Blight Control

Table of Contents

Calculating when to spray for blight using a spreadsheet. 

An Excel spreadsheet that can be used as a tool to decide when to spray for blight was originally developed at Lincoln University.

The spreadsheet has been used over a number of years by Tim Armitage, a walnut grower and member of NZWIG.

Tim has modified and improved the spreadsheet and a copy of his version, that you can use, can be requested below.

To be successful it is necessary to input hourly data for air temperature and relative humidity from your closest Weather Station or you own datalogger.

The first page of the spreadsheet contains information and notes on its use.  

Disclaimer. It is likely that using this spreadsheet is the most accurate method of deciding when to spray commercial walnuts for blight in New Zealand but neither Tim Armitage nor the New Zealand Walnut Industry Group guarantee the results on any particular orchard.  

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Guide to spraying for the management of walnut blight

At the time that this paper was written Mankocide DF was considered to be the best spray. In recent years other less expensive chemicals, and chemical mixes have been used. 


NZ Walnut Industry Group[1] & David Manktelow[2]



This guide provides a brief introduction to:

  • choosing spray concentration.
  • determining chemical application rate requirements for different tree sizes.
  • methodology for setting spray nozzle flow rates and groundspeed.
  • achieving an even coverage of your canopy.
  • assessing spray coverage.
  1. What concentration to use?

Mankocide DF is a mixture of copper hydroxide and mancozeb active ingredients formulated as water dispersible granules.  A mixing concentration of 250g of Mankocide DF per 100 litres of water is recommended when applying high volume “dilute” sprays, as we are commonly doing in walnut orchards. (Note: If you prefer to measure your Mankocide DF by volume, not weight, the equivalent concentration is 350ml of Mankocide powder per 100 litres of water).


“Dilute” spraying is usually taken to be the volume required to wet the outer (easily accessible) canopy to the point at which spray droplets begin to coalesce and start to drip off leaves (“the point of runoff”), with the inner and upper (less accessible) canopy seen to be reasonably well wetted with spray droplets.  This guide explains how to calculate application volume for dilute spraying.


“Concentrate” spraying: It is also possible to apply products like Mankocide DF at application volumes below the point of runoff, if they are mixed to higher concentration.  For example, if the application volume required to wet a particular canopy to the point of runoff was 1000 litres per hectare and a helicopter application was made at only 250 litres per hectare, it would be standard practice to concentrate the chemical up in the spray tank by a factor of between 3X and 4X the dilute rate to ensure that sufficient chemical deposits were achieved on the crop to provide effective control.  The use of low volume spray applications can be very efficient in that less chemical is lost to the ground as runoff, hence the potential to use slightly less chemical in low volume sprays (usually 20% less) to achieve the same outcome as a higher rate at high volumes.


However, the use of higher spray volumes (as described in this guide) compensates for a multitude of application sins, and low volume spraying should only be attempted using a well setup sprayer that is capable of projecting the spray plume into the tops of the trees.


  1. What application volume per hectare is required for different sized trees?

If you are spraying small trees with a backpack sprayer or similar, you can easily spray “to runoff” by just observing the leaves, nuts and branches as you go.  You will be adjusting the spray volume by eye at each tree because you can see the coverage you are achieving on each one.  However, once you are spraying larger trees from a tractor-mounted rig or similar, you will be traveling at a constant speed and applying a constant application volume (you can’t adjust by eye at each tree) so it is necessary to calculate the required application volume in advance, and then set up your spray rig to achieve this.


There are several different methods for working out spray application volumes for different sized trees.  However, the logic is simple – think of painting the exterior of a house: the more wall area your house has, the more paint you will need.  The larger the floor area of the house the larger the wall area will be.  Likewise, the more storeys the house has the larger the wall area will be.  Trees are obviously a little more complicated to “paint” with spray because they have targets (leaves, nuts, buds etc) distributed throughout a volume of space.  However, it is reasonably easy to estimate the required spray application volume based on the volume of space occupied by your trees.


Step 1:   Estimate average height of tree canopy (excluding bare trunk) in metres eg. 6 m

              and estimate average spread of tree canopy in metres eg. 5 m

              then calculate the tree profile through which the spray is moving = height × spread = 30 m2

              This tree profile is the area of canopy you would observe by viewing a row end-on.


Step 2:   Calculate the summed length of rows per hectare by:

              Measure the row spacing in the driven alleyway eg. 8 m

              then calculate the row length per hectare = 10,000 ÷ row spacing, eg. 10,000 ÷ 8 = 1,250 m


Step 3:   Then the canopy volume per hectare (as if the trees form a constant hedgerow – don’t worry if they don’t)

              = tree profile area × row length per hectare  eg. 30 × 1,250 = 37,500 m3


Step 4:   Finally, estimate the spray application volume required to wet this canopy volume evenly with spray.  Work on other crops has shown that one litre of dilute spray can effectively cover between 11 and 17 cubic metres of canopy volume to the point of runoff.  In open (relatively hollow) trees the larger figure will probably apply, while in dense canopies (usually younger trees that have not hollowed out) the smaller figure is likely to apply.  To calculate the spray application volume, simply divide the cubic metres of canopy volume calculated in Step 3 by the appropriate coverage factor.  Say our trees are quite open, so we will use a coverage factor of 17 cubic metres of canopy volume per litre of spray.  Then in the example canopy above:

  1. 37,500 ÷ 17 = 2,200 litres per hectare required.

Now you need to set up your sprayer to deliver this target application volume onto your trees, as described in the next section.  If your trees form a continuous hedgerow you will need to run the sprayer down the row with a constant emission.  However, if there are gaps between the trees you should turn the sprayer off in the gaps.  Turning off in the gaps gives big spray savings.  In this case the sprayer still needs to be set up to deliver the calculated target application volume per hectare (as for continuous operation), but the actual volume delivered will be less than this because part of your hectare was gaps that did not need to be sprayed.


  1. Setting your sprayer to deliver a target application volume

To calibrate your sprayer to deliver a desired application volume, you have two practical options:

EITHER choose how fast your sprayer will travel then calculate the total flowrate of spray required (sprayer output)

OR choose your flowrate (ie, your spray nozzles and pressure) then calculate how fast you must travel


To do the calculations, there are four variables to consider:

  1. The target application volume (litres per hectare), which we calculated above.
  2. The width of the strip (swath width) which you are spraying in one pass (metres)

This is usually the row spacing (assuming your sprayer sprays both sides at once), but if you are spraying from only one side of a sprayer work on the distance from the nozzles to the tree trunks.

  1. The output flowrate from your sprayer (litres per minute)

This is the total volume delivered by your sprayer pump out of all of the operating nozzles.  Nozzle flow charts tell you the litres per minute output from different sized nozzles at different operating pressures.  Alternatively you can measure the flowrate from each nozzle by timing how long it takes to fill a 10 litre bucket.  You will find that pump capacity often limits your application volumes and/or travel speed options.  As a rule of thumb it is important to keep at least 20% of your pump output volume in reserve to maintain tank agitation.

  1. The forward speed of your sprayer (kilometres per hour)

To measure your forward speed, time how long it takes to drive a given distance (eg. 50m).  For example, if you travel 50m in 40 seconds, you are traveling at 50x3.6÷40=4.5km/hr.  Forward speed is usually limited by the ability of the sprayer to throw the spray plume into the tops and inner parts of tree canopies.  As trees get larger travel speeds will typically need to be held below 5km/hr and may need to be below 3km/hr.  Coverage assessment will tell you how fast you can travel.  The faster you try to go the larger your pump output needs to be to achieve your target application volume, as the following calculations show.


There is a simple relationship between these four variables – provided you know any three of the variables the fourth can be calculated.  The methods for calculating the two variables you might be interested in are given below.


#1 Use target volume per area (litres per hectare), speed and swath width to work out the required nozzle output rate (litres per minute)

Calculating total sprayer output required (l/min)

Output (l/min)  =  app. volume (l/ha) x width (m) x speed (km/hr) ÷ 600

You would do this if you already have a tractor (or other spray rig) and are already comfortable travelling at a particular speed, but you still have the freedom to choose the number and type of your spray nozzles or the pressure at which you run them.

eg, for a target application volume of 2,200 litres per hectare, a swath width (row spacing) of 8 metres, and a travelling speed of 4 km/hr, the required output = 2,200 x 8 x 4 ÷ 600 = 117 litres per minute


#2 Use target volume per area, sprayer output and swath width to work out required travel speed (kilometres per hour)

Calculating required travel speed (km/hr)

Speed (km/hr) =  output (l/min) X 600 ÷ width (m) ÷ app. Volume (l/ha)

You would do this if you have already set up the number and flowrate of the nozzles on your sprayer, but you have the freedom to choose how fast you will drive while spraying.

eg, for a target application volume of 2,200 litres per hectare, a swath width (row spacing) of 8 metres, and output of 90 litres per minute, the required speed = 90 x 600 ÷ 8 ÷ 2,200 = 3.1 km/hr.


Reality check: You may find that your traveling speed and sprayer output (flowrate) are constrained by practicalities, so are not exactly in line with the figures you have calculated in the preceding steps.  In this case, do a quick ‘reality check’ to make sure that the speed and flowrate at which you are operating are capable of producing at least approximately the volume of spray you require, using the following calculation.


Measure output, speed and swath width to work out actual volume applied per area (litres per hectare)

Calculating spray application Volume (l/ha)

App. volume (l/ha) = output (l/min) X 600 ÷ width (m) ÷ speed (km/hr)

eg, for a swath width (row spacing) of 8 metres, a measured output of 105 litres per minute, and a measured travelling speed of 3.7 km/hr, the application volume achieved is 2128 litres per hectare.


A well calibrated sprayer should deliver a spray volume that is within 5% of that targeted and any differences of more than 10% should be fixed.  For example, if a sprayer is calibrated to deliver 1,000 litres per hectare, it should consistently be able to deliver 950 to 1,050 litres per hectare and not go beyond 900 to 1,100 litres per hectare.


  1. Achieving even coverage

Once you have worked out the sprayer output required, you need to distribute this between the different nozzle options on the sprayer to give an even coverage through the trees.  This may seem daunting, but is actually quite easy if you follow the following rules.

Rule 1:   Park the sprayer in a typical row the distance out from the trees that it will be operated.  Turn off any nozzles that will not directly target the tree canopy (allow at least one nozzle to spray the area immediately above large canopies to “rain” down onto it).

Rule 2:   Divide the available nozzles into those that target the upper half of the tree and those that target the lower half.  Select nozzles to deliver 60-70% of the pump output through the upper set of nozzles into the upper half of the trees (use nozzle flow charts that tell you the output from different sized nozzles at different operating pressures, or directly measure the litres per minute from each nozzle by timing how long it takes to fill a 10 litre bucket).

Rule 3:   The further a nozzle has to throw the spray the narrower its output angle needs to be and vice versa (again nozzle flow charts should give you output angles).  Typically aim for 20-30o output angles from nozzles that need to throw the spray any distance and for wider angles on nozzles closer to the canopy.


  1. How to test whether you are achieving an even coverage of your canopy

It is very important to check on spray coverage achieved by your sprayer.  An easy and inexpensive check is to get someone else to drive the sprayer and stand back and look at how the spray plume interacts with the canopy.  Does it reach the tops of the trees?  Are there any obvious gaps in the output between different nozzles?  If you can see misses like these adjust (or change) the nozzles, try slowing down, try spraying closer to or further away from the canopy etc.


The next check on coverage is to look at the spray wetting of the foliage and branches immediately after the sprayer has passed through the canopy (best done with water not chemical!)  Again look for obvious dry spots and try tweaking the sprayer to fix them.


For more detailed observation of your sprayer performance, we recommend placing water sensitive papers in your tree canopies.  These paper “tags” are typically about 25mm wide by 75mm long strips of yellow paper that turns blue on contact with water – most agchem merchants will sell them (expect to have to pay about $70 for a packet of 50 unfortunately).  These can be stapled directly onto leaf top and bottom surfaces.  Alternatively they are easily deployed and retrieved by attaching them to a vertical pole in your canopy at various heights (say 1500mm, 3000mm, 4500mm etc from the ground up to the top of your canopy).  They should be folded over so that both sides will be sensitive to water, and attached two at each height with one aligned horizontally and one vertically.  Refer to pages 5–6 with the person in the red coat setting them up in a vineyard.  Clothes pegs or bulldog clips (with the odd extra hole drilled) can be made to work well for attaching the tags to the pole.


Place the pole in the tree-line well inside the canopy (so not mid-way between two trunks where the canopy may be thinner but also not too close to a trunk where it may be shielded from spray).  If your sprayer sprays both sides, place a pole in the tree-line on both sides, since performance of the sprayer can be different on the two sides.  Then drive down the row between the trees and spray them and the tags on the poles.  When you have finished, staple the tags carefully on the assessment sheet (see blank sheet and instructions on pages 7–8.  Pages 9–10 of this guide show the water sensitive tags being used in a walnut orchard and explain how to interpret the result.



[1] Clive Marsh, [email protected]

[2] Applied Research and Technologies Ltd, PO Box 3415 Napier Mail Centre,Napier, [email protected]

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New Zealand Walnut Industry Group