VG01082 Broccoli Head Rot

White blister – Albugo candida – is the most important foliar disease of brassica crops in Australia. The disease can affect all stages of plant growth, but its greatest impact is on infected flower heads, resulting in substantial losses to yield and higher costs in sorting.

This Horticulture Australia project aimed to evaluate fungicides for white blister control and to expedite the registration of suitable fungicides in Australia for white blister control on brassica and leafy brassica crops.

In Tasmania and Victoria, favourable field conditions, consecutive plantings and multiple plantings combine to create a constant disease pressure over a prolonged period of time, as well as prolonged exposure of fungal populations to a fungicide.

The systemic fungicides, Amistar, Cabrio, Ridomil Gold MZ and Ridomil Gold Plus have a high risk of resistance development if they are exposed to constant disease pressure.

Failure to follow resistance management guidelines carefully will likely result in the loss of these new fungicides as effective control measures.

Growers should adopt an integrated disease management strategy that incorporates the use of fungicides, along with other management practices such as resistance cultivars, crop rotations and reducing multiple plantings, for long-term and sustainable disease control.

Conclusions :

For long-term and sustainable white blister control growers should adopt an integrated disease management strategy that incorporates the use of fungicides along with other management practices such as using disease tolerant cultivars, crop rotations, management of crop debris, avoid consecutive brassica crops and reducing multiple planting.

• Currently, the most effective fungicides for white blister control are phenylamide (metalaxyl-M) and strobilurin (azoxystrobin and pyraclostrobin). These fungicides have a high risk of resistance development.

• It is essential to use a management program to prevent or delay the build-up of resistant fungal populations, in order to maintain and prolong the useful life of the most effective fungicides. Attempting to manage resistance after it has developed is far more difficult than prevention.

• Systemic fungicides should be used early in the epidemic when the pathogen population is low. High-risk fungicides should be used at the manufacturer’s recommended full rate and application interval.

  Therefore, Amistar, Cabrio, Ridomil Gold MZ and Ridomil Gold Plus should be used early; when infections are first noted or when field conditions are conducive to infections.

• A spray mixture of Amistar or Cabrio with copper fungicide should also be considered for fungicide resistance management.

• It is best to limit high-risk fungicide to no more than two consecutive sprays during the crop or as recommended by their manufacturers.

• Copper fungicides applications should be considered, following the early systemic fungicide applications, in order to prevent head infections.

• The number of spray applications will depend on locations, disease pressure, climatic conditions, cultivar susceptibility and crop growth.

  The disease risk prediction model that was developed in UK based on the temperature and duration of leaf wetness has been shown to have the potential to improve fungicide application timing for white blister control in Australia.

• In commercial practice, it is typical to apply up to two systemic fungicide applications in each planting at the initial flower head formation.

  Therefore, in order to reduce long-term exposure of the pathogen to a fungicide and hence reduce the risk of fungicide resistance, a block of two spray applications of a systemic fungicide should be considered.

  For example, apply two applications of Ridomil Gold MZ in one planting, followed by two applications of Amistar SC in the next planting.

Acknowledgments :

This project has been facilitated by Horticulture Australia Limited, and has been funded by voluntary contributions from Syngenta Crop Protection Pty Ltd and the Australian Government.

Nufarm Australia Limited and Agrichem Industries Pty Ltd also contributed voluntary contributions in the first year for preliminary screening of their fungicides.

Key researchers involved in the residue trial studies include : Rodney Burns, Jane Floyed, Kate Williams, Paul Florissen, Elizabeth Fields, Scott Winner.

The assistance of broccoli growers who provided the trial sites in Tasmania and Werribee South, Victoria, is gratefully acknowledged.