HRI-East Malling, West Malling, Kent ME19 6BJ, UK

Background and objectives
The main UK apple cultivars Cox, Jonagold, Gala and Bramley are susceptible to a range of fungal diseases which reduce yield and quality. The major UK diseases are scab (Venturia inaequalis) and powdery mildew (Podosphaera leucotricha), and control of these relies on routine applications of fungicide at 7-14 day intervals to produce the blemish-free fruit required by the market. Such practices are usually reliable and preferred by growers, but increased public concern about the use of pesticides and rising costs to growers have led to a re-appraisal of their use. The development of disease warning systems offers scope to optimise fungicide use, but such systems must integrate with orchard practice in order to be adopted by growers. ADEMTM (Apple Disease East Malling) [1] is a PC-based system which forecasts the risk of apple scab, powdery mildew, Nectria fruit rot and canker (Nectria gailigena and fireblight (Erwinia amylovora). The objectives of this study were to (i) validate the scab and mildew models in ADEMTM and (ii) to develop practical strategies for their use in orchards.

Materials and methods
The models were validated for 4 years by comparing the appearance of new scab or mildew lesions on leaves on labelled apple shoots in an orchard with predictions from ADEMTM. Practical strategies for using ADEMTM were evaluated using replicated large orchard plots (17x12 trees) of apple cv. Cox. In these the disease control achieved in 1994-97 by a routine programme of fungicide sprays, applied from bud-burst at 10-day intervals until harvest, was compared with that achieved in plots where sprays for scab control were applied curatively in response to ADEMTM warnings or managed according to a key-stage strategy. In the latter, sprays for scab control were applied routinely at key growth stages (bud-burst and petal fall), but at other times fungicide choice and spray interval were managed according to scab warnings. Other spray requirements in the orchard for pests, nutrients and other diseases were also taken into account, as well as practical considerations such as holidays, when spray operators may be unavailable. Untreated plots were included. Weather data for driving the models in ADEMTM were recorded on a METOS weather station (Gottfried Pessi, Weiz, Austria) and downloaded to the PC as required. The incidence of scab and mildew was assessed at 7-14-day intervals for decision-making, with more detailed assessments monthly and at harvest to evaluate the effectiveness of the programmes.

Results and conclusions
The appearance of new scab or mildew lesions on leaves agreed closely with predictions given by ADEMTM. The scab predictions were more accurate than scab warnings based on Mill's Table. In each of the trial years the use of a key-stage strategy resulted in similar or better disease control compared to the routine programme, but with reduced fungicide inputs of 20-50%, depending on disease risk. Maximum savings were possible by following the curative strategy, but this resulted in poorer disease control and significant fruit scab (up to 5%) at harvest in 2 out of 4 years. In 1996-97 ADEMTM was used to assist in spray decisions on 15 orchards at East Malling. Fungicide use was reduced by up to 40% compared to routine-sprayed orchards on the same farm.

The trial clearly demonstrates the potential for optimising fungicide use in apple orchards by using ADEMTM with a keystage strategy. ADEMTM has been available commercially since 1996. Current work is aimed at encouraging uptake of the system by growers by working with fruit consultants. The feasibility of developing a Met network for growers is also being examined.

1. Butt DJ, Xu XM, 1996. ADEMTM Version 2.0. Computer Software Manual, HRI-East Malling.