Strawberry Diseases and Pests

BOTRYTIS (GREY MOULD)

Strawberry affected by botrytis or grey mould

Grey mould, caused by the fungus Botrytis cinerea, is a very common disease, causing a growth of fuzzy grey mould. It infects many plants, especially those grown where conditions are  so humid.
SYMPTOMS 

  • Under humid conditions fuzzy grey mould grows on affected buds, leaves, flowers or fruit
  • If humidity is low, infections may be contained within discrete spots, but if it is high they can spread rapidly
  • Above-ground parts of many plants, particularly buds and flowers, shrivel and die
  • Small black seed-like structures form in infected material (these are often overlooked)
  • On soft fruit, particularly gooseberries, Botrytis infection kills branches, but the fuzzy grey mould is seldom evident
  • On strawberries, grapes and sometimes other fruits, Botrytisinfection leads to a soft brown rot, often as the fruit is ripening

CONTROL

  Non-chemical control

  • Hygiene is very important, especially under glass. Remove dead and dying leaves, buds and flowers promptly 
  • Do not leave dead plant material lying around
  • Reduce humidity by improving ventilation and do not overcrowd plants

Chemical control

No fungicides are approved for use against grey mould by gardeners. Products containing plant and fish oil blends (Vitax Organic 2 in 1) may be used, but are not recommended by the manufacturers for grey mould control and are unlikely to have much impact. Use of other fungicides to control other disease problems may give some incidental control of grey mould, but this is not guaranteed by the manufacturers.

POWDERY MILDEW

Strawberry affected by powdery mildew

Powdery mildew is a fungal disease of the foliage, stems and occasionally flowers and fruit where a superficial fungal growth covers the surface of the plant.
Powdery mildews usually have narrow host ranges comprising of just a few related plants. For example, the powdery mildew affecting peas is a different species from the one attacking apples.

 SYMPTOMS

  • White, powdery spreading patches of fungus on upper or lower leaf surfaces, flowers and fruit
  • Tissues sometimes become stunted or distorted, such as leaves affected by rose powdery mildew
  • In many cases the infected tissues show little reaction to infection in the early stages, but in a few specific cases, for example on Rhamnus, the infection provokes a strong color change in the infected parts, which turn dark brown
  • Sometimes the fungal growth is light and difficult to see despite discoloration of the plant tissues, e.g. on the under surface ofrhododendron leaves

CONTROL
Non-chemical control 
Destroying fallen infected leaves will reduce the amount of infectious spores next spring. Mulching and watering reduces water stress and helps make plants less prone to infection. Promptly pruning out infected shoots will reduce subsequent infection.

Most powdery mildew fungi have a host range restricted to a relatively few, related plants, but these can include wild relatives which can be sources of infection, e.g. wild crab apples may be sources of infection for apple orchards.

Seed producers sometimes offer powdery mildew-resistant cultivars of both vegetables and ornamental plants, check catalogs for details.

Chemical control
Because most of the growth of powdery mildews is found on the plant surface they are easily targeted with fungicides.

Edibles and ornamentals: Myclobutanil (Bayer Garden Systhane Fungus Fighter concentrate) can be used on ornamentals, apples, pears, gooseberries and blackcurrants.

Ornamentals only: tebuconazole (Bayer Garden Multirose Concentrate 2) and triticonazole (Scotts Fungus Clear Ultra) can be used on ornamentals. Some formulations of myclobutanil, tebuconazole and triticonazole contain insecticides to control pests. Avoid these unless an insect pest problem is specifically identified.

GLASSHOUSE RED SPIDER MITE
Red spider mites
Glasshouse red spider mite is one of the most troublesome pests of greenhouse plants, houseplants. It can also attack garden plants in the summer. It is a sap-sucking mite that attacks the foliage of plants, causing a mottled appearance, and in severe cases, leaf loss and even plant death.SYMPTOMS

  • On leaves: Plants infested with glasshouse red spider mite show a fine pale mottling on the upper leaf surface. The underside of the leaves have many tiny yellowish green mites and white cast skins and egg shells. These are more easily seen with the aid of a x10 hand lens
  • On plants: In heavy infestations, you may see fine silk webbing on the plants, and the leaves lose most of their green color and dry up or fall off. Heavily infested plants are severely weakened and may die.

CONTROL

Glasshouse red spider mite can be difficult to control as it breeds rapidly in warm conditions and some strains of the mite are resistant to some pesticides. Biological control is a viable alternative to using pesticides, it can give good control and as it avoids resistance problems and the risk of spray damage to plants.

Non-chemical control
Cultural.
Remove severely infested plants from glasshouses in late summer before lower temperatures and shorter days induce the females to seek sheltered places where they will remain dormant for the winter period.  To reduce overwintering mites to a minimum, clear out plant debris, old canes, stakes and plant-ties before the spring. 

Empty glasshouses can be washed down thoroughly with a glasshouse disinfectant.  Weeds in and around the glasshouse should be kept down as these can act as hosts for the pest.Plants grown at high temperatures in dry, overcrowded glasshouses are more liable to severe infestation.  Regular syringing of plants with clear water and maintaining a high humidity reduce the danger of severe attacks, but will not on its own control this pest.

Biological. 
A predatory mite (Phytoseiulus persimilis) feeds on the eggs and active stages of glasshouse red spider mite. It needs good light and daytime temperatures of 21ºC (70ºF) or more if it is to breed faster than the pest. Its effective period of use is normally April to October in greenhouses; June to September outdoors.

As the predator is susceptible to pesticides, biological control cannot be used in conjunction with most chemical controls. The exceptions are those with very short persistence, such as plant oils or extracts (e.g. Vitax Organic 2 in 1 Pest and Disease Control, Bug Clear for Fruit & Veg) or fatty acids (e.g. Bayer Organic Pest Free, Doff Greenfly and Blackfly Killer, Bayer Natria Bug Control) or urea/mineral lattice (SB Plant Invigorator), which can be used to keep mite numbers down before it is time to introduce the predator.

Note that Phytoseiulus persimilis will not control other species of red spider mite, such as fruit tree red spider mite, citrus red spider mite, box red spider mite and conifer red spider mite.

Phytoseiulus and compatible biological controls for most other greenhouse pests can be obtained by mail order from specialist suppliers.

Chemical control

  • Pesticides containing  acetamiprid (e.g. Bug Clear Ultra) or thiocloprid and methiocarb (e.g. Bayer Provado Ultimate Bug Killer, Aerosol)  are available for use on ornamental plants only
  • Edible plants can be sprayed with plant oils, plant extracts or fatty acids. These pesticides may require frequent applications to control the mite.
Other diseases and pests that affect strawberry include:
Fungal leaf spot
Strawberry black eye
Birds
Red core
Strawberry seed beetle
Verticillium wilt
Vine weevil
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