Controlling tomato leaf mould

Understanding the importance of timing, hygiene and the implementing of environmental controls can help you prevent outbreaks of tomato leaf mould.

• Monitor tomato crops for tomato leaf mould symptoms from April onwards and remain vigilant
• Train staff to recognise symptoms
• Maintain excellent hygiene standards throughout the entire season
• Identify and monitor disease hotspots where fulva infections occur early each year, and treat accordingly
• Minimise periods of relative humidity (RH) above 85% and keep the crop well ventilated
• Remove the lower leaves, where possible, to decrease inoculum levels and improve crop ventilation
• Fungicides with translaminar effects offer the best control
• Early use of biofungicides can extend conventional spray intervals and/or reduce the number of fungicide sprays required
• Ensure maximum crop coverage when treating. Treat the whole plant, angling the nozzles to target the underside of leaves
• Grow resistant varieties where possible, placing these in areas with a history of fulva infection
• Comprehensive clean-up, including the removal of all plant debris, will reduce the risk of future infections occurring in the following crop
• All disinfection products, such as Unifect-G and Menno Florades, should be used at their recommended, regulated rates. Check regulations before use

Figure 1. Severe tomato leaf mould infection with lesions and leaf wilting. Copyright Dave Kaye, RSK ADAS

Disease symptoms usually appear from April onwards, so it is important to monitor the crop from this time. Train your staff to do so, but be aware – the spores it produces can also cause human health issues in the eyes and lungs, leading to sickness or the need for additional PPE when working.

If an infection is likely to occur, and especially if it might reoccur, preventative measures should be implemented early, either to stop the disease taking hold or to reduce its effects.

If the disease is spotted early, when it is present on relatively few plants, pick off the affected leaves, directly into a bag, and dispose of them carefully.

During routine de-leafing of an affected crop, remove leaves directly from the house, or as soon as possible. Do not leave them on the floor.

Using a polythene floor covering, replaced each year, to act as a physical barrier preventing soilborne spores and moisture getting into the crop has showed good levels of disease control. But using certain types of floor cover (e.g. Mypex), which allow soil and dust through, could reintroduce infection to glasshouses and serve to create a humid microclimate.

Impermeable plastic sheeting can result in pools of water forming from leaky equipment. Fix all leaks, where possible, and slit the plastic to ensure that pooled water can drain away.

Crop monitoring

Crops should be monitored frequently for disease, especially at known hotspots or high-risk areas, and action should be taken before the disease becomes an epidemic. Train your staff to do this and recognise symptoms and points of risk.

Disinfection

Effective crop clean-up and glasshouse disinfection is key, and all remaining plant debris must be removed from the glasshouse.

Sites that have experienced severe disease levels will need all equipment, including irrigation lines and pegs, to be disinfected and all surfaces of the glasshouse structure treated, misting where appropriate. Also use disposable gloves and wash and disinfect hands regularly and certainly if moving between glasshouses.

The most effective products, such as Unifect-G and Menno Florades, should be used at recommended rates to ensure thorough disinfection.

Disclaimer. Please note:

The products named are not necessarily authorised as biocides across all UK cropping situations. Biocidal and plant protection products must only be used in accordance with the authorised conditions of use.

Regular changes occur in the authorisation status of biocides and plant protection products. For the most up-to-date information, please check with your professional supplier, BASIS-registered adviser or the Chemical Regulation Division (CRD) of HSE. 

Prevention of disease by managing the glasshouse environment is much easier than managing the disease. In fact, good environmental management will reduce incidence and severity of tomato leaf mould and has the potential to control the disease to levels in which no fungicide sprays are required.

Humidity

Glasshouses with relative humidities (RH) optimal for P. fulva (above 85%) are at the greatest risk.

It is essential that the crop is well ventilated and relative humidity (RH) maintained as low as feasible, with periods of humidity above 85% minimised. Throughout the year, it is important to maintain high standards of glasshouse and crop hygiene. Good environmental management will reduce incidence and severity of tomato leaf mould and has the potential to control the disease to levels in which no fungicide sprays are required.

Outbreak ‘hotspots’

On sites with less severe infection, the disease tends to break out in humid microclimate ‘hotspots’, e.g. where there are leaks and pooling water, or where condensation occurs at the edges of glasshouses. The current practice of humidity control in edible crops is generally based on humidity deficit (HD). The use of HD allows optimised plant development and growth through promotion of transpiration. Relative humidity (RH) gives a better indication of the risk of condensation developing, and therefore, in terms of fungal growth and disease, RH is more relevant than HD.

Dutch growers and some UK nurseries are now using RH as well as HD to both achieve good yields and control disease. Growers using HD should consider routinely checking RH levels, especially on the lower leaves. Nurseries with computerised systems that automatically control humidity below 85% RH are less likely to experience issues than those without.

Age and condition of the glasshouse

Disease levels at several sites were monitored during 2016. The age and condition of glasshouses was found to have an impact on tomato leaf mould.

Generally, older houses are leakier than new ones, creating favourable conditions for leaf mould development.

A grower reported that their site’s initial P. fulva outbreak occurred in a glasshouse with old, low glass before spreading to other, more modern houses. New glass can create a more sealed environment, resulting in higher humidities, whereas old glass is often more ventilated.

Glass condition is an important factor in the likelihood of disease establishment, but its impact is influenced by how each grower manages the crop, the environment and the quality of end-of-season clean-up/disinfection. A balanced management strategy needs to be developed to encompass all these factors, minimising disease risk while reducing overheads.

If you would like to order a hard copy of the Tomato leaf mould best practice guide, please contact:

Email: publications@ahdb.org.uk

Telephone: 0247 799 0069

Tomato leaf mould web pages originally authored by Sarah Maybe and Dave Kaye (RSK ADAS).