Grey mould of tomato
A very common disease, which can cause severe decay
What is it?
Grey mould is a very common tomato disease, caused by the fungus Botrytis cinerea. It is particularly damaging during wet or humid conditions, causing leaves, stems or fruit to rot. Both outdoor and greenhouse-grown plants can be affected. Botrytis cinerea is a ubiquitous fungus with a very wide host range, and the potential to cause severe damage. Affected tissues die rapidly, and if lesions girdle the stem the entire plant may die. Huge numbers of infective airborne spores can be produced on affected plant parts. The fungus will readily colonise damaged plant tissue, and spread from here into healthy growth.
What does it look like?
Symptoms vary according to conditions. In high humidity, a fast spreading rot develops, covered in a fuzzy grey-brown layer of fungus. In low humidity, plant parts may still rot and shrivel, but without the grey fungal growth. In addition to causing fruit decay, a very common Botrytis symptom seen on the fruit is ‘ghost spot’. This develops when a spore germinates and infects the fruit, but the infection does not progress further. The result is a tiny brown spot surrounded by a whitish halo. Grey mould is important to commercial growers as it affects fruit quality, but affected fruit is perfectly edible.
How is it spread?
The fungus can overwinter within the greenhouse, and new infections on tomato may begin in the spring as soon as weather conditions are favourable for disease development. As grey mould is such a common disease, with a wide host range, spores of the fungus are always likely to be present in the air. Botrytis can colonise dying flowers, leading to subsequent infection of the fruit. It also has the ability to remain dormant in developing tissues, only to break out and cause rotting at a later stage.
How can I control it?
Regular monitoring and testing of plants with symptoms will enable prompt remedial action to be taken. Affected leaves or fruit can be picked off and disposed of, but it may be best to remove severely affected plants completely. Place a plastic bag over affected plants or plant parts prior to removal to avoid spreading spores. Good cultural practices are critical to preventing disease - avoid excessive plant damage, remove promptly any dead or damaged tissue, and reduce humidity whenever possible by opening doors or vents. Wash hands and / or disinfect tools when removing affected material. No fungicides are available to gardeners to control this disease.
When do I test?
Grey mould symptoms could appear at any time during the growth of the crop. Testing should be carried out as soon as symptoms are detected and monitoring for symptoms should continue until cropping is complete.
What do I test?
Where possible, select areas of leaf or stem which show symptoms of disease. For best results include material where healthy and diseased zones meet. Do not use completely dead plant material. As a general guide, use ssamples of leaf material approximately 25mm x 25mm. For stem samples, use small pieces approximately 5mm across.
It is unlikely that fruit with ‘ghost spot’ symptoms will give a positive result, as little or no fungal material is present within the spots.