3.7.40
THE EPIDEMIOLOGY AND CONTROL OF RHODODENDRON POWDERY MILDEW

AM HALL and PA BEALES1

Environmental Sciences, University of Hertfordshire, Hatfield AL10 9AB
1now at CSL, Sand Hutton, York

Background and Objectives
Rhododendron Powdery Mildew has been in this country for twenty years[1], but even though it can cause serious epidemics and loss in specialist nurseries and gardens, little was known about it's biology, epidemiology and effective control.The disease is therefore causing considerable financial loss to some growers and they are often spraying bushes every 2 to 3 weeks for seven to eight months a year for 2 or more years.This project aimed to investigate the biology of the fungus and the epidemiology of the disease. It also aimed to recommend a more environmentally acceptable control strategy for the disease.

Materials and Methods
Epidemiological studies were carried out in the field by monitoring disease levels on individual bushes throughout the year at Exbury Gardens, Savill Gardens and at Hatfield House.Temperature, humidity and rainfall were monitored electronically both in diseased bushes and in the surrounding area.In vitro studies were carried out on the effects of temperature and humidity on spore germination.A study was also made of varying cultivar susceptibility to the disease.Electron microsopy was used to verify disease and monitor disease levels on individual leaves.

Results and conclusions
The studies of cultivar susceptiblity showed that there is considerable variation in susceptibility between different cultivars. Some are apparently unaffected whilst others exhibit severe symptoms.Severe infection results in premature leaf drop and subsequent poor growth and flowering of the bushes. The conditions which favour epidemic build up and spread of the disease were characterised as follows: 2 consecutive days at <210C and 40%rh with corresponding nights of <110C and 83%rh.The disease spread fast on young leaves, but the epidemic was slowed if night time temperatures dropped to 7.50C or less.In vitro studies showed that spores could survive exposure to 0 0C or less, and electron microscope studies showed that some of the elongating secondary hyphae survived throughout the winter as living hyphae amongst dead fungal debris on the leaf surface[2].These survival hyphae initiated the continuing epidemic build up on the evergreen leaves the following year.The disease built up in faster on young leaves, whilst older leaves escaped initial infection.

The conditions necessary for epidemic build up were established. It was proved that the most common means of over wintering was as fragments of living hyphae amongst dead fungal material. It is recommended that these observations are used to devise an intregrated disease control programme by using fungicides as soon as possible in the Spring thereby reducing the initial inoculum and slowing down the epidemic build up. This should reduce the spray load for much of the rest of the growing season.

References
1. Beales, PA, (1997) PhD Thesis, University of Hertfordshire

2. Beales,PA and Hall AM, (1994) Proceedings of the BCPC. Pests and Diseases 1994 p949-950.