Ascospore monitoring of apple black spot - another way to improve control
Three things are needed for a disease such as black spot to develop :
These three aspects are known as the disease triangle and must all be present for infection to take place. Therefore to monitor and control apple black spot (Venturia inaequalis), we must understand each of the above factors.
Up until now, much of the black spot research has revolved around one of these factors - monitoring the weather conditions. Weather information takes much of the guess-work out of fungicide timing, but used on its own it only forms part of the picture.
Another corner of the disease triangle is understanding the disease itself. An excellent summary of the disease cycle of apple black spot can be found in the Orchardist article: "Improved management of apple black spot" by David Manktelow and Robert Beresford (September, 1993).
The primary source of black spot infection is from the release of ascospores in the spring. Ascospores are microscopic spores which develop during the winter on dead, fallen leaves that were infected the previous season. From budburst onwards, rain triggers the release of ascospores into the air. Ascospore release will continue until early December, or later in a dry season, and this is known as the primary infection season.
If there are few ascospores present, then the risk of black spot is low regardless of the weather conditions. Alternatively a high number of ascospores can cause infection even under a marginal infection period.
At present little is known of the timing and amount of ascospore release of Venturia inaequalis. A practical method of measuring these spores is one of the key factors missing from the black spot management programme. By understanding when and how many ascospores are produced during the spring, it should be possible to time fungicides to the periods of highest risk.
Monitoring Methods
Dr Robert Beresford and David Manktelow have evaluated two methods of monitoring black spot ascospores - the glass slide and forced discharge methods. Both methods have proved to be practical and could potentially be used by consultants.
1. Glass slide method
Picture 1: Set up for glass slide method.
The glass slide method is simple and has been well tested. Glass microscope slides are placed on wire meshes containing leaves. As the ascospores are discharged from the leaves, they are collected on the microscope slides. Each week the slides are removed and assessed under a microscope. This method provides information on the relative numbers of ascospores available. It can be used to identify the beginning, peak, and end of the ascospore release season. Because it is done regionally it is not suitable for identifying numbers of ascospores released in individual infection periods on individual orchard blocks.
Beresford and Manktelow investigated the glass slide method over three seasons (1992-94) on different cultivars in Auckland, Hawke’s Bay, Canterbury and Central Otago.
They found very little difference in ascospore release between the three cultivars tested (Gala, Braeburn and Fuji). This was unexpected considering, for example, that Gala leaves fall up to one month before Braeburn leaves. It does mean that only one cultivar per region needs to be monitored and these results can be applied to other varieties.
Likewise, as the timing of ascospore production did not vary greatly between sites, only a few sites per region would have to be monitored. Thus monitoring within a region is very straightforward
Ascospore timing of production did vary between districts. The regional differences arose mainly from ascospores being released later in colder regions. There was not so much difference in the length of release but, in general, spores were released over a greater period of time further south.
Monitoring will need to be done every year as there is a huge variation in ascospore release, and hence potential infection risks, between seasons. For example, in Hawke’s Bay the date at which 5% of the ascospores were released varied from 8 September to 3 October over the three years of the trials. For 95% release, the dates were as early as 19 October and as late as 5 November, 17 days apart. Therefore in a year where the release starts late (or finishes early) there would be little point in applying excessive fungicides. The 5-95% release dates can probably be used to define the period when fungicide protection is needed.
The glass side monitoring system can be applied commercially. All that is required is a compound microscope and a short period of training so assessors can accurately identify the black spot ascospores. The labour cost required for spore counting has been estimated at $2,000-$3,000 per season.
2. Forced discharge method
During the 1994/95 season, the HortResearch team investigated the possibilities of using the forced discharge method for measuring ascospores. Fallen apple leaves were collected from six sites around the country at weekly intervals from 28 September to 26 October. The leaves were wetted and sealed in plastic bags overnight. Ascospore release began when the leaves were exposed to light. The ascospores were collected, counted, and the spore count per gram of leaf litter was calculated.
The highest numbers of ascospores per gram were collected in early-mid October. Large differences in the number of ascospores were found between the six sites. These differences were thought to be due to previous disease history.
By using the forced discharge method, it should be possible to gauge how many fungicides should be used and whether curative fungicides are needed for marginal ascospore infection periods. This will be important for the future management of DMI fungicides which are at risk from black spot resistance. Knowledge of ascospore numbers also provides additional information on disease risk later in the season which could be used to determine the need for late season fungicides. As late season fungicides cause residues, they should only be applied if necessary.
The forced discharge method requires further development and evaluation, but it is seen as an important further component in the arsenal of methods to control fungicide use in apples.
Which method is better ?
Each of the two methods evaluated serves a different purpose. Glass slide monitoring assesses the timing of spore release but not the actual amount. Forced discharge assesses the number of spores available, but not specifically when they are released. The glass slide method looks to be the more useful because of variability in ascospore numbers with the forced discharge method.
Both methods could be used for deciding when fungicide applications should be made, how infection periods should be responded to and what late fungicide use will be required. When used in conjunction with weather forecasting, they should give us a clearer picture of disease risk.
Acknowledgments
HortResearch acknowledges ENZA New Zealand (International) and the Foundation for Research, Science and Technology for their support.
References
Manktelow D.W.L and Beresford R.M. (1995) Evaluation of an ascospore monitoring method for Venturia inaequalis to improve apple black spot fungicide management. Proc 48th NZ Plant Protection Conference. 1995 : 78-82.
Manktelow D.W.L And Beresford R.M (1993) Improved management of apple black spot. The Orchardist September 1993. Vol 66 No. 8 :59.