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5. Pests and Diseases of Hibiscus by F.D. Hockings
Hibiscus are attacked by a number of pests and diseases. Some are not very important but others require treatment to prevent the leaves and flowers from being destroyed.
Correct diagnosis of problems is of utmost importance because fungicides are effective against fungal diseases only and pesticides are even more specific. Each pesticide is effective against only a particular class or group of pest. If you use the wrong chemical you are wasting your time and money and your plants will continue to be damaged.
Always read the label on garden chemicals. Be aware of what the actual chemical is (the active ingredient) and not just the trade name. Carefully follow the recommended dilution rate stated on the label; too weak may not control the problem and too strong may harm the plants or more importantly you. Metricated measuring receptacles are inexpensive, so do not guess quantities. Addition of a non ionic wetting agent such as Agral or X77 will improve the effectiveness of sprays, but thorough coverage of both sides of the leaves is the key to successful pest and disease control. Spray materials should be mixed fresh for each spraying operation; do not use solutions that have been mixed and left overnight. If the plants are drought affected or if the temperature at the time of spraying is too high (32°C or 90°F or higher) plants may be damaged by sprays. Water plants thoroughly a day or two before spraying and apply sprays in the early Chewing Pest morning or late afternoon.
Wettable powder forms of chemicals are safer for plants than emulsifiable concentrates and further more, two or three wettable powders may be mixed with relative safety. Malathion, Lannate and several systemic insecticides sometimes cause injury to hibiscus plants.
In general, plants that are grown in clean
surroundings and are properly fertilised and regularly (but not too
frequently) watered, are less likely to be attacked by diseases and pests.
These include caterpillars and grasshoppers. Caterpillars are the larval stage of moths and butterflies. They devour foliage, sometimes stripping a plant of all its leaves. The Heliothos grub and the cabbage looper (commonly called inch worm) are the most prevalent worm pests of hibiscus buds, flowers and foliage. The Heliothos grub is nearly 5 cm (2 in) long when full grown, yellowish, green, or brown or lengthwise light and dark stripes. It is usually found in the bud or flower. The cabbage looper is greenish, the body tapering to the head, with a thin white line along the body. It is more often found on the leaf.
Grasshoppers and katydids consume large
quantities of foliage. Katydids are green and feed at night. Neither are
found in great numbers. Control to eliminate immediately, before they
become too numerous. They can frequently be removed by hand.
These include aphids, jassids or leaf hoppers, mealybugs, scale insects, bugs and mites. Their mouth parts limit their feeding to piercing the surface tissue of the plant and sucking the sap.
Mealybugs are important pests. They excrete
honeydew which attracts ants and serves as a medium for the development of
sooty mould. Mealybugs are soft bodied and scale like insects usually
covered with powdery or cottony, wax like material. They are around 6 mm (
%4 in) long when mature and make cottony nests at twig joinings and under
Aphids or plant lice are small, soft bodied
insects about the size of a pinhead which attack new growth, causing
leaves to curl and blossoms to be malformed. They are green to brownish,
with one type being black. Ladybugs and aphid lions are natural enemies of
Thrips are tiny, fast moving, yellow winged
insects. They leave silvery patches on leaves and cause buds to drop.
Close examination with a
Snails and slugs are night feeders and
injure plants, especially in damp, shady areas. Shiny trails may be seen,
and snail baits should be applied when the pests are most active, e.g.
dewey mornings or in wet weather.
Ants are fond of the honeydew excreted by aphids and mealybugs, and they may protect and move these pests around from plant to plant. Chlordane and Baygon are effective controls directed to a broad area around the ant hills as well as in the centre of the hill.
Root rots and collar rots: Several
species of fungi may cause soft rotting of roots and sometimes also stems.
Infected plants will often wilt as though they are short of water. In
general, root rots are associated with over wet conditions brought about
by overwatering or poor drainage. Treatment involves watering less
frequently and improvement of soil drainage as well as soil drenches with
fungicides such as Terrazole (Terrazole WP) or Fongarid (Fongarid).
Bud drop: This is one of the most common physiological disorders of hybrid hibiscus. The problem is more severe in some seasons than in others and some varieties are more severely and more regularly affected than others. Sometimes an improvement can be effected by shifting the plant to another part of the garden. Sometimes if a variety is severely and continually affected by bud drop, it is better removed and replaced with another variety. The most common causes of bud drop appear to be lack of food or lack of water. Excessive amounts of nitrogen, particularly when associated with foliar fertilisers, have been known to cause bud drop. Double flowered forms seem more affected than singles. Regular watering and fertilising is the best cure.
Yellow leaves: Hibiscus bushes
naturally discard their old leaves several times a year. The larger older
leaves on the bottom of the bush will turn bright yellow a few days before
they drop. This is perfectly natural and should cause no concern, unless
of course the yellowing and dropping continues up the stems to the top of
the bush. If this happens it is usually caused by excessive fertiliser or
moisture around the root zone. The yellowing is usually triggered off by
an excessively hot day, cold snap, or by spraying with unsuitable
insecticides such as Malathion or Lannate. A number of the systemic
insecticides will also cause yellow leaves, and could lead to complete
defoliation if used constantly. Worrying is of no help; if you are really
concerned don't look at your bushes for a couple of days, by then the
leaves will have dropped and the plants be looking good again. Remember
the old leaves must die for the new ones to take their place.
When spraying on a regular basis, it is advisable to cover every part of your body for self protection. Cover ails or overalls, boots or shoes, safety gloves, respirator and safety glasses are adequate protective apparel. Do not smoke while handling poisons or spraying. Shower well after spraying, and wash clothes separately from normal washing. Obtain a poison and antidote list from your Department of Agriculture or Primary Industries. It's a must to have on hand. Make sure your spraying equipment is in good working order and condition at all times. Thoroughly wash the equipment after use so it is ready for next time.
Do not use your spraying equipment for any weedicides at all, particularly herbicides containing hormone 2 4 D and 2 4 5 T. Have a separate unit for this as these herbicides are particularly toxic to hibiscus and other plants, causing malformed leaves and flowers. It will take your plant a long time to get over this type of damage if it ever does. Remember when spraying with herbicides that any slight drift can cause severe damage, so keep well away from your precious hibiscus. Never use white oil sprays and fungicide sprays mixed together as this will have toxic effects. Always read the compatibility chart on the label before mixing two or more sprays together! Never use left over sprays that were pre mixed the day before. It is desirable not to pour excess sprays down drains where they can be washed into creeks, streams and rivers. Burying this waste is one safe method of disposal.
There are many different types of sprayers
on the market, some are excellent products whilst others barely do the job
for which they were intended. Choose a sprayer that will do the job for
you with the least amount of effort. Remember hibiscus are reasonably
large plants and require fairly frequent spraying and if this turns into a
heavy chore or major operation then the task is more apt to be put off,
resulting in more damage to your plants. A Big Boy spraying attachment
which uses the pressure from your water supply is an easy method of
applying sprays, particularly when there are plenty of large hibiscus
bushes in the garden. It does use a little more spray but the savings in
effort and time more than compensate for this. The Rega gen spray and uni
spray are also ideal units. Their adjustable spraying heads make it easy
to reach high branches. A little extra time spent in choosing the right
sprayer for your needs will result in less time spent in spraying.
When pests appear, the first thing to do is
identify them. The next step is to select the right spray for that
particular pest. In the past we had certain knockdown sprays that simply
killed everything, poisoned the garden beds and would not break down for
many years. These chlorinated hydrocarbons were accepted as the great
breakthroughs in their time, saving people time and money by only spraying
once. We know now that the war on pests is a constant one. Instead of
killing off harmless predators by indiscriminate spraying, selecting the
right spray for the right pest keeps the predators around to help us in
the war against the pests.
Past experience tells us not to spray hibiscus regularly with the same insecticide. Always try to alternate your sprays, e.g. if you use endosulphan this week use diazinon the next time you spray, then change to carbaryl and so on.
While the best time for spraying is generally not during the heat of the day, for a bad infestation of hibiscus beetle the middle of the day is best. At this time the blooms are wide open and you can direct the spray onto the beetles it is just a waste of spray and energy if you do the spraying when the flowers are not fully open. If you have a problem with army worms or heliothos caterpillars, spraying toward evening is advised.
Always read the instructions on the label carefully; if in doubt ring your local nursery or Department of Agriculture for correct dilutions. Many insecticides have very small hard to read instructions (a good magnifying glass is handy for reading labels on bottles). A well defined medicine glass is also necessary for correct dilution rates. It is always a good idea to use a wetting agent when spraying as this helps the spray `stick'. White oil is good for this purpose; however, it is not recommended on hot days and never with fungicides.
Do not spray for the sake of spraying; sometimes only a couple of caterpillars are causing the damage and they can be picked off by hand. Aphids can often be hosed off, and snails captured by using an old terracotta drainpipe.
Do not put off spraying either. An infestation of beetle or army worm can gallop away and it will take a lot of spraying and a long time for the plants to recover. Look at your plants regularly; neglect is the biggest cause in the outbreak of insect pests. Remember also, healthy plants can repel insect pests better than unhealthy ones. Keeping your plants well watered and nourished keeps them free from pests and diseases as well.
Investing in a good sprayer is well worth a few dollars extra for the time saved. Learn to use it properly for each pest. Spray where the pests are, e.g. under leaves for loopers and aphids, in the flowers for beetles and caterpillars, around the base of plants for army worm. Do not be frightened to ask questions regarding the use of certain sprays on certain pests; most nurseries these days have good qualified staff who are only too happy to offer expert advice on insecticides and spraying.
Never mix insecticides together, even though many insecticides are compatible homemade 'cocktails' are definitely not the way to go. Mixing insecticides and fungicides together can be hazardous to hibiscus. Different plants are attacked by different pests there is no `wonder insecticide' available to combat all the pests on all the different plants. Use the insecticide applicable to your problem and you will find better results obtained each time.
Excessive spraying is just as damaging as not spraying at all. Use the right insecticide and spray only when necessary. Read the instructions carefully and clean your equipment thoroughly after use. Encourage the predators who in turn will help keep a lot of pests down.
Not many fungicidal problems are encountered in growing hibiscus; however, an infection of Phytophthora or Pythium can be fatal. While there are fungicides to help combat these diseases, the control is often more expensive than replacing the plant with a new and healthy specimen. When a plant succumbs to these diseases it is good practice to remove the infected soil and replace it with fresh soil before replanting. Selecting plants that have been grafted onto good disease resistant rootstock, such as H. albolacinatus `Ruth Wilcox', will ensure that the need to spray for such diseases is kept to a minimum. Culture that keeps the plants vigorous and healthy will aid in warding off many fungus diseases.
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