Pests, Diseases And Disorders Of African Violets

Insect pests

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.

Pests

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.

APHIDS

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

  • Skins cast by the moulting insects remain on the African violet leaves
    and look like whitish-colored insects. In fact these skins are often taken
    to be live whitefly.
  • Live aphids can be seen on leaves, and are difficult to pick off because
    their mouthparts are held firmly in the leaf.
  • Honeydew, which is readily produced by the aphids, makes lower
    stalks and leaves sticky. The surface the plant stands on will also be sticky.
  • Black sooty mould appears on the lower leaves and feeds on the honeydew.

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.

BROAD MITE

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

  • The leaves turn yellowish in color and become brittle, with the
    edges curling under.
  • The underside of the leaves develops a bronzed appearance.
  • The flowers become distorted and discolored.
  • Damaged leaves continue to grow, but they crack and split, giving a
    ragged appearance.

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.

CYCLAMEN MITE

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

  • The centre crown leaves remain very small, they stop growing and
    appear very hairy because of the damage caused to live tissue.
  • The centre crown becomes grey, brown or yellowish in color and dries up.
  • The centre crown leaves become twisted, bunched and brittle.
  • Flower stalks appear short, thick and deformed, the flowers fail to
    develop and those that do have streaks and/or blotches of darker hues.

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

  • Small, creamy-white, mealy cotton wool patches appear, normally
    found on leaves or in leaf axils.
  • Cream-colored grubs or larvae can be seen within the wool.
  • Leaves become yellow and the plants wilt.
  • Buds and flowers drop prematurely.

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

  • Because its roots are damaged the plant cannot take up water, so it
    becomes limp even though the compost may be very moist.
  • Very fine white threads and the insects can be seen around the roots and through the
    root ball.
  • Soil mealy bugs do not move even when touched with a needle.
  • A sure sign of infestation is the white waxy patches of webbing on the
    inside of the plant’s pot.

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

  • Leaves become speckled and yellowing, and will drop.
  • Red spider mites can be seen in their webbing on leaves and young shoots.

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.

SCALE INSECTS

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

  • Honeydew will be evident on the lower leaves and the surface on which the plant is standing.
  • Small, discolored spots appear on the upper surface of a leaf
    indicating where scales are attached on the underside.
  • A discolored and pitted area appears on the underside of a leaf where
    the scale is attached near to the main vein.

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.

SCIARID FLY

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

  • Tiny black flies which can be seen around plants.

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.

SPRINGTAILS

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.

THRIPS

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

  • Pollen scattered on flowers and leaves. Check the flowers by vibrating
    one, and watch for tiny scuttling insects.
  • The flower color becomes streaked and has a silvered appearance.
  • Leaves are discolored and spotted by faeces, rasped areas are silvered and scarred.
  • Heavy infestations mean that pests are feeding continually, and this
    may cause curled or deformed leaf growth.

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.

VINE WEEVIL

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

  • The plant suddenly collapses and will be seen to have no roots.
  • Larvae will be found in the compost.

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.

WHITEFLY

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

  • Clouds of tiny whiteflies which arise when plants are disturbed.
  • Honeydew and sooty mould are present on a plant’s lower leaves, and
    on the surface where plants are standing.
  • Nymph scales can be seen on the backs of leaves.
  • Plant leaves become yellow.

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.

Diseases

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.

BOTRYTIS

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.

Control

  • Any water lodging in a crown should be mopped away with a soft
    tissue and the crown allowed to dry.
  • Judicial watering should be carried out with either the contact
    fungicide benomyl, or the systemic fungicides bupirimate and triforine.
  • Affected leaves of the crown, provided they are not badly affected,
    should be dusted with green or yellow sulphur dust.
  • With root rot, provided the rot has not extended into the stem, the
    entire root ball should be cut away and the crown re-rooted.

MILDEW

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.

Control

  • Flowers and buds should be taken off the plants and the foliage
    sprayed with the fungicide benomyl or watered with one containing
    bupirimate and triforine.
  • Plants should be placed more widely apart for better air circulation.
  • In hot weather use an electric fan to give gentle air circulation.

PYTHIUM

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.

SOOTY MOULD

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.

Physiological disorders

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.

FERTILIZER BURNS

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.

INCORRECT pH

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.
This disorder is rarely suffered by enthusiastic growers and exhibitors,
because they root prune and repot their plants so frequently. Normally
this procedure is carried out every six months on standard and large-sized
hybrids, and every three or four months on miniature and semi-miniature
hybrids. For the grower with just a few plants on the window-sill, root
pruning and repotting should be done at least once a year in spring; that
way there should be no problems with incorrect pH.

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