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The management and life cycle of black spot (Venturia inequalis)

Pest and Disease Management

On 18 August, APAL was glad to host a webinar with insights from Dr Peter Triloff and Fanny Le Berre (RIMpro) into the management and life cycle of black spot (Venturia inequalis). 

A range of topics were covered, highlighting key factors in black spot development, techniques to reduce inoculum levels, tree management, spray timing and selection and the capability of models to assist in spray timing and selection.

Tree management

Given the susceptibility of young growing tips to the disease, Peter highlighted the necessity to ensure termination of growing tips by mid-summer as well as reducing the use of practices that encourage late summer growth (early summer pruning). Where terminal buds are not formed, significant overwintering inoculum can be held on the tree producing a mass of conidia in the following season starting you off with the potential for large secondary infections due to overwintering conidiospores. 

Suggestions to manage trees included: 

  • Use of dwarfing rootstocks (particularly with more vigorous scions).
  • Minimal cuts where possible. 
  • Tie down steep shoots if not removing. 
  • Avoid early summer pruning that may see extended late season growth into winter. 
  • Consider root pruning and growth regulator usage.  

Inoculum management

The disease pressure is largely linked to the level of overwintering inoculum from the previous season. Where spore loads are high (such as where the disease was established in the previous season) the efficacy required from fungicide is significantly higher and unlikely to provide control relative to orchards with lower inoculum levels.  

The solution? Reduce the inoculum. 

Given the majority of the overwintering spores are located in leaf litter on the orchard floor, examples were given from vacuuming inoculum with specialised implements, to blowing to pick up leaves to the simple sweep/rake and mulch.  The key message was that a lower inoculum orchard will have less disease pressure, increasing the likelihood of adequate control. 

Sprayer coverage

Whilst tree and inoculum management are both key factors in controlling the disease, effective coverage of fungicides is also critical. Peter provided a range of examples of why high angles of incidence from the nozzle to the canopy present challenges to adequate coverage of fungicides, with narrow rows combined with axial fans regularly providing inconsistent spray coverage from the bottom to the top of the canopy (worse in the top) in a range of studies he has been involved in. 

For individuals grappling with disease issues in the upper canopy, Peter’s essential advice was straightforward: When dealing with tall crops planted in narrow row distances, there are two primary courses of action to consider. One option is to employ tall fan types, such as tall tower sprayers, to maintain an angle of nozzle incidence of less than 40 degrees. Alternatively, one can opt for a reduction in tree height to guarantee effective disease control.

Fungicide choice and timings

In the final section of this webinar, Peter highlighted the key types of fungicide, their broad modes of actions and the timings in which they are effective as well as some of their key benefits and limitations as well as their relative risk of developing resistance. Whilst fungicides play a key role in disease control, Peter highlighted that ensuring tree management, inoculum reduction and sprayer calibration/choice are all addressed are absolutely critical. Understanding the actions of fungicide and the appropriate timings were discussed in length with the use of protectants at the appropriate timings (taking care of leaf growth and rainfall) both before the onset of rain and during the germination window should be considered.  

For more information on resistance management in Australia, please see: CropLife Australia | Apple, Pear – Apple and Pear scab) 

Peter’s discussion on fungicide choice and timing was further supported by a video recording of Fanny Le Berre from RIMpro demonstrating the capability of the modelling system to identify infection windows, their severity and plan for control options by planning sprays and their relative impact based on weather (ie. rainfastness) and choice of product. Infection periods for both primary (ascospores) and secondary (conidiospores) can be viewed/forecasted in the model highlighting the severity of each infection period allowing targeted timing of sprays for these infection events. For those interested in using RIMpro, feel free to contact Marcel Veens.

We thank Peter and Fanny for their involvement in this webinar and look forward to the next one. 

Peter’s notes are available here and growers are strongly encouraged to read them for additional information about the above key messages. 

Have a suggestion for a webinar topic? Feel free to get in touch with Nic Finger [email protected] with your suggestions. 

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