Leaf spots and fungicides: one size doesn’t fit all

A recent webinar designed to boost the nursery industry’s knowledge of leaf spots and fungicides has highlighted that prevention is key, particularly when it comes to correct identification and treatment of leaf spots.

The webinar, ‘Leaf spots and Fungicides’, is one of six produced by the nursery levy-funded project, ‘Building the resilience and on-farm biosecurity capacity of the Australian production nursery industry’.

Presenter and project manager, Dr Andrew Manners, provides production nurseries with key tips and tricks on how to identify leaf spots and understand which application to use during the prevention, treatment or eradication phase.

Correct identification

Correctly identifying the cause of leaf spots is critical, so as to manage the problem cost-effectively, says Dr Manners.

For problems that occur regularly, proactive management strategies will be far more profitable than reacting to a problem after it occurs.

In almost every case, it’s better to prevent the problem than to treat it with chemicals, which can have plant health implications down the track.

Those unsure of the cause should seek further information from industry factsheets, the pest ID tool and the other webinars in the series.

Some key examples include:

  • Anthracnose and many other fungal leaf spot pathogens: Management actions include removing heavily infested material, reducing leaf wetness, increasing ventilation and applying the fungicides to protect healthy plants.
  • Botryosphaeria dieback: Tends to be a stress pathogen and can sometimes be pruned out in early stages. Once it becomes systemic, remove the plant. Fungicides are unlikely to be beneficial.
  • Phytopthera: Phytopthera occurring as a leaf spot is likely to be present in the growing media and the root system. Growers should remove all infected material and apply pesticides to healthy plants only, and put in place rigorous hygiene practices to prevent further infections.
  • Bacterial infections: It’s recommended that growers remove infected plants and use copper products to protect healthy plants. Note, there are no antibiotic products available for application on plants in Australia.

Protectant Fungicides

The disease cycle is centred on infection, colonisation, symptom development and spore production.

Protectant fungicides can provide a protective barrier to stop infection from occurring in the first place.

The barrier (product) must be applied to healthy tissue and often involves repeated applications during periods of high pest pressure.

Protectant fungicides do not need to be applied to all crops and should not be applied prophylactically throughout the year.

Curative/Penetrant Fungicides have mild systemic properties, moving into the plant to a limited extent.

They generally act against plant pathogens during the infection and early colonisation stage and may also provide a protective barrier to stop further symptom development.

Eradicant fungicides act on the later stages of colonisation and are often referred to as “anti-sporulants”.

They are designed to stop fungal growth and production of reproductive structures, i.e. they stop spore production.

Growers are reminded that product efficacy may vary, depending on a range of factors. These include:

  • Pathogens
  • Host plants
  • Time after infection
  • Inoculum load
  • Product concentration
  • Environmental Conditions.

The overall message is that whilst fungicide treatment is worthwhile, it’s not a ‘one size fits all approach’ to every pest or pathogen.

While it’s not always easy to identify which fungicides are best at what stage, there are a great range of resources below to assist, including this webinar.

More information