result. To prevent this occurring, always remove any droplets with a soft tissue or allow the plant to dry, well out of the way of bright light or draughts. Brown spots also represent permanent damage.
YELLOW BORDERS ON LEAVES
Plants that are kept in an ambient temperature of below 60°F (15°C) for a prolonged period are likely to show a yellowish border on their leaves with yellow spots on the edges which may also curl under. These symptoms are signs of chill and are permanent. As the damaged outside layer of leaves will not recover its good green colour, it should be removed. This disorder is more likely to be seen towards the end of winter in plants that are kept in a room that is not heated overnight; it is therefore good practice to keep plants a little warmer if at all possible.
YELLOW EDGES ON LEAVES
Yellow edges and pale green leaves may also be the result of underfeeding. This disorder can happen at any time of the year and so should not be confused with the yellow borders caused by chilling. Feeding with a high nitrogen fertilizer at half the recommended strength at every watering for four weeks should improve the condition; an indication of this is that the leaves become darker green.
Obviously this is caused by over-feeding, symptomized by the young leaves in the centre of the crown bunching together and their hairs appearing to be brown-tipped because the fertilizer salts which exude from the leaves crystallize on the hairs. Unsightly permanent damage to these leaves will be done, but they should not be removed until they have grown out. Meanwhile, flush the compost with copious amounts of warm water to wash out excess fertilizer, and also wash the centre crown with warm water, drying it afterwards with a soft tissue. Any later watering should be with small amounts of plain tepid water to lessen the possibility of root rot, and no fertilizer should be given for four to six weeks. Then very weak feeding should commence, and once the condition has been corrected normal feeding should begin.
Initially this disorder may appear to have the same symptoms as cyclamen
mite attack in that the leaves in the centre crown cease to grow and have a
bunched appearance. However, it is the easiest to diagnose because all
that is necessary is for the pH of the compost to be checked by a meter,
which will show whether it is too acidic or too alkaline.