Pests and diseases
Strawberry plants are susceptible to threat from various pests and diseases. A number of precautionary and protective measures are required in order to achieve maximum production.
The fungus penetrates into the plant at the moment when there are wounds. The root neck of the affected plants shows a reddish brown discolouration. The progress of the disease may be quick; the plants will be wilting and die. Remarkable thing is, that the roots are still fully healthy then. To prevent similar attacks your are advised to use healthy planting material. In case susceptible varieties will be planted, you should immerse them before planting.
Red root rot
Growth of the plants will slow down and they will become dull bluish green. In spring the plants will convalesce somewhat. An affected plant will form no or only few flowers. The small fruits will dry out. The root-hair of the roots is lacking. When cutting the main roots, it will appear that the central cylinder has discolored red. To prevent this harmful effect you should buy certified planting material.
Colletotrichum is a water-loving fungus species, which may strongly spread, especially at higher temperatures (20°C is optimum). The fungus will spread from the soil through splashing water drops to fruit and crop. Consequently, preventive control should be carried out before rainfall. The first symptoms can be observed on the leaves. All three leaves will curl up. On the runners and leafstalks the attack can be observed in the form of ellipse shaped sunk small black spots. On the damaged fruit round sunk brownish black spots will appear. In an infected plant the fungus will spread. By observing utmost hygiene during activities in the fields quick spreading in the establishment can be avoided. If an infection should be established on a parcel, it should be worked or picked, as the case may be, at the end of the day.
The fungi of this vascular disease will penetrate into the plant through its roots or through its stolons. Damaged plants lag behind in growth, which manifests itself especially on hot, scanty days. The damaged plants will be slack then. In a later stage the plant will creep as it were into the soil and the fruits it will still bear are small and dry. Diseased leaves will discolour dull yellowish green.
The type of rhizoctonia which will be described here may not be mistaken for black root rot, which is also caused by a Rhizoctonia fungus. In principle Rhizoctonia can occur throughout the year, however, no spreading will take place when it will be freezing. If a plant has been attacked, it will show reddish brown discolouration. In almost all cases erwinia is involved, as a result of which the entire heart will disappear and the sleeping eyes will start coming out. This will result in the formation of a bushy plant. To prevent this, planting should be done not too deep and not on wet parcels.
Blossom-end rot and stem rot
This fungus occurs especially in long-range crops. In an early stage small brown spots can arise on the buds. After flowering the calyx tails will turn brown, the fruits will discolour brown and will dry. The fungus will hibernate on dead parts of plant above ground level. Spreading will occur by precipitation.
Botrytis is a wound parasite. Stamina, which are breaking off and petals falling off prematurely are ideal attacking points for Botrytis. Characteristic is the whitish grey fungus fluff on the attacked fruits. They are traces of the fungus germs at high humidity of the air and temperatures ranging from 15 to 20 C. They are spread by the wind.
In case of an attack coming on, the leaves will curl, forming white fungus fluff on the underside. Subsequently light purple spots will form. Also the fruits can be attacked by mildew. Especially during hot weather the fungus can spread quickly. Some varieties are especially sensitive. In growing tunnels mildew can be avoided partly by preventing from arising. Aeration should be done by opening just the side that is out of the wind.
Purple spot disease
Typical characteristic of this type of fungous disease are the purple spots which appear on the leaves of the attacked plants. These round spots, measuring 2 to 5 mm, are having a red or brown spot in the middle. Just a single spot on the stem of a leaf or a cluster of flowers will be enough to let it die.
Every year again various kinds of insects can cause much damage in strawberries. To obtain a crop with good results, effective fighting of these parasites should be part of the necessary measures to be taken. Eelworms, caterpillars, plant lice, red spiders, bugs, thripses and beetles should be controlled in the best possible and most correct way. Besides insect pests also snails are often a nuisance, to which much attention should be paid. Especially in glasshouse cultivation various biological killers of insects can be put into action. To this end the best thing you can do is to contact your local supplier.
- Foliar nematodes - Foliar nematodes do not only live on strawberries but they also occur on a large number of plant species. They are especially in the centre of the plant. At the outside of the plant they can not live very long. The leaves are deformed, scalloped, minor and have sharp points. The leafstalks are considerably thinner than they ought to be. The number of flowers is poor and sometimes the whole production of new buds is lost. The buds on the side often sprout (the plant is falling apart as it were).
- Stem nematodes - In contrast to foliar nematodes, stem nematodes can live for a long period of time without host plant. If a sensitive plant will be planted on infected soil, it will be attacked. If stem nematodes are involved, the attack can be best observed in spring. The leafstalks and flower stems will remain short and are strongly thickened and bumpy. The leaf rims will curl up and the leaves are covered with a bluish bloom and often strongly deformed. The flowers remain small and produce deformed fruits. The infection often occurs spot wise. To make sure about the actual presence of the infection, a soil sample is necessary
- Free living root nematodes - (Longidorus elongatus and Xiphinema diversicaudatum) - Both species of nematodes have an extensive series of host plants. Plants which have been attacked by root nematodes, lag behind in growth. The root system has badly developed, the roots are stunted and have swollen root tips. What is more important than this direct damage, is the carrying on of viruses. In this way strawberry plants can be infected with viruses.
- Root cystnematodes - (Pratylenchus penetrans) - The root cyst nematode can live on a large number of plants. For strawberries it is the most dangerous nematode species. Nematodes spend their entire life cycle in the roots of the plants. One generation will last about six or seven weeks. Attacked plants show interrupted growth, produce fewer runners and will die. As a result of this, so-called eelworm patches form in the crop. Especially on light soil this eelworm may cause damage. The development of the roots of attacked plants is far poorer. The ends of the roots are thickened and the root hair is lacking. An attacked plant is growing as it were on the soil and can be easily pulled out of the soil. Through the damaged roots fungi - for example black root rot - can penetrate into the plants.
- Root-knot - (Meloidogyne hapla) - This nematode can be found especially on light soil. It provides a thickening (small tuber) on the fine roots. Plants which have been seriously attacked, are strongly lagging behind in growth and are worthless for future use. So far this nematode causes hardly any problems in strawberry plants.
Subterranean caterpillars are round, grey, twisted caterpillars, the size of which varies from a few millimeters to 3 centimeters. They can be found all the year round. They are caterpillars of different kinds of owl butterflies. Most species have one generation per year. If the caterpillars are still small, they live aboveground on young leaf green. However, they pass on rather quickly to a way of living underground. They eat from roots and different underground young parts of plant. Besides, they eat from the aboveground parts of plant during the night time. Subterranean caterpillars can often be easily found in the soil, because they take along parts of plant to their places of shelter.
Leather jackets are the larvae of the daddy longlegs. This insect is present especially in ploughed up grassland. One daddy longlegs will lay three or four hundreds eggs. Leather jackets do not have legs and they are measuring a few millimetres to 3 centimetres. Young leather jackets are white. As they grow older they, colour grey and their form is getting flat. Their head is not clearly visible.
Wire worms are the larvae of the click beetle. Because of their yellowish brown colour wire worms are also referred to as click beetles. They eat themselves into the rhizome of the strawberry plant, as a result of which the strawberry plant will start to become slack. They occur, just like leather jackets, especially in ploughed up grassland. The beetles do not do any harm.
Plant lice live on plant juices and exude honeydew. As they suck plant juices, leaf deformations and sometimes fruit deformations are caused. Besides, some lice pass on viruses. The strawberry aphid and the shallot aphid are the major plant lice species, occurring on strawberries. Both species pass on virus diseases. Plant lice are having many natural enemies. The best-known are: ladybirds, lacewings, syrphus ribesii, earwigs and ichneumon flies.
- Strawberry aphid - (Chaetosiphon fragaefolii) - The strawberry aphid is a small whitish green louse, which exclusively occurs on strawberries. The club-shaped hair on the back of this plant louse species can be easily observed under a magnifying glass. Besides, its dark eyes are conspicuous. There are several generations per year.
- Shallot lice - (Myzus ascalonicus) - Shallot lice are small and bluish green. These plant lice suck on leaves and young flower stems, thus producing a bushy plant. The fruits are formed on short fragile stems close to the plant. Sometimes they are deformed. In summer plant lice infect different crops. In autumn they fly back to the strawberry again.
Beetles are insects which have two pair of wings. The first pair is not really a set of wings, but it is armoured. The second pair is situated under these wing covers. Both the beetles and the larvae have biting mouth parts. Consequently they are causing damage. The larvae have a distinct head and three pair of legs. The larvae of snout beetles and those of the strawberry blossom weevil are exceptions in this respect. They do have a distinct head but no legs.
- Strawberry blossom weevil - (Anthonomus rubi) - The strawberry blossom weevil is a small dull black beetle (3-4 mm). The beetle feeds on leaves and pollen. In spring the beetle lays its eggs in the flower buds. When doing this, it pierces the flower stem at the same time, just under the flower. Consequently the flower will bend and start hanging. From the egg a legless white larva will develop, which will pupate in the flower. After some time the beetle will appear. In September the beetles will take shelter under the ground or under dead plant material. Comment: during the flowering season you should not use any pesticides that are harmful for bees!
- Strawberry seed beetle - (Harpalus rufipes) - The strawberry seed beetle is a black beetle having a length, ranging from 10 to 17 mm, with yellowish red legs. It is a nocturnal animal which can often easily be found under black plastic. The beetles feed on the seeds that are on the strawberries. As a result of this damage the strawberries will become worthless.
- Strawberry rhynchites - (Rhynchites germanicus) - Considering its size (2.5 - 3 mm) and form this beetle resembles the strawberry blossom weevil very much. However, its colour differs somewhat; dark, bluish green and glossy. Besides, the strawberry rhynchites has bent antennae and a short snout. The female will lay 1 to 4 eggs in the flower cluster stem. Subsequently she cuts the flower cluster stem off, as a result of which the whole cluster will die. The difference with the strawberry blossom weevil is, that this species cuts the flower stem and the strawberry rhynchites cuts the flower cluster stem.
- Nettle weevil - (Phyllobius pomaceus) - This beetle is measuring bout 8 mm and its colour is yellow to bluish green with golden glossy patches. After their hibernation the yellowish white legless larvae will pupate in April. After a couple of weeks the adult beetles will come out. Eggs will be laid in May. The larvae feed on plant roots. The damage is comparable to that caused by the black vine weevil.
- Black vine weevil - (Otiorhynchus species) - The grooved black vine weevil is a snout beetle of about 8 to11 mm long with dark, greyish brown, grooved wing covers. The beetles eat round bites from leaf rims. The actual damage is caused by the larvae, which are living subterraneously. The larvae are white to pinkish in colour, legless and have brown heads. They feed on the young roots of plants, as a result of which plants will die. The larvae overwinter in the soil or in the rhizome of the plant. In the latter case the plants will die.
Just like insects, mites are arthropods. They have a skin of chitin. This skin serves as external skeleton. Although we can distinctly discern in insects their head, thorax and rump, in mites the corpus is forming one whole. Insects always have 6 legs. We can distinguish three species of mites: 1) harmless species, which feed on moss, algae and fungi, for example: moss mites; 2) harmful species, which suck the leaves of plants. Important species occurring in strawberry plants are the strawberry mite and the two spotted spider mite; and 3) useful species, which feed on harmful mites. They are the so-called predatory mites. If there is no animal feed (read mites) available, they will live on pollen, fungi, etc.
- Strawberry mite - (Phytonemus pallidus spp. fragariae) - Strawberry mites are living exclusively on strawberries. They are very small (0.25 mm) and stay in curled up leaves in the centre of the plant. Looking through a magnifying glass they look like fine water droplets. However, they are moving! The adult females overwinter in sheltered places in the centre of the plant. If temperature rises about mid March, the mites move to the young leaves which are still folded. Subsequently, transparent eggs are laid in long lines along the vein of the leaf. The larvae which come out the eggs shed their skin e few times. After two or three weeks the adult mites appear. Mites can be observed in the folded young leave in the centre of the plant. This young leaf will shrink. The growing point of the plant will become dull and bluish green in color. The centre of the plant will turn brown. Attacked plants will produce short, stocky flower stems, which will not develop.
- Two-spotted spider mite - (Tetranychus urticae) - The two-spotted spider mite can live on strawberries, but also on numerous different crops. It is also present in different weeds, such as Black Nightshade, small stinging nettle and gallant soldier. The adult females overwinter in sheltered places in the crops. In spring they become active. From mid April eggs are laid on the underside of the leaves. The larvae vary in color, from yellow to light and dark green. Sometimes they are even orange in color. Recognizing them, not the color is decisive but rather the two black spots on their body. The larvae and the mites will prick in the plant cells on the underside of the leave and suck them out. An initial attack is observed as yellow patches on the top of the leaf.
Thrips are also referred to as thysanoptera. This name refers to their wings, which are fringed with setae, small hairs. They are also known to be specially active in flying during sultry weather. Their size is maximum a few mm so that they can only be accurately observed when using a magnifying glass. Thrips have a few generations per annum. They are present in a large number of crops. Many kinds of thrips occur in The Netherlands, however, one single species is responsible for problems in horticultural crops. The problems they cause vary from sucking out of plant cells, as a result of which leaves die. Besides, a plant can react to this by providing rampant growth of tissue, which results in deformations of leaves. When leaves are sucked out, also viruses can be carried on. In strawberries especially Californian thrips (Frankliniella occidentalis) causes major damage.
Besides Californian thrips also tobacco thrips (Frankliniella fusca) and rose thrips (Thrips fuscipennis) occur in strawberry cultivation, however, these species are less harmful. It is hardly possible to recognize with what species of thrips you have to do in specific cases. This is only possible if the thrips involved will be enlarged 250 times. Eggs are laid in plant tissue, both in flower bottoms and in leaves. Subsequently, the larvae develop in stages. This development also occurs both in the leaves and in the flower bottoms of the plant. In the prepupating stage the greater part of the thrips drop on the ground and look for some dirt or creep under plastic to pupate. When the pupae have come out, the adult thrips will move to the plants again and after a number of days they will start laying eggs. The best way to find thrips is by removing petals and the stamina of the flowers. Larvae and adult thrips will be sucking on the fruits. This causes brown scaly sucking patches round the seeds.
This bacterial disease mostly occurs in the hot climate of southern Europe. However, it can also occur in central Europe. A very infectious bacterial disease is involved here, which can destroy complete plantings, unless stringent hygiene measures are taken in good time. The attack can be recognized as angular light green almost transparent patches on leaves, which will produce mucus after some time on the underside of the leaves.