Lettuce Fusarium wilt and root rot

An outbreak of lettuce Fusarium wilt and root rot caused by an aggressive strain F. oxysporum f. sp. lactucae was reported on protected lettuce in the UK for the first time in early October 2017 in Lancashire. The pathogen was identified as Race 4, a particularly aggressive strain of the fungus with no known treatment or varietal resistance available to date.

The disease has also been reported for the first time on lettuce in Ireland. However, the pathogen race has not yet been confirmed.

Now that the disease has been reported in the UK, propagators and growers are advised to review their production protocols, particularly regarding crop hygiene.

AHDB commissioned the University of Warwick to produce a technical review on lettuce Fusarium wilt with detailed information on disease management options to help minimise the impact of this disease on the UK lettuce industry.

Monitor crops frequently so that the disease is identified at an early stage. If Fusarium wilt symptoms are suspected, it is recommended that a sample is sent promptly for diagnosis at a UK research organisation or via your seed company. 

Under the AHDB project Fusarium: Investigations into the control of basal rots in crops (FV POBOF 452), Dr John Clarkson at Warwick University will accept lettuce samples from growers who suspect symptoms of Fusarium wilt, for free testing of Fusarium species present. He will respond to individual growers with results of Fusarium species and race found, but is unable to provide related agronomic advice. Growers should send several affected lettuce plants with the stem base fully intact and some roots if possible. Details of grower contact details and location should be included but will remain anonymised. 

John can be contacted on: john.clarkson@warwick.ac.uk

Details for sending samples are as follows:

FAO Andrew Taylor/Alison Jackson, Warwick Crop Centre, School of Life Sciences, University of Warwick, Wellesbourne Campus, Wellesbourne, Warwick, CV35 9EF.

A wilt disease of lettuce caused by Fusarium oxysporum f. sp. lactucae was first described in Japan in 1955. This disease was then detected in other Asian countries, in the USA and finally in Europe (initially Italy and Portugal, and since confirmed in France, the Netherlands and Belgium). For over three years, protected lettuce growers in the Netherlands and Belgium have been battling with a particularly aggressive strain of the pathogen known as Race 4. Up until now, the pathogen has been contained in mainland Europe.

UK lettuce crops with severe symptoms of Fusarium wilt

Lettuce wilt and root rot caused by Fusarium oxysporum f. sp. lactucae is listed on the Defra pest risk register. Defra have confirmed that, in line with other European countries where this disease has occurred, no statutory action or surveillance is planned to check imports of lettuce seed or propagation material or to monitor outbreaks of lettuce wilt in the UK. There is no requirement to alert PHSI if outbreaks occur. 

Affected plants show leaf yellowing and wilting. When cut open, crown tissues and upper roots usually show reddish-brown necrosis followed by decay. Necrosis of vascular tissue in leaf veins can often be seen. Severely infected plants are stunted and often die.

Another pathogen, Pythium tracheiphilum, can also cause vascular wilt with similar early symptoms. Therefore, it is important to get laboratory analysis to ensure which pathogen is responsible.

In mainland Europe, F. oxysporum f. sp. lactucae is considered as one of the limiting factors for commercial production of lettuce during the summer season. In Italy, for example, up to 70% losses in contaminated lettuce fields have been observed. Under favourable climatic conditions (the optimum soil temperature is between 24–28°C although the disease can occur at temperatures as low as 8°C), an infection may kill the crop. In the Netherlands, some growers have been obliged to stop growing lettuce in infested structures during the summer months.

Symptoms of lettuce wilt and root rot

Lactuca sativa and Valerianella locusta (lamb’s lettuce) are known hosts. Most cultivar groups of lettuce, such as butterhead, iceberg, oak leaf, romaine, and Batavia, are susceptible to the disease.

 
Lettuce wilt affects both field-grown crops (where temperatures are high) and protected soil-grown crops. Soilless hydroponic production has not been significantly affected to date unless heavily infected plant material has been used. However, the pathogen can be transmitted in water so poses a potential threat to all UK lettuce production systems.

F. oxysporum f. sp. lactucae is primarily a soilborne disease. It is thought that the fungus penetrates the plants via natural apertures or wounds on the roots, and that its chlamydospores (resting spores) can remain viable in the soil or on plant debris for several years. In addition, research in Italy has shown that lettuce Fusarium wilt can be seed-borne, although the relative importance of disease transmission via seed has not been confirmed.

Over long distances, transmission occurs via young plants (which are likely to be symptomless at this stage), infested soil and potentially seed. Over smaller distances (i.e. within a country or region) infected transplants can rapidly spread the disease, and on a smaller scale (from field to field, or between protected structures) the disease is spread by water, or soil and crop debris attached to machinery, agricultural tools, trays and clothes/footwear.

Disease control is extremely difficult since spores survive for a long time in soil and crop debris; there are no effective fungicides, no available resistant varieties and soil sterilisation has limited efficacy. Therefore, avoidance is the best strategy.

Review hygiene measures on your nursery and with your plant propagator to avoid the disease getting on to your site.

AHDB has received guidelines and protocols regarding hygiene for the prevention of lettuce Fusarium wilt from Teagasc, Ireland and a research programme in Belgium. Growers are recommended to read the documents and to conduct a risk assessment to identify potential sources of disease that could infect your nursery and to help identify how to prevent spreading the disease between your sites and onto other nurseries. 

A contaminated seed is a potential source of disease transmission. Propagators should check with seed suppliers whether seeds have been cleaned and request results from seed testing for F. oxysporum.

Ensure hygiene measures during propagation are rigorous. Sterilisation of equipment and glasshouse structures is particularly important. Tray washing and disinfection are vital to remove soil from trays that have been stood outside and ensure that contamination from infested soil or debris from a previous crop is eliminated.

Before disinfecting a surface, remove peat, plant debris or any other organic matter as far as is reasonably practicable (by sweeping, vacuuming or washing) as such contaminants are likely to reduce product efficacy.

Do not use disinfectants on plants, soil or growing substrates – such use is illegal unless the chemical is registered as a pesticide or basic substance for that particular use.

Disinfectant concentration, contact time and the level of organic matter contamination are likely to be the main factors influencing the activity of a given disinfectant against a particular microorganism. However, check the product label carefully as other factors, such as temperature, pH and water hardness, may also have a significant influence. Some disinfectants are combined with detergents which have good wetting powers and increase contact with surfaces.

To learn more about disinfectant use, including results from previous research on disinfectant efficacy against F. oxysporum, please refer to Use of chemical disinfectants in protected ornamental plant production

Reducing the risk of pathogen survival between crops and disease spread

• Fusarium wilt on lettuce is primarily a soilborne disease and can survive in soil and crop debris. It is highly likely to reoccur if lettuce or lambs lettuce are planted in an area where the disease was severe in the previous crop

• If the disease has been confirmed on-site, soil from affected beds should not be rotovated or spread to other areas of the nursery. If only a few plants are affected, remove them from the crop (bag them in situ) before spore production occurs on the basal leaves of the plant. If the disease is widespread, plant material should be uprooted and burned, or put in a covered skip for landfill. Do not bury the material in soil or add to discard piles or compost areas

• Avoid overhead watering of affected crops to minimise disease spread by water-splash. Once the crop is removed, flaming using a propane burner may be effective and useful for killing the pathogen in remnants of crop debris and soil at the surface, but is unlikely to control the pathogen at depth

• Control of Fusarium wilt using fungicides alone is unlikely to be effective since it is a soilborne disease that colonises the vascular tissue within roots and stems, making it a difficult target. In project PC 213, Amistar (azoxystrobin) significantly reduced Fusarium wilt in artificially inoculated plug plants of stocks but was not completely effective when a programme was applied to a crop with high inoculum pressure. More recent trials on stocks (PO 005a) did not identify any conventional or biological products with efficacy against Fusarium wilt. Results from recent Belgian trials suggest that Amistar and the biofungicide T34 Biocontrol (Trichoderma asperellum strain T34) were the only products to show activity against lettuce Fusarium wilt.

• Information from mainland Europe suggests that soil disinfestation techniques have limited efficacy in eliminating the lettuce wilt pathogen from soil. Previous AHDB research looked at soil steaming options for controlling F. oxysporum in soil.

In summary:

• Soil steaming (sheet, vacuum or plough) gave moderate to good control of F. oxysporum in woody stem pieces of stocks
• Soil steaming treatment efficacy is likely to be improved by prior removal of large pieces of crop debris
• Where steam treatments are used, ensure that soil temperature has exceeded 90˚C for 30 minutes to at least 25 cm depth
• Take care not to re-contaminate newly disinfested soil through the use of dirty equipment or shoes for example

Further information on this research, including the advantages and disadvantages of different types of soil steaming, can be found at:

• Factsheet 08/07: Integrated management of stock fusarium wilt
• PC 213 - Protected stock: aspects of the biology and control of Fusarium wilt, a new disease problem
• PC 213a - Protected cut flowers: evaluation of two steaming methods for disinfesting soil of Fusarium oxysporum and Sclerotinia sclerotiorum (extension to PC 213)

Research is being carried out on disease transmission, the effects of crop rotation (probably five years without lettuce crops would be necessary), crop management (e.g. planting dates), soil disinfestation (e.g. steaming, solarisation, chloropicrin) and biological control measures (e.g. Streptomyces griseoviridis, Trichoderma harzianum and hypovirulent strains of Fusarium) as well as breeding for resistance. Since the fungus can potentially be seed-transmitted, there is a risk of spreading the disease across continents. It is thought that the use of healthy planting material (seeds, transplants) produced in the framework of certification schemes could be an essential tool to prevent further spread of the disease through trade.

Dr Andrew Taylor presented an update at the Brassica and Leafy Salad Conference, in Peterborough in January 2019. You can see his presentation here.