Post category: List of garden plant diseases
A fungal disease, rose blackspot is the most serious problem of roses. It overwinters on dead leaves and infected shoots. In spring the emerging leaves are infected and the cycle re-commences. It causes black or brown spots on the leaves which often turn yellow and fall off. The plant is seriously weakened after a few years.
Collect and burn affected leaves in the autumn. Spray in late March or early April, with Roseclear, or Multirose to break the cycle. Continue spraying, at least once a month, until July, and longer in a wet summer. Some varieties are more susceptible. Alternate the spray chemical to reduce the chances of resistant strains building up.
Brown rot is a fungus disease of plums and occasionally cherries, apples and pears. It causes a brown rot, often with white spots, which destroys the affected fruit. It is rarely a very serious problem, destroying only a few fruits usually. It can also cause ‘shot-hole’ of the leaves, tiny round pieces dropping out to leave a hole.
Brown rot disease on plums.
Pick off all old ‘mummified’ fruits left on the trees in the autumn. These are the main sources of infection. The sprays for apple scab help in prevention.
Blossom wilt disease causes severe damage to some varieties of flowering cherry, such as Prunus triloba and Prunus subhirtella ‘Autumnalis’. The blossom bunches including the associated leaves wilt and wither when the flowers are open. The result can be parts of branches with withered blossoms and healthy flowers.
Shot-hole symptoms caused by blossom wilt disease.
Twigs die back and larger branches occasionally. The leaves of the surviving buds often show ‘shot-hole’ symptoms later in summer. The disease is more severe after a wet summer. There is no satisfactory remedy and badly affected trees may have to be removed.The trees recover somewhat following a dry sunny year. Sheltered trees are more affected. Try a feed of potash, at 30 grams per square metre of the area to the spread of the branches – it can help by hardening the foliage.
Bacterial canker is one of the few serious bacterial diseases. It causes flowering cherry and plum trees to fail gradually and eventually die. The signs are branches dying; foliage with little holes, descibed as ‘shothole’; and gum exuding from the main limbs.
The bacteria is prevalent in winter, and it is most important to prevent injury to these trees in that season. For that reason, pruning is done only in summer. Once affected, there is no remedy. If a single branch has been attacked, it can be removed before the disease progresses to the main limbs and trunk.
See also silver leaf disease and blossom wilt disease.
Apple scab is a very common disease, caused by a fungus that attacks the young leaves as soon as they emerge in spring. It grows on the leaves, producing spores. When the fruits form they are attacked too, small brown or black scabs being the result.
Apart from direct damage to the fruit, apple scab seriously weakens the tree, predisposing it to apple canker disease. Pears are affected by a similar scab disease, but less frequently. Apple scab is most severe in wet years and in localities with high rainfall.
Use resistant varieties such as ‘Discovery’, ‘Katy’ and ‘Lord Lambourne’. Spray susceptible varieties at bud-burst, in late March or early April, with Captan or Systemic Fungicide, such as Fungus Clear 2. Repeat the spray at least three times between then and the end of June. In a wet year, and in wet localities, more applications can be necessary.
Potato blight spreads by spores from old, dumped potatoes to new plants in mid-summer. The foliage quickly rots with brown spots appearing on the leaves. The fungus needs moist conditions and it is worst in a wet year and in areas of high rainfall. The spores spread rapidly from plant to plant, sometimes causing all the foliage and stems to rot.
Apart from bringing growth to a premature end and reducing yield, the blight spores are washed into the soil to affect the tubers, which later develop a bronze rot, and become unusable. Some varieties, such as ‘Rooster’ have a reasonable level of resistance but will still suffer damage.
Spraying is essential in most years, except for early crops. Spray with Dithane, Bordeaux Mixture, or Liquid Copper. Old potatoes should be burned or buried deeply, not just dumped in a corner where they will grow and send up spores.
Neck rot is a serious fungal disease of onions, causing them to rot in store. The fungus enters the bulb during its period of growth but only causes the rot later. It is worst in wet years and in cases where the onion plants have been grown too strongly with too much manure or fertiliser. Heavy feeding also leads to ‘thick neck’ onions, which do not store well.
Destroy all rotted onions – do not simple dump them – they are a source of infection for the following year. Dress seed and sets with a dusting of Systemic Fungicide before sowing.
Honey fungus is the most serious fungal disease of trees and shrubs. It moves through the soil between woody plants, invading them through the roots. The growths in the soil resemble black boot-laces, hence the alternative name ‘boot-lace fungus’.
Affected plants usually have the root system destroyed, causing them to wilt and die. Yellowish mushrooms are sometimes produced at soil level beside the trunk. The fungus spreads from old stumps to living plants and it is very common in old woodland and ditches.
Some plants can be affected but live on for years, resisting the fungus, and only succumbing when other pressures such as summer drought make the burden too great. Single trees and shrubs die off over a period of years.
Remove old stumps and dead plants. Do not plant near the site of old ditches or use resistant species. Susceptible plants include privet, apple, griselinia, cypresses, lilac and willow. Reasonably resistant plants include hawthorn, yew, ash, beech, laurel, box, clematis, honeysuckle and holly.
A barrier of heavy polythene placed vertically in a 60 centimetres deep trench between an infected area and new planting is considered to be of some assistance.
The name, ‘grey mould’, describes the masses of grey spores produced on the rotted parts of plants affected by this fungal disease. The fungus involved is mainly a decomposer of dead plant tissue but it can invade living tissue as well, especially if the plants are unhealthy, and late in the growing season.
Grey mould disease on tomato
Any plant that has some dead tissue attached, such as withered flowers or leaves, can develop this disease. Old plants in cool, damp conditions are very susceptible. Most common in a wet season, it often affects soft fruit, french beans, celery, lettuce, cucumbers and tomatoes, but it is implicated in die-back of branches of gooseberries, skimmia, choisya and elaeagnus too.
Avoid damp air for indoor plants, especially in the dull months. Remove dead plant parts before they are invaded. Strawberry and raspberry fruits are often affected, especially in a wet summer, and more than half the crop can be lost. Remove the affected fruits as they are a source of infective spores. Balanced feeding with high potash reduces the problem.
Better air movement through the plants, growing strawberries on a raised drill and more thinning out of raspberries will give good control. If the cultural methods of control prove inadequate, spray at least twice, once when the first flowers open, with Systemic Fungicide and again when flowering has finished.
Gooseberry mildew is a fungal disease that causes a white or brownish deposit on the leaves, shoots and fruit of gooseberries, and it also attacks the leaves and shoots of currants, but does not affect the fruit. It sometimes attacks flowering currants. The bushes can be seriously weakened in a bad attack.
Prune surplus shoots of gooseberries to keep the centre of the bush open, allowing air to circulate. The fungus mostly affects fruit at the centre of the gooseberry bush. The affected fruit can be wiped clean with a dry cloth and is perfectly good to use.
Prune out any affected shoots in summer, on both gooseberries and currants. In areas of persistent attack, to prevent fruit mildew, spray gooseberries with Systemic Fungicide when the first flowers open, and again when the fruit is the size of a pea.
Fireblight disease is a bacterial disease that causes susceptible plants to die back at the tips and eventually to die completely. It mostly affects pyracantha and some kinds of cotoneaster but apple and pear trees can be affected, as well as hawthorn and mountain ash. The shoots tips wither and yellowish ooze appears on the stem in warm, muggy weather.
This disease is notifiable to the Department of Agriculture and the plants affected must be burned.
Fairy rings on a lawn are caused by a soil fungus feeding off the roots of the lawn grasses. A ring of vigorous green grass may surround a broken ring of bare spots. The rings expand slowly outwards, occasionally producing a few pale brown, small mushrooms.
Fairy rings occur most frequently on good lawns, and can be very disfiguring. Toadstools or inkcap mushrooms are not lawn diseases as such – these are fungi feeding on dead roots, bits of timber or other organic material in the soil – and can be removed by brushing away.
Control of a fairy ring is difficult to achieve. The grass over the rest of the lawn should be well fed to disguise the ring.
Clubroot is a fungal disease that causes finger-like swellings on the roots of the members of the Cabbage family – hence the alternative name, ‘finger-and-toe’. It occurs frequently on wet, acid soils but very seldom on limy soils. Affected plants are stunted and fail to develop. Spores can survive in the soil for many years.
Improving drainage, and applying lime to raise the pH to over pH7 are two ways to reduce the disease.
These are general names given to the damage caused by the fungal and bacterial diseases that kill twigs and branches. The bark often dies with a sunken, cracked or papery appearance. Many kinds of trees and shrubs can be affected by these diseases, which are most common in wet localities, on heavy soil and in over-sheltered gardens.
Apples, willow, roses, beech, ash, laburnum and clematis are commonly affected. Prune out the cankers and twigs that have died-back, because these are a source of fresh infection. An application of potash fertiliser helps to toughen the wood.
Three separate fungi cause the canes of raspberry and loganberry to die back. Cane blight kills entire canes by invading at ground level. Spur blight causes individual spurs on the canes to die. Cane spot causes purple spots on canes and leaves, and part of canes can die in a bad attack.
Avoid over-feeding the plants, which makes them soft and more easily attacked. While these diseases are serious for commercial growers, they result in a smaller crop in gardens. If the damage is severe, spray at bud-burst for cane spot and when the new canes are 15 centimetres high for spur blight. For cane blight, avoid injury to the canes, such as hoeing.
Leaf spot diseases are caused by fungi that invade the leaf tissue causing it to turn brown or purplish, often with a concentric ring and perhaps a tiny centre spot. Mostly, leaf spot diseases are not very virulent, causing only spotting but not much tissue loss. Plants often respond to leaf spot diseases by shedding leaves, and this can be debilitating
Tar spot on sycamore
Almost every kind of plant has its own particular leaf spot disease. Most are not very damaging, being no more than curiosities like sycamore tar spot, but blackcurrant, celery, iris, beans, hellebore, honeysuckle, poplar and cineraria are among the plants that suffer serious leaf spot diseases.
White blister disease affects the Cabbage family, including weeds. It causes raised yellow bumps about a centimetre across on the upper surface of the leaves, and white spots on the underside. It spoils the foliage but does not greatly weaken the plant. Since it usually affects old leaves, the crops are generally edible. Remove old crops.
The major source of infective material for leaf spot diseases is the fallen leaves and a good measure of control can be achieved by raking these up and destroying them. In the case of repeated severe attacks, a spray of Bordeaux Mixture or Liquid Copper in late spring and early summer might give some control. There are so many different species of leaf spot fungi involved that precise recommendations are not possible.
Onion white rot is a serious disease of onions in some gardens. It is soil-borne, rotting the roots and making the bulbs unusable. On a badly infected site, every plant will be lost.
To keep out the disease, avoid planting onion sets – use seed instead.
Onion white rot damage
If the disease is already in a part of the garden do not grow onions there, and avoid transferring soil on tools and boots to other parts. A seed dressing with Systemic Fungicide may work but the best solution is to avoid infected ground. Destroy all affected plants by burning.
Powdery mildew diseases are very common, affecting plants of many kinds. For most plants, there is a form of powdery mildew, caused by fungi that are specific to the host plant. The symptoms are obvious – a chalky, dust-like coating on the leaves, flower buds or stems, and sometimes the fruit as well.
The most seriously affected garden plants are apples, rhododendron, roses, clematis, asters, forget-me-nots, sweet pea, foxglove, courgettes, swede turnips, strawberries and grapevines. These plants can be seriously weakened by the feeding of the fungus; leaves destroyed; flowering and fruiting stopped or spoiled.
Powdery mildew on apple
Plants growing in greenhouses, and on light, dry soils in a sunny position, are more prone to mildew attack. It is common in dry seasons, but can also make a strong attack in a dull season when plants have grown soft. Young trees such as oak and maple are often whitened by powdery mildew but this appears to have little effect on growth.
Apple mildew can be controlled by pruning out the affected shoots in early summer. Rhododendron mildew is very severe on some varieties and affected varieties are best avoided, even rooted out – it may not be practical to spray repeatedly.
Swede turnip mildew is not controlled by these but late sowings usually escape damage. Courgette mildew looks more serious than it is but can stop growth. Over-feeding should be avoided. Grape mildew is very severe and difficult to control. Improve ventilation and do not over-feed the plant.
If mildew is severe, such as grape mildew, spraying with suitable chemicals such as Systemic Fungicide with myclobutanil can be effective. This level of spraying may be considered too onerous.
Several types of soil-living fungi attack the roots of a wide range of plants, causing the root system to rot. The plant above ground first stops growing, then leaves can go yellow and wilt, and total collapse eventually occurs.
Collapse from root rot is usually sudden and often occurs in mid-summer when the damaged root system proves inadequate to supply moisture needs, or in spring when the plant, having had its root system killed during the wet season, fails to commence growth.
Phytophthora on lawson’s cypress
Commonly affected plants include house plants, tomatoes, cucumbers, lettuce, lawson’s cypress, strawberries, heathers, rhododendrons, pieris and japanese maples. When seedlings die of root rot, it is called damping-off.
A range of related fungus species are involved, such as phythophthora and pythium. Root rot fungi are moisture-lovers that thrive in conditions of water-logging outdoors, and over-watering indoors. Use clean trays, compost and water when raising plants from seed.
Cheshunt Compound can be used to stop damping-off moving across a tray of seedlings. Outdoors, drainage should be improved. Affected plants should be disposed of.
Many kinds of plants have a rust disease caused by a specific fungus. Brown, yellow or reddish spots appear on the undersides of the leaves, often with a pale green or yellow ‘shadow’ on the upper surface.
Most of these diseases are not fatal to the plants affected, but usually spoil the foliage and reduce growth and flowering. Commonly attacked plants include roses, leeks, hollyhocks, carnations, beans, geraniums, mint, poplars and willows.
Rust diseases have a curious habit of appearing every few years for just one season. If they appear more frequently, and the plants are weakened, spraying might be considered. Usually only roses need to be sprayed. Other plants can be replaced.
Rust disease on raspberry
Suitable chemicals include Dithane and Liquid copper. Use these sprays as indicated above, in early summer, when the disease is first noticed – not as a routine. Repeat the spray twice. Rusts are more common in hot years.
Virus diseases can affect any type of garden plant. They cause stunting, mottling and curling of foliage, and reduce flowering and fruiting. Commonly affected plants are strawberries, raspberries, potatoes, courgettes, cucumbers, spinach, tulips and blackcurrants. But many other plants are affected too.
There is no cure for virus diseases, except to use resistant varieties, for example of courgettes. Affected plants must be removed and destroyed. Virus diseases are spread by greenflies and other sap-sucking insects, by eelworms under ground, and on pruning tools.
Strawberry plant stunted by virus
Controlling greenflies helps, especially on strawberries and raspberries. Avoid the use of secateurs or knives on healthy plants after pruning infected ones, for example, when harvesting courgettes.
Silver leaf disease is a major killer of plum trees and cherries, including flowering cherries. The fungus gets in through wounds during the cool, wet part of the year and gradually kills the tree by blocking its water-carrying vessels, and the tree wilts. Usually a single branch is first affected and dies. The disease spreads until the whole tree dies.
Silver leaf disease on cherry
Never prune these trees in winter. Prune away any sick-looking, or silvered branches in summer, and immediately seal all pruning cuts or injuries.
Willow canker is a severe disease of some kinds of willow. It is a fungal disease that attacks the leaves and twigs soon after leaf-break in spring. It causes spotting, which sometimes join up and the subsequent drying of the leaves causes curling and distortion. Badly damaged leaves wither and are shed.
It can affect twigs too causing them to die at the tip and fall off. It can severely affect the appearance of a tree. Although spraying with copper sprays, potato blight sprays or Bordeaux mixture might reduce infection, this is not practical. It is worst in damp weather and less in a dry year. The weeping willow variety ‘Chrysocoma’ is very susceptible. Persistent attacks might require removal of the tree. An application of potash at 30g per square metre in spring for three years might help to toughen the bark and foliage.
Leaf spot disease of escallonia has appeared in many parts of the country in recent years, causing significant or severe leaf drop. Spraying for this disease is not really practical and all you can do is leave it be and see how it progresses. This disease is generally worst in wet weather, high rainfall areas, and on the plant close to the ground where the leaves stay damp longer.
Sometimes diseases have a very favourable spell of weather and afterwards, they do not present the same problem again, but often the problem becomes chronic.
It appears to be quite virulent and causes severe defoliation and weakening, leading to death, of plants.
It would appear to be a fungus called Mycosphaerella or related Septoria leafspot.
Escallonia leaf spot is here to stay and will continue to be a problem. Try applying a fertiliser with high potash that might harden the leaves but it is unlikely to do much and spraying is not a solution, as it is not possible to spray large bushes, or to spray often enough for control.
