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Grey mould (Botrytis) in greenhouse tomato crops | AG0577 |
Gray Harrison, Knoxfield
July, 1996 |  |
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Scientific name
Botrytis cinerea
Introduction
Grey mould is a common and often serious fungal disease of tomato plants in greenhouses. Once established it is difficult to bring under control and it may be present in greenhouse crops all the year round. Severe infection of stems can often kill the plants.
Figure 1. Grey mould on tomato fruit.
Symptoms
Grey mould can occur on all above-ground parts of the tomato plant and it often starts at a point of damage or on any decaying tissue. Fallen flower petals resting on leaves, and pruning wounds on stems are examples of infection points. The most characteristic symptom is a grey-brown furry mould, which are masses of spores of the grey mould fungus, covering the infected area. When shaken, clouds of spores are released from these infected areas. The infected areas can expand rapidly covering whole stems, leaves or petals. Stem infections can girdle the whole stem and cause wilting of the plants above the infected area. Flower petals are particularly susceptible. The fungus may grow from the infected petals into the fruit. Another symptom of grey mould is the production of halo or ghost rings on the fruit. These are caused by partial infections which stop developing before the fruit is rotted.
Biology
Survival
The grey mould fungus survives on plant debris and in the soil.
Dispersal
The source of contamination to a new crop is from spores carried by wind coming from host plants outside the greenhouse or spores spread on air currents in the greenhouse.
Environmental conditions
Grey mould development is favoured by cool and humid conditions. These conditions also stress the plants, making them susceptible to disease. Spores are developed only under very humid conditions. Four to six hours of free water on the plant surface is required for spores to germinate and infect the plant. Condensation on the plant surface is the most common source of free water in a greenhouse. Condensation occurs when the plant surfaces are colder than the surrounding air. The chance of condensation forming increases with high humidity of the air and when the air temperature decreases quickly. Condensation may also form on the interior surfaces of the greenhouse and drip on to the plants.
Host range
A wide range of plants host grey mould fungus.
Figure 2. Grey mould on flowers.
Control
1. Ventilation
Good ventilation, both day and night, to keep plants dry and air humidity low is critical to preventing and controlling grey mould. Chemicals should not be be relied on for control because without proper environmental controls the grey mould will continue to develop and spread. Open the vents in the greenhouse when temperatures rise to replace warm moist warm air with cool air . Heat the greenhouse at night to reduce humidity. When heated, air absorbs moisture. Plant transpiration (releasing water vapour) at night causes humidity levels rise in greenhouses. To maintain low humidity a cycle of heating and venting must be practised during the night. Control of the ventilation is best achieved by the installation of a humidity sensor to allow automatic opening and closing of the vents.
Circulation of the air within the greenhouse
Maintain good air circulation in the greenhouse to improve ventilation and reduce the chance of condensation forming. Moving air through the crop helps keep the plant and air temperatures the same.
- Fans can be used to move air within the greenhouse.
- Choose a plant density that will allow good air movement between the plants.
- Prune regularly to remove laterals and old and dense leaves to assist air movement through the plants.
- Avoid plant wetness by never spraying crops in the afternoon or at any time if the weather is such that drying by night will be difficult.
2. Hygeine
- Cut out stem infections and remove dead plant parts before pruning operations begin. This will prevent the build up of spores and the spread of infection.
- Cut out stem infections before the whole stem is damaged to save the plant from dying.
- Remove all plant debris from the previous crop - this material carries Grey mould spores that can infect the next crop.
- Dispose of these plant residues away from the greenhouse property.
- Avoid having bare soil by covering the floor of the greenhouse - the grey mould fungus survives in the soil and become a source of infection for the next crop.
- Wash down walls and floors after each crop.
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Figure 3. Early symptoms of Grey mould on pruning wound.
3. Chemical Control
Chemicals alone cannot be relied on to give control of grey mould. It is difficult to spray all surfaces of the plant where infection may occur. The fungicides currently registered are protectant types and do not have a systemic action of control. Protectant fungicides only provide a protective barrier to the outside part of the plant that discourages fungal spore development. They do not cure the disease once it has developed. Chemicals help reduce the risk of infection along with good ventilation and hygeine.
Resistance management
When fungi survive doses of fungicides that would normally control them they are called resistant to that chemical. The grey mould fungus is prone to developing resistance to the dicarboximide group of chemicals products such as Rovral. To help avoid the build up of resistant strains alternate the use of the different groups of chemicals.
Spray application
When applying chemicals aim to coat both sides of the leaves and all other parts of the plant. For a full grown tomato crop using high volume spraying about 300 litres of spray per 1000 square metres of greenhouse should be used.
Amount of product
For spraying to be effective the correct amount of product must be applied to the crop being treated. The following gives guidelines for different spraying methods.
High volume (dilute) spraying
Refer to the label to obtain the amount of product to be used for 100 litres of water.
Low volume spraying
The amount of the chemical product used in low volume spraying should be the same as used in dilute spraying regardless of the amount of water that will be used from the low volume machine. As a guide for 1000 square metres of greenhouse use the same amount of product that would be used for 250 litres of dilute spraying. If the label only specifies the amount of product to be applied per hectare use this rate regardless of whether high or low volume spraying is to be used.
Management to assist chemical application
- Plan the row spacings so there is sufficient room between the rows to provide easy movement for the person spraying.
- Select a plant spacing between the plants so the spray can be directed at the targeted parts of the plants.
- Deleaf or prune so the sprays can be applied to the parts of the plants that are difficult to reach.
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Consider the use of pruning tools that have an attachment that sprays the pruning wounds with a fungicide .
Table 1. Products registered in Victoria for the control of grey mould.
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| Poison schedule | WHP | Chemical group |
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| 6 | 1 | Phtalimide |
Use regularly as a protectant before the disease appears.
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| 5 | 1 | Sulfamide |
Use regularly as a protectant before the disease appears.
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| Ex. | 0 | Aromatic hydro |
Use regularly as a protectant before the disease appears.
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| Ex. | 7 | Dicarboximide |
Use 3-4 weeks after transplanting or at the onset of the disease.
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Use appropriate strategies to minimise the risk of fungicide resistance developing.
Always read the label before using any of the products mentioned
Correct diagnosis is essential for effective pest and disease control. A commercial diagnostic service is available at the Institute for Horticultural Development (IHD). For further information, contact Crop Health Services on (03) 9210-9222 or fax (03) 9800 3521.
For further information on registered chemicals, phone DPI Chemical Information Service.
The advice provided in this publication is intended as a source of information only. Always read the label before using any of the products mentioned. The State of Victoria and its employees do not guarantee that the publication is without flaw of any kind or is wholly appropriate for your particular purposes and therefore disclaims all liability for any error, loss or other consequence which may arise from you relying on any information in this publication.
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