In areas where the inoculum source at the start of the season is almost exclusively ascospores from overwintered leaves it is possible to rationalise the early season fungicide programme based on an assessment of the likely ascospores available for infection or potential ascospore dose (PAD).
- Where PAD is low, then the early season sprays can be delayed until an action threshold is reached based on the growth stage, the PAD for the orchard and scab infection periods.
- The PAD for an orchard is calculated from PAD = LD x PD x AD x LLD x n where LD = scab lesions per square metre of leaf tissue at leaf fall; PD = pseudothecia density or mature ascocarps per visible lesion; AD = asci per ascocarp; LLD = leaf litter density prior to bud burst; and n = number of ascospores per ascus.
- In practice only two measurements are made per orchard; scab incidence and severity are assessed just before leaf fall by recording the number of scab lesions on a random sample of 200 shoots per orchard; and leaf litter density assessed at bud break.
- The leaf litter density is assessed by selecting four trees in the orchard at random and running a measuring tape diagonally across the orchard from each tree and recording the presence or absence of leaves under the tape at 30 cm intervals.
- The LLD is then calculated from the percentage of points under which leaves were found.
- The information on scab incidence and leaf litter density is then input into a computer model which calculates the PAD for the orchard.
- This system is being developed for commercial use in the USA.
- In the UK, while the information collected could be used similarly, it assumes that the early season scab infection only arises from ascospores, which may not be true for UK orchards, particularly following wet autumns and mild winters.
- Therefore, basing decisions on early season sprays purely on PAD could result in a significant scab level on some trees in the orchard.
- However, at present PAD gives the best forecast of ascospore production in an orchard.
Predicting ascospore maturity and discharge
Ascospores are usually the main means by which the scab fungus can infect new growth in the spring.
- Therefore, the ability to predict when ascospores are mature and able to infect is an important part of scab management.
- In addition, prediction of ascospore discharge, the proportion of the season’s ascospores that are mature on a given date, and the date from which ascospore discharge is almost complete are also useful for managing apple scab in an orchard.
- Such information can be obtained manually based on using a simple spore trap placed over overwintered scab leaves and monitoring ascospore discharge.
- Such monitoring is usually not suitable for individual growers but can be provided centrally by consultants or research stations.
- In other countries, e.g., Belgium, such information is provided by research stations.
A dot-ELISA diagnostic kit has been developed in the USA that combines a spore trap with an antibody-based procedure that stains the mucilage (slime) that surrounds ascospores of scab.
- The kit has potential for use by growers or consultants because the staining procedure requires little time (30 mins), is easy to perform, and the red-stained dots are countable using a hand lens (30x).
- The kit could be used to determine the start of ascospore release and to monitor ascospores through the season.
Alternatively, the information can be obtained from computer simulation models developed from data on temperature and rainfall and ascospore maturation and discharge collected over many seasons.
- Several such models have been developed in the USA, Canada, South Africa and various parts of Europe.
- RIMpro is the model most commonly used in the UK.
- Such models are useful but do not always give accurate forecasts especially when rain follows long dry periods which result often in massive ascospore release.