Post category: garden troubles
The outer surface of a plant – bark, skin or wax layer – is the main line of defence. If this is damaged, and the plant is too weak to heal the injury, fungi, bacteria and viruses can get in. When the outer defences are breached, plants react by producing natural chemicals that inhibit fungal and bacterial growth.
The plant’s ability to make these defensive chemicals depends on its state of health. Weak plants, growing in unsuitable conditions, are less able to defend themselves, and therefore more likely to suffer disease, and this can be very commonly observed in gardens. Strong vigorous plants with balanced growth are very good at warding off disease attacks.
Many diseases are specific to the plants they attack, other plants being resistant. Some varieties of a particular plant have resistance to certain diseases. In some cases, plants have been bred specifically for disease resistance. For instance, most modern tomato varieties are resistant to tomato mosaic virus and leaf mould, formerly both serious diseases of greenhouse tomatoes.
Just as plants have conditions in which they thrive, the disease-causing fungi and bacteria have conditions that favour them. Moist conditions generally favour fungal diseases. Some kinds, such as apple scab and potato blight, need moisture on the plant leaves for a period of hours in order that their spores can successfully germinate and grow.
Powdery mildew on maple
Some soil-borne fungal diseases, such as root rots, prefer wet soil and actually travel through the soil water to reach new hosts. Bacteria generally need warm weather and this explains the small number of bacterial diseases in this country.
Do not splash water about indoors, or stand pots in trays of water. Outdoors, plant disease susceptible plants so that the foliage dries out more quickly. Take care not to buy diseased plants and do not leave old plants as sources of infection. Removing diseased plant parts gives good control in some cases.
The principle of control of plant diseases using chemicals is to parallel the plant’s own defences. Just as the natural wax layer on a plant leaf prevents the germination of fungal spores, chemicals – mostly based on copper or sulphur – form a protective layer on the plant’s surface. Some chemicals work within the sap of the plant, giving it systemic protection, in the same way as the plant’s own internal defence chemicals.
Control of plant diseases using chemicals depends on the accuracy of placement of the chemical and its efficacy in preventing entry or development. If a barrier chemical is placed on the leaves after the fungus has already entered, it will have no effect.
If the barrier is not complete, some leaves or parts of leaves not having been sprayed, the fungus will enter through the undefended area. If the attacking fungus has mutated a new strain that is not susceptible to the chemical, it will not be hindered in its development.
To achieve correct coverage, even using an effective chemical in good time, is not all that easy to achieve. Results using chemical disease control in amateur gardening can be quite poor.
The question arises in the amateur context of the desirability of depending on chemical disease control, given that applying chemicals routinely is time-consuming, carries some expense, is not likely to be hugely effective, and exposes the user and the environment to some risk, however small.
A question mark must be placed over any plant that routinely needs spraying. If spraying is the only way to grow a plant successfully, it should be applied properly and to a limited number of plants, only those considered essential.
Plant viruses cannot be treated by chemicals at all because they operate within the plant cells.
Tiny mites feed inside the buds of blackcurrants, causing the tissues to swell. The bud, normally elongated, takes on a rounded shape. Swollen buds are most noticeable in winter. In spring, they usually fail to open.
The direct damage – reducing the number of fruiting buds – is usually not too serious. However, the mites spread ‘reversion’, the most serious virus disease of blackcurrants, by migrating from the swollen buds at around flowering time. Leaving a bush with the virus disease and moving to a healthy one, they bring particles of virus with them.
The best way of controlling them is to pick off all swollen buds during the winter and burn them. Over a few years, this will achieve control.
Ants are not strictly a garden pest because they do not damage plants. However, their presence in large numbers can be a real nuisance. Ants are social insects, living in nests in dry soil under stones and paving.
They actually farm colonies of greenflies for the honeydew they excrete and they often move greenflies to new plants. Burrowing by ants while building nests sometimes undermines small shrubs, leaving them high and dry and prone to wilting in a warm summer.
There is no effective method of physical control. Pouring hot water into the nests does not work. If the nest is hard to find, leave some sugar in a little heap where the ants have been seen and a couple of days later there will be a trail of ants back to the nest.
Apply ant-killers at the entrance to the nest. These include Anti-ant Powder, Murphy’s Antkiller. They can need to be renewed several times until the ants are no longer seen.
Wireworms are hard, reddish or yellowish grubs about 1 centimetre long and feed on the roots of grass and weeds. The grubs are usually only a problem in new ground that has been broken out of old grassland, occasionally attacking the roots of new plants, including bedding plants.
Damage to potato tubers is common after old grass, the tubers tunnelled by wireworm grubs. In a lawn the damage is rarely significant. The larvae mature within three years of the old grassland being ploughed up and rarely appear in cultivated ground afterwards.
Whitefly is a tiny fly – about the size of a grain of sugar – that is often confused with the skins shed by greenflies when they moult. But the two are easy to tell apart – the whitefly has white wings and it flits about of the plant when the foliage is touched.
Though small, whitefly is a serious pest of greenhouse plants and houseplants, especially tomatoes and fuchsias. The flies weaken the plants by sap sucking and excrete sticky honeydew on which black moulds grow, rendering the plants very unsightly.
Avoid buying plants that are infested. Effective control can be achieved using a parasitic wasp that lays its eggs in the whitefly eggs – available from suppliers of biological control products.
Alternatively, spray three times, at weekly intervals, with a general garden insecticide, until control is achieved.
Slugs and snails are closely related molluscs; snails have shells, slugs do not. These attack many kinds of plants, especially young plants and the soft, succulent growth of herbaceous perennials and vegetables. Slugs provide a useful function by eating dead plant tissue and they are more prevalent on soils with high organic matter where manure and compost have been applied. They are more active in wet years.
Slugs and snails can be simply collected and disposed of. Collection is made easier by laying out traps of old cabbage leaves or fruit skins. Stale beer is attractive too and a shallow dish at soil level allows them to enter and drown. Apart from this general control, it is particularly important to protect young plants and strawberry fruit.
A barrier of soot or egg shells gives mixed results. A copper strip is quite effective, giving a kind of electic shock. Otherwise, chemical slug-killers can be used, based on metaldehyde, in pellets. Some of these are liquids or powders with the advantage that birds or pets cannot eat them.
Mini-pellets are smaller and more difficult for birds and pets to find than large pellets. These should be covered by a slate supported on a few stones.
Red spider mites are very tiny mites, just visible to the naked eye. Cucumber, melons, peppers, peaches, apples, strawberries, datura, blackeyed susan, busy lizzie and ageratum seem to be especially attractive but more or less any plant can be attacked, even potatoes. Outdoor plants can be affected.
The sap-sucking of thousands of mites causes stunting and yellowing of plant leaves, which often fall off. The most obvious symptom is small, yellow flecks on the upper surface of the leaf, although the pest feeds beneath. Very fine webbing can also be detected when there is a heavy infestation.
Control is difficult to achieve. Be careful to avoid buying plants that are infested. Healthy, strong plants are fairly resistant, so feed and water indoor plants properly. Spraying twice a day with clear water helps to control the mites but this may not be practical. Outdoors, the mites have many predators, usually becoming troublesome only in warm summers.
There is a very efficient predatory mite that may be introduced to control the pest in greenhouses. The predator can be ordered form garden centres for delivery by post. Reasonably good control can be obtained by spraying with a 5% solution of vegetable oil in water, dispersed with a few drops of washing up liquid. The oil coats the mites and makes movement difficult; the application will need to be repeated.
If colonies begin to build up, indoors or outdoors, a spray of a general garden insecticide gives some control but might need to be repeated.
Some of these sprays can damage certain plants so test them on a few leaves first. Also, red spider mite has built up resistance to certain chemicals, in some places. Sprays cannot be used in conjunction with the predator.
Raspberry beetle is a fairly common pest of raspberries, blackberries and loganberries, being the white grubs often found in wild blackberries. The grub enters through the stem end, which will be brown instead of white on an affected berry. It is most common in rural areas.
If the pest has attacked in the past, spray after flowering finishes, with a general garden insecticide. This spray is timed to prevent the grubs entering the fruit.
Rabbits are major pests of trees and shrubs in rural gardens, also damaging fruit trees and vegetables. The only complete remedy is to fence the garden with a 90 centimetre high fence of close-mesh netting wire, sunk 15 centimetres down and 15 centimetres towards the outside in an L-shape underground.
A deer fence needs to be two metres high to prevent deer accessing a garden and this is usually too obtrusive an expensive.
Sleeves of strong polythene stapled together around the trunks of young trees give good protection. Around flower beds, string dipped in creosote and suspended on 15 centimetres high pegs is a useful, temporary deterrent.
It has been estimated that six rabbits can consume the same quantity of fodder as one sheep, and if a sheep was in your garden every morning you would be more conscious of the damage. The damage caused by rabbits is often not fully realised. It is a gradual nibbling that can be easily missed. It is much more noticeable on some plants. The damage is very gradual. Initially only one rabbit or a few is active, but as the population grows, the level of damage can be very severe. Rabbits feed on a very wide range of garden plants. New plants and soft growth in spring may be eaten, even on plants that are not susceptible at other times. And the main season of damage is fast approaching.
Herbaceous plants can be grazed down to ground level. Foliage and soft shoots of woody plants can be grazed up to a height of about fifty centimetres by rabbits standing up on their hind legs. Bark may be gnawed away from the base of trunks, especially in winter when snow or frost makes other vegetation unavailable. Damage of this kind can kill a tree if the bark is removed all the way around the trunk. If it is noticed in time the partly gnawed trunks should be wrapped with strips of black polythene to encourage the damaged area to callus over. Rabbits also dig holes in lawns and flower beds and leave their droppings in some places.
Damage is usually caused by rabbits coming in from surrounding land. It is very rare for a rabbit problem to be contained within the confines of the garden itself. A rabbit warren nearby is the usual base for garden attacks. Warrens are usually located in dry, sandy soil, often with some cover, such as a ditch or scrubby bushes. There are a number of ways of controlling rabbit damage. One way is to create a wire mesh barrier around the garden in the form of a fence 120cm high, 15 cm below ground level and 15cm out from the garden to prevent the rabbit burrowing underneath. Apparently 15 cm is enough, they do not have the brains to dig further back. This can be placed around the entire garden site, using one-inch mesh. It will last for many years if good quality wire is used and it is a complete solution – the only complete solution in fact.
For individual plants netting can be put up as a deterrent, without the need to bury part of it underground, although netting does not look great. Wire netting or spiral tree guards can be put around the base of young trees to prevent bark scraping. Animal repellents can be applied, but are generally less than satisfactory because they do not give complete protection, particularly during wet weather or when plants are growing actively, and they have to be repeated. Rabbit baits are available for farm use and can be used if there is a warren on the land. This usually gives a temporary solution because the ground will attract new rabbits if it is suitable.
A good approach is to avoid planting particularly susceptible plants and select those that survive. A bit of investigation in neighbouring gardens often gives a good clue as to a few more plants that might survive, and it might be possible to do some swapping. A list of about fifty rabbit-resistant plants is given below, and there are others. Some plants are rabbit-proof because they are poisonous and no grazing animal will touch them, such as daffodil, snowdrop, aconitum, euphorbia and kalmia. Others are not poisonous but unpalatable. Unfortunately, it is the older stems and leaves that become unpalatable and younger shoots are often quite sweet and tasty. So it is often necessary to protect young shoots for just a few weeks. String dipped in creosote or other strong smelling substance is sometime used around rose beds, set about 20cm about the ground. Rabbits will eat unpalatable items if they are hungry enough. Growing plants that are resistant is a good solution, but it means that some plants will not be possible. However, the choice of resistant plants is quite good.
List of rabbit-proof and deer -proof plants.
Acanthus
Aconitum
Anemone
Aquilegia
Astilbe
Bergenia
Choisya
Clematis
Cornus
Cortaderia
Cotoneaster
Daphne
Deutzia
Elaeagnus
Epimedium
Eryngium
Euphorbia
Fatsia
Gaultheria
Geranium
Helleborus
Hemerocallis
Hydrangea
Hippophae
Hypericum
Ilex
Kalmia
Kniphofia
Laurel
Ligustrum
Lonicera
Pernettya
Persicaria
Philadelphus
Polygonatum
Rhododendron
Rhus
Ribes
Rosmarinus
Ruscus
Sambucus
Saxifraga
Sedum
Skimmia
Spiraea
Symphoricarpus
Syringa
Taxus
Ulex
Viburnum tinus
Vinca
Plum fruit moth and plum sawfly are fairly common pests of plums in some parts of the country. Once the pest appears, attacks usually continue in subsequent years. The caterpillar bores into the centre of the fruit, destroying it and often causing it to fall early.
The sawfly entry wound is very obvious, the fruit moth less so, just a pin-point and a tiny bluish sunken mark. The caterpillars pupate in a cocoon and emerge the following year.
Destroying fallen plums helps to reduce numbers but control will not be good but adult moths can fly in from neighbouring gardens. Spray with Hygeia Caterpillar Spray as blossoming finishes, and again three weeks later, to kill the caterpillars before they enter the fruit.
The tiny grubs of this pest feed inside developing pears, causing the fruitlets to swell and blacken. Each fruitlet can contain several grubs when they fall off the tree prematurely in June. Pick up the fallen fruitlets and destroy them. This usually gives control of this not-so-common pest.
These little beetles are difficult to find on the plant but the damage they cause is obvious – U-shaped, bite-like pieces are eaten from the edges of the leaves of peas and beans.
They are not worth controlling unless the young seedlings are very severely damaged. Derris Dust can be used.
The larvae of this insect bores into onion bulbs, destroying them. It is not common but can be a problem in some localities. Destroy wilted plants and cultivate before winter to expose the pupating maggots.
Millipedes are soil-living insects that normally provide the useful function of eating dead plant tissue. However, they can attack living plants too, especially seeds and seedlings of peas and beans, potatoes, strawberries and bulbs. Millipedes have two pairs of legs per body segment, compared with centipedes which have one. Centipedes are fast-moving, beneficial predators of pests especially slug and snail eggs and small snails.
Millipedes are most common on wet soils and in wet seasons, especially on soils high in organic material. Remove debris or incorporate it thoroughly into the soil to rot quickly. Use a little paraffin on early sown peas and beans.
Mealy bugs and scale insects, like greenflies, are sap-suckers. Mealy bugs are covered with white wax and are mainly pests of indoor plants, especially cacti and succulents. Scale insects resemble tiny shellfish in shape and colour. They affect indoor plants, especially citrus and other glossy evergreens, and outdoor plants, usually woody types, for example, beech, lime, japanese maple, lilac and bay laurel.
Chestnut scale on lime tree
Black sooty moulds grow on the honeydew excreted by both kinds, but especially scale insects. The blackened leaves of affected plants are located under higher leaves that have colonies of the pest.
The main control method indoors is to avoid introducing them on new plants. Otherwise, if there are only a few insects, brush them off with some methylated spirits on a cotton bud, or use systemic insecticides such as Hygeia Greenfly Spray.
Chestnut scale should be sprayed with a systemic insecticide such as Provado.
Brown, legless maggots are the larvae of leather jackets or daddy-long-legs. They sometimes eat the roots of a wide range of plants, including grass, weeds, vegetables and strawberries. They are usually only a problem in lawns and ground newly-broken out of grass and weeds, especially after wet summers, and near existing meadows.
Treading heavily on affected patches of lawn gives some control. If black polythene is spread over the affected area for 24 hours and lifted early in the morning, the grubs will be found on the surface, and can be brushed away or exposed for birds to eat. The application of Draza slug pellets is effective too.
Many trees and shrubs, especially apples, plums, lilac, holly and flowering cherry, are attacked by tiny flies, the maggots of which tunnel between the surfaces of leaves. Small twisting and widening tracks can be seen. Damage is rarely severe, or worth controlling.
Flea beetles are small, black or striped beetles that jump away from cabbage family seedlings when touched. These are often seen on cabbage family weeds such as shepherd’s purse and these weeds keep the population of beetles going. The flea beetles eat small round holes in the seed leaves.
Usually, the plants grow out of it and no control is necessary. If damage is severe – more than ten holes in a seed leaf – growth can be checked. A shake of Derris Dust along the rows of seedlings gives good control.
Eelworms of a large variety of species attack a wide range of plants, especially chrysanthemums, onions, strawberries, potatoes and tulips. The species are specific to the host plant in most cases, or a narrow range of hosts.
The tiny, microscopic worms get into the leaf, stem and root tissue of plants and feed in the sap, the plants often taking on a swollen or ‘bloated’ look, especially bulbous plants. Affected plants fail to flower, crop poorly and should be lifted and destroyed. Eelworms often spread virus diseases.
Potato cyst eelworm is the most common and troublesome garden pest. The eelworms feed on the roots and attach themselves. The pregnant females swell to form white or yellow spheres, just visible to the naked eye, or with a hand lens, standing out from affected potato roots. The symptoms are severe stunting of the potato plants and the production of small tubers. There are two kinds, white and yellow, the latter more in the south.
The problem builds up year after year and eventually no crop is obtained. The cysts remain in the soil, infecting new potato plants as soon as they begin to grow. The only remedy is to cease growing potatoes for at least five years, or grow resistant varieties. ‘Sante’ and ‘Valor’ are notably eelworm resistant, carrying resistance to both kinds of eelworms.
Earwigs are not really pests, only occasionally eating parts of the petals of flowers such as dahlias, delphiniums and pansies. They are often responsible for ‘mystery’ damage to flowers, but this is generally so slight as not to need control. Besides, they are beneficial predators of certain kinds of greenflies and red spider mites.
The removal of debris reduces their hiding places. Upturned pots with wood-wool or dry leaves make good traps and the earwigs can then be removed. Tapping flower heads is usually enough to dislodge them.
Considerable damage is caused to lawns, by female dogs, and to small shrubs, by male dogs, urinating on them. Keep gates closed and fence off the garden. Solutions of strong-smelling disinfectant have a temporary effect. Dogs often wear tracks on lawns and can be diverted by placing obstacles such as dead, leafless boughs, in their path.
Cats are attracted to freshly disturbed soft soil, in the same way that they are attracted to cat litter. Firm freshly cultivated soil and water it heavily to settle it down and make it unattractive to cats. Small twigs, especially with spiny leaves, deter cats without hurting them – they recognise the spines and go elsewhere. Be sure to remove the spiny leaves before they wither, because they can make weeding a painful business!
High frequency sonic cat scarers can work in some circumstances.
Cutworms are caterpillars – the larvae of night-flying moths – but they are unusual among caterpillars in that they live just below the soil surface, attacking young plants just at, or below, soil level. Lettuce and young cabbage family plants are those most commonly damaged.
The seedling stems are usually eaten through at soil level – hence the name. The affected plant then falls over and withers. Cutworms can be brown, green or yellowish, and a single caterpillar often works its way along a row of seedlings, eating as it goes.
Search in the top 2 to 4 centimetres of soil near the most recently damaged plants for the caterpillar and remove it. Controlling weeds helps too, as fewer eggs are laid. Generally, no chemical control is necessary.
Codling moth is a common pest of apples in some parts of the country, especially in town gardens. Pears are occasionally attacked. The caterpillar bores into the centre of the fruit, destroying it and often causing it to fall early. The caterpillar pupates in a cocoon and emerges the following year.
Destroying fallen apples, and using bands of sacking to trap the pest in the cocoons, helps to reduce numbers but control will not be good because the adults fly in from neighbouring gardens.
Spray with Hygeia Caterpillar Spray as blossoming finishes, and again three weeks later, to kill the caterpillars before they enter the fruit. Plum sawfly and plum fruit moth have a similar life cycle and can be controlled in the same way.
