Download PLANT CLINIC FACTSHEET

Grey mould infection of grapes
Key facts
l A fungal disease with a very wide host range
l Causes decay or dieback of all parts of the plant
l Prefers damaged or senescent material in order to colonise a plant
l High humidity is critical for spore germination
l Active over a wide temperature range
l Nursery hygiene, and manipulation of the environment to reduce humidity,
is critical to prevent outbreaks
l Protectant or curative fungicides available for many crops
l Anti-resistance strategies required.
Introduction
Grey mould, caused by the fungus Botrytis cinerea, is one of the more easily
recognised plant diseases. Unfortunately, it can also be one of the most difficult
to prevent or control. Grey mould can cause dieback and decay of most plant
parts, and a huge number of plant species can be affected.
The fungus is actually quite a weak pathogen, in that it usually requires some form
of damage to, or senescence of, part of the plant before it can infect. Once within
the plant, however, decay can be very rapid and severe.
High humidity is essential for the germination of Botrytis spores. Nursery hygiene and
cultural control techniques are critical to try and create conditions less conducive to the
disease. Whilst a range of fungicides with activity against the disease is available there
are problems with fungicide resistance to some chemical groups, so anti-resistance
strategies are also very important.
Sand Hutton, York
YO41 1LZ, UK
Tel +44 (0)1904 462 324
E-mail [email protected]
www.fera.co.uk/plantClinic/
PLANT CLINIC FACTSHEET
Grey Mould (Botrytis cinerea)
Symptoms
Grey mould can affect leaves, stems, flowers, fruit, and even tubers and roots (e.g. potatoes and
carrots in storage). The type of decay caused will vary according to the type of plant and the part
affected. It can range from a very soft breakdown of fleshy leaves to a firm dieback of woody stems
and branches. At high humidity the typical brownish-grey, furry, fungal growth will be very obvious.
Huge numbers of spores are produced on this growth, and it is common to see 'clouds' of spores
being released if the air around an affected plant is disturbed. In some cases, particularly on the stems
and branches of woody plants, a 'target' or 'tiger-stripe' symptom may develop as the fungus
undergoes alternating phases of slow and rapid growth due to varying environmental conditions.
Ghost spotting (of tomatoes) and petal spotting (of cyclamen and some other plants) are symptoms
caused when the spores of the fungus germinate and damage a few plant cells, but progress no
further. This results in small (a few millimetres), circular areas of discolouration on tomato fruit. Spots
of a similar size develop on cyclamen petals, but in some cases these may later enlarge to cause a
more extensive decay.
Grey mould sporulation
Ghost spot of tomatoes
Biology
The asexual spores or conidia, produced in large numbers on the typical grey mould growth, are
readily dispersed and carried by air movement and water splash. Botrytis cinerea is such a ubiquitous
pathogen that spores of the fungus are always likely to be present in the air. As mentioned previously,
damaged or senescent plant tissue is generally required for colonisation, but once within the plant the
fungus can destroy healthy tissue very rapidly by producing a range of enzymes.
High humidity or leaf wetness is critical for spore germination. Grey mould is therefore most common
during the cool, damp weather of autumn, winter and early spring, although it can occur throughout the
year. Research has been shown that at 10-20˚C, a relative humidity of more than 95% for at least
three hours will lead to spore germination.
Once within the plant the fungus often causes a rapid decay, but it may also remain in a latent or
symptomless state, sometimes for many weeks. Changes in environmental conditions or the chemical
composition of the plant will eventually cause the fungus to resume activity. In raspberries and
strawberries, for example, infection often occurs at flowering, with the fungus remaining latent until
changes in sugar levels within the ripening fruit lead to the develop of typical grey mould symptoms.
Botrytis cinerea can cause decay over a very wide temperature range. The growth rate of the fungus
will vary according to the temperature, but even in cold storage a slow, progressive decay may occur.
The fungus may also produce long-lived resting structures called sclerotia as a means of surviving in
the absence of a host plant. These may be visible to the naked eye as small (a few millimetres), black,
hard, flattened structures within the decaying plant tissue.
Contamination of seed by Botrytis can be an important source of the disease in crops as diverse as
linseed and primroses. In many crops, however, the significance or even occurrence of seed-borne
infection is unknown.
Diagnosis
Microscopic examination of affected plant parts will usually enable the characteristic spores of Botrytis
cinerea to be identified. If no spores are present, incubation at high humidity will encourage their
production. Fera has also developed a molecular technique (real-time PCR) to detect latent infection
and help predict potential crop wastage. Some plants (e.g. onion, lily, paeony) are affected by other,
host-specific Botrytis species as well as B. cinerea, in which case more detailed examinations are
required.
It is very common to find Botrytis cinerea as a secondary coloniser of plant tissue damaged by another
disease or pest, or by cultural problems (e.g. scorch, nutrient deficiency). In such cases the underlying
problem must be identified before effective control measures can be implemented, and laboratory
examination of affected plants may be required.
Fungicide resistance is a common problem in grey mould, particularly on crops grown under
protection. Resistance testing of Botrytis (and some other pathogens) is a service offered by Fera. We
can also test seed lots and other planting material for the presence of Botrytis and other pathogens.
Chemical control
There are a large number of fungicides available to growers with activity against grey mould. These
will vary according to the crop, and are too numerous to list in this factsheet. They range from
protectant products, which need to be applied before the disease is present, to fungicides with some
eradicant activity. In general, applying fungicides as a protectant programme before grey mould is
seen in a crop leads to better control.
Because Botrytis cinerea produces huge numbers of spores, there is always the chance of mutations
arising, some of which could lead to the development of resistance to previously effective fungicides.
Fungicide resistance is already a common problem with fungicides from some chemical groups (e.g.
MBC's, dicarboximides) and could develop with others. An anti-resistance strategy is therefore very
important when formulating a fungicide programme. Detailed information is beyond the scope of this
leaflet, but can be obtained from the Fungicide Resistance Action Group (FRAG) or downloaded from
www.pesticides.gov.uk/rags_home.asp.
Preventative measures
l Reduce humidity and avoid leaf wetness. Ventilate crops grown under protection (with heat if
required)
l Increase air movement by the use of fans, adequate spacing, open sites for outdoor crops, etc
l Use sub-irrigation if available
l Monitor crops regularly to identify outbreaks as early as possible. Consider quarantining bought-in
material to allow time for any latent infections to develop
l Dispose of affected material carefully to avoid spreading the spores of the fungus
l Removed damaged or moribund tissue (e.g. dead flowers) regularly. Avoid damaging plants. Deep
planting of crops such as cyclamen and primroses can lead to infection of lower leaves and should
be avoided
l Ensure that there is a thorough clean-up to remove all debris after a crop. Consider using
disinfectants to treat standing areas
December 2014
© Crown copyright 2014