C6 – Wilts

Introduction

Fusarium oxysporum and Verticillium dahliae cause wilt diseases on a range of herb crops. Both fungi infect plant roots, then invade and block the vascular system (water pathways within the plant), resulting in wilting.

In the UK, the most common Fusarium wilt disease on protected herbs is caused by F. oxysporum f. sp. basilici which can be seed- and soil-borne, and is specific to basil. Fusarium wilt of basil has been reported in Russia, the USA, Italy, Israel and France as well as the UK. In the UK, the disease is sporadic but can cause severe crop losses when it occurs. Because of its seed-borne nature, it can affect basil irrespective of production system. F. oxysporum f. sp. basilici is specific to Ocimum basilicum (sweet and lemon basil cultivars) and does not affect other members of the Lamiaceae. In other countries, F. oxysporum is reported to cause wilt diseases on other herb crops such as cumin, coriander and sage.

V. dahliae is predominantly soil-borne (seed-borne on some hosts) and attacks perennial herbs such as mint (peppermint and spearmint), sage and chamomile as well as a range of other crop and weed hosts. As well as reducing yields of spearmint for culinary use, Verticillium wilt can also have a deleterious effect on essential oil yields from peppermint. The disease is most common in field-grown herbs, but could also affect for example, soil-grown herbs in polytunnels.

 

Symptoms

For basil Fusarium wilt, typical external symptoms include stunting of the plant, severe wilting (Figure 1), premature defoliation without chlorosis, and dark longitudinal streaks on the stems and petioles (Figure 2). Internal browning of the vascular tissue starting from the stem base often occurs and is a useful diagnostic feature (Figure 3). Disease development is more rapid in young plants, which may die 4-7 days after symptoms are first observed, compared with longer survival for older plants. In addition to wilt symptoms, abundant production of air-borne dispersal spores, seen as white or pale pinkish masses on diseased stems, often occurs (Figure 4). Occasionally, symptoms of crown and root rot (dark brown) on basil may also occur. At an early stage, the disease may be easily confused with Botrytis but internal stem discolouration and white/pale pink spore masses on infected stems help to distinguish plants affected by Fusarium wilt.

Of protected herb crops, mint species are most commonly affected by V. dahliae in the UK. Stunted plants with smaller, twisted (sickle-shaped) top leaves develop in patches. The plant foliage turns yellowish to red or bronze (Figure 5) and the vascular tissue becomes dark brown/black. Plants wilt and die (Figure 5) progressively from the base upwards. Microsclerotia (survival structures) develop in diseased plants and then remain several years in the soil.

Disease sources and spread

F. oxysporum f. sp. basilici is known to be seed-borne but can also persist in soil and crop debris. Seed-borne inoculum is the most likely source of outbreaks on UK basil crops. For example, a survey in the USA of 28 commercial seed lots from four countries revealed that 59% of the lots were contaminated with F. oxysporum. There is also potential for soil infestation to occur if Fusarium wilt develops in soil-grown, cut basil crops. While seed-borne inoculum is responsible for long-distance transmission of the pathogen, local spread of the disease can occur via movement of infested soil, dust and crop debris. Moreover, in situations where spores develop on the surfaces of infected stems, these can be readily dispersed by air currents, water splash, and re-cycled irrigation water.

F. oxysporum from basil does not infect herbs other than basil in the family Lamiaceae, but can colonise the vascular tissue of other herb species (e.g. rosemary and thyme) without causing symptoms. Colonisation of non-host herbs may provide a mechanism for survival in the absence of basil particularly if debris of these crops is incorporated into soil.

A life cycle for wilt caused by V. dahliae is shown in Figure 6. Verticillium wilt is initiated by small black survival structures in the soil called microsclerotia. Under favourable environmental conditions, the microsclerotia germinate to produce fungal strands (hyphae) which infect susceptible plant roots. The fungus invades the vascular system of a host plant and causes wilt symptoms by reducing the ability of the plant to conduct water. Microsclerotia may be produced in large numbers on an infected plant and then survive in soil for up to fifteen years. The fungus may also survive as mycelium or spores in the vascular system of perennial plants. Verticillium spreads readily via diseased planting stock and soil-contaminated equipment.

Although V. dahliae infects a wide host range (e.g. potato, strawberry and some tree species), there is evidence that different strains of the fungus exist, which are adapted to different hosts. Research in the USA indicated that strains of V. dahliae that infect potato and strawberry are less aggressive to spearmint and peppermint. However, it is also thought that populations of V. dahliae that can infect mint may be maintained at low levels in the soil by superficial or symptomless infections on many plant species.

 

Conditions for infection

Assuming the presence of inoculum in seed or soil, Fusarium wilt on basil is most likely to occur under warm conditions (25-28°C). Once infection has occurred, symptom development tends to be most rapid when water demand is high.

Verticillium wilt is favoured by moist soils and a temperature range of 21-27°C. Microsclerotia are stimulated to germinate by root exudates of both host and non-host plants. Symptoms of Verticillium wilt may be more severe in soil infested with the root lesion nematode, Pratylenchus penetrans. This nematode may increase the severity of the disease by altering the host physiology, thus making the plant more susceptible to damage.

 

Integrated disease management

Cultural control

Planting material
For basil, ensure that seed is obtained from a reputable supplier and that seed has been tested and found free of F. oxysporum. In particular, basil seed health should be confirmed before introducing a new variety to the nursery. Hot water treatment of basil seed at 56-58°C for 20 minutes was found to reduce significantly contamination due to F. oxysporum without affecting seed germination rates, although optimal conditions for treatment could vary with seed batch.

For mint and sage, check with the supplier to ensure that planting stock is free from Verticillium wilt.

Varieties
Most basil varieties show some degree of susceptibility to Fusarium wilt, with certain varieties such as ‘Spicy Globe’ miniature being particularly susceptible. However, two commercially available Genovese varieties, ‘Nufar F1′ (also known as ‘Nofar’) and ‘Aroma 2′, are reported to be resistant to basil Fusarium wilt.

Spearmint is less susceptible to Verticillium wilt than peppermint. Varieties of peppermint that are less susceptible than the commonly used Black Mitcham peppermint include Todd’s Mitcham, Murray Mitcham and Roberts Mitcham. However, in the presence of very high V. dahliae soil populations, even resistant cultivars may exhibit symptoms of Verticillium wilt.

Glasshouse and crop hygiene
With basil Fusarium wilt, it is important to monitor the crop regularly. If early symptoms are observed on a few plants, these should be bagged in-situ and disposed of outside the glasshouse in covered bins or skips. The aim is to remove infected plants before spore development results in secondary disease spread to other plants.

After a severe outbreak of basil Fusarium wilt, spores produced on infected stems and leaves are likely to contaminate benches, the glasshouse or polytunnel structures, machinery and other equipment, growing media/soil and irrigation water. Several disinfectant products offer good potential for clean-up of structures and equipment. The product Unifect G (QAC + glutaraldehyde) could be particularly useful for these disinfection purposes, since experiments on F. oxysporum from stocks showed it to give complete kill of spores and mycelium (fungal strands), even when peat contamination was present (HDC Project PC 213). Ensure that new growing media or disinfested soil is used for subsequent basil crops and that re-cycled irrigation water has been treated (e.g. by UV light). For further details on nursery hygiene and use of chemical disinfectants, see Section A – Principles of IPM and HDC Factsheet 15/05, respectively.

For Verticillium wilt, crop rotation should be seen as a preventative rather than a curative disease management option, since the host range of V. dahliae is wide and the survival structures (microsclerotia) can persist in soil for many years, even in the absence of a susceptible host. Rotation programmes should be initiated early before inoculum builds up to high levels in the soil. If an outbreak of verticillium wilt occurs, it is important to remove infested crop debris before microsclerotia are incorporated into the soil and to disinfest the soil before planting another crop of the same species.

 

Soil disinfestation

If there is an outbreak of either Fusarium or Verticillium wilt on soil-grown protected herbs, there is a high risk that the disease will re-occur if the same herb species is planted in the affected area, due to long-term survival of these pathogens in soil. A range of options for soil disinfestation is available using either chemical products or steaming techniques. The Factsheets listed in the ‘Further Information’ Section provide detailed information on the relative advantages and disadvantages of the various methods. Another soil disinfestation option includes flaming with a propane burner after harvest and prior to soil cultivations. Flaming will kill fungi in plant material and soil at the surface, but is not effective for eliminating pathogens at depth.

Biological control

Prestop and Prestop Mix (containing Gliocladium catenulatum) are biological fungicides approved for use on protected herbs as a drench, foliar spray or for media incorporation. There is reported to be moderate product efficacy against Fusarium root rots but efficacy against Fusarium wilt (especially if seed-borne) is not known. Research in the USA and Italy has identified non-pathogenic strains of F. oxysporum that can reduce the incidence of Fusarium wilt but these are not commercially available or approved for use in the UK. There is also evidence that soil amendments with compost or bark can reduce Fusarium wilt severity in cases where the fungus is soil- rather than seed-borne. There are currently no biological fungicide products approved for use against Verticillium wilt in the UK.

 

Chemical control (protected herbs)

Wilt pathogens such as F. oxysporum and V. dahliae colonise vascular tissue within roots and stems making them a difficult target for which to achieve effective fungicidal control. Research on cut flowers (HDC projects PC 213 and 213a) indicated that of the fungicides approved for protected herbs, azoxystrobin (Amistar) or prochloraz (Scotts Octave) applied to foliage then irrigated in, reduced the severity of Fusarium wilt but did not provide effective disease control. Scotts Octave was also moderately effective in reducing the incidence of hebe Fusarium wilt (HDC project HNS 146). Both azoxystrobin and prochloraz are approved for use on protected herbs. Note that for azoxystrobin, the harvest interval is 28 days for crops harvested in November to April and 14 days for crops harvested in May to October.

Before selecting products to use for wilt control on protected herbs, test treat a small batch of plants before widespread application if using a product for the first time, to ensure crop safety.

Refer to Section A – Principles of IPM for details on strategies to minimise the risk of selecting resistant strains.

• Full details for the use of biological control agents and compatibility of pesticides are available from biological control suppliers or consultants.
• Pesticide approval information in this guideline is current at 31 March 2013.
• Regular changes occur in the approval status of pesticides arising from changes in pesticide legislation or from other reasons. For the most up to date information, please check with a professional supplier or the CRD website http://www.pesticides.gov.uk/.  General enquiries on pesticides and detergents are now being handled by the Defra Helpline (as of April 2013): Tel: 08459 33 55 77.
• Always follow label recommendations or statutory conditions for use on Extension of Authorisation for minor uses (EAMU, previously SOLA) notices of approval.
• Always follow instructions for Pesticide Resistance Management guidelines given on the label or EAMU.
• Growers must hold a paper or electronic copy of the current EAMU before using any product under the EAMU arrangements. Any use of a pesticide with an EAMU is at grower’s own risk. Relevant EAMUs are sent to HDC members by HDC, or are available from CRD (see above) or from consultants.
• Use pesticides safely.

 

Further information

HDC Factsheet 08/07. Integrated management of stock fusarium wilt.

HDC Factsheet 09/07. Soil disinfestation options for cut flower growers

HDC Factsheet 15/05. Use of chemical disinfectants in protected ornamental production.

HDC Project PC 213 and 213a. Protected stock: aspects of the biology and
control of Fusarium wilt, a new disease problem. Final Report, January 2007.

HDC Project HNS 146. Hebe: aspects of the biology and control of Fusarium wilt. Final report, 2009.