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Mill’s Periods are not the entire story for apple blackspot management. Now you can interpret weather information for the likelihood of ascospore release events throughout the season, and get a better definition of when the blackspot season begins and ends. Combine with information from MetWatch on Mill’s Periods for a more complete picture of Blackspot on your orchard leading to better control, and more peace of mind. For further information on blackspot and the importance of understanding ascospore maturation and release follow this link.
Screenshot from the Ascospore Release Model. Simply use File |Open to select weather station file to work with, and click on the "lightbulb" icon to run the model.
Daily data available in tabular format as well as graphs by clicking on View | Table:
For MetWatch users, use View | Season Summary and click on "MetWAtch Copy" to paste discharge data into MetWatch... very very useful!
Background To The Ascospore Release Model
Blackspot Disease
Managing black spot disease in your orchard means dealing with ascospores - the
means by which the black spot fungus reproduces. If ascospores land on your trees or fruit
they may germinate if conditions are right, and cause damage to your crop.
Ascospores mature gradually between September and mid-December. The overall rate at which
the ascospores on your orchard mature is dictated by temperature accumulation with higher
air temperatures leading to faster maturation. When a group of ascospores is mature, they
will be released into the air when they experience at least 0.2mm of rainfall. If they
then land on fruit or leaves which are wet, and the temperature is right, the spores will
germinate causing an infection event unless you control infection with fungicide sprays.
To fully understand the behavior of black spot on your orchard you need to consider:
1. How heavy an inoculum load is present on the orchard - see ENZA IFP manual section
Potential Ascospore Dose (PAD) for information on how to assess this.
2. What proportion of the inoculum is mature at any time, and how large a release will
occur when the orchard receives rain - use the HortPlus™ implementation of the
Beresford Ascospore Release Model to estimate the percentage of ascospores in your orchard
which will be mature at any given time, and when they will be - or were - released in
response to rainfall as monitored by a weather station.
3. If a release of mature ascospores does occur, how conducive are weather conditions to
the ascospores causing an infection. Use the Mill's Periods model in HortPlus™ MetWatch to assess this.
Estimating Ascospore Maturity and Release
Select a weather station [File|Open Met File] and then run the Beresford Ascospore Model
[click the light bulb symbol] you will see a two graphs. The top panel of the screen shows
ascospore maturation and release, and the bottom panel shows information about the weather
which is driving ascospore behavior, including daytime and nighttime rain and accumulating
temperature. The horizontal axis shows the date. 22 August has been statistically
determined to be the date on which to start the temperature accumulation for ascospore
maturation in New Zealand.
In the top panel you will notice a smooth S-shaped curve. This is the percentage of
ascospores on your orchard which have matured up to that point in the season. The warmer
the climate, the more quickly the S-shaped curve will climb and the earlier the date at
which all ascospores will have matured.
The vertical bars in the top panel mark ascospore release events predicted by the model.
Whenever daytime rain occurs and a release of mature ascospores is triggered, then all of
the spores which have matured since the last release event will be released. The longer
the period between release events, the greater the buildup of mature but unreleased
spores, and the bigger the release event, as marked by the height of the bar.
Beneath the S-shaped curve you will see a line which periodically comes up to meet the
S-shaped curve. This is the cumulative proportion of the ascospores on your orchard which
have been released up to that point in the season. Whenever a release event occurs (marked
by the bars) then the cumulative release line moves up to indicate that all mature spores
up to that point in the season have been released.
You can view the information in table form by selecting View|Tables from the menu.
This will bring up a table of values showing for each day:
· Temperature sum (Temperature accumulation: Growing Degree Days base
0°C)
· Cumulative percentage of ascospores matured to this point in the
season
· Percentage of ascospores mature but unreleased at that point in the
season
· Percentage of all ascospores released during a release event
Orchard Management Considerations
Remember that the information provided by the model is based on weather station data and
is an approximation of the biology of ascospore behavior. Use the information as a guide
only.
Some of the things to consider in relation to your orchard management:
Time of Season
The buildup of mature ascospores per day is highest mid season (when the S-shaped curve is
rising most rapidly), approximately between the dates when 20-80% of the ascospores are
mature. Earlier and later than these dates the rate at which ascospores are maturing is
decidedly reduced.
The size of a release event following periods of similar buildup is smaller as the
S-shaped curve flattens, ie when the S-shaped curve is rising more rapidly, more spores
will be maturing every day than earlier or later in the season.
Length of dry period
If there is a dry period in your orchard, no ascospores will be released, but there will
be a buildup of mature ascospores waiting to be released. When you do get a rain event
after a dry spell, a large pulse of mature ascospores will be released.
In other words, your biggest risk for a large release of ascospores is generally mid
season (when the S-shaped curve is rising most rapidly) and after several days with no
daytime rain.
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