VG402 Development of an onion downy mildew forecasting system R O'Brien, ¥ Galea, G MacManus Department of Primary Industries and University of Queensland VG402 This report is published by the Horticulture Australia Ltd to pass on information concerning horticultural research and development undertaken for the vegetable industry. The research contained in this report was funded by Horticulture Australia Ltd with the financial support of the vegetable industry and Queensland Fruit and Vegetable Growers. All expressions of opinion are not to be regarded as expressing the opinion of Horticulture Australia Ltd or any authority of the Australian Government. The Corporation and the Australian Government accept no responsibility for any of the opinions or the accuracy of the information contained in this report and readers should rely upon their own enquiries in making decisions concerning their own interests. Cover price: $22.00 (GST Inclusive) ISBN 0 7341 0186 4 Published and distributed by: Horticultural Australia Ltd Level 1 50 Carrington Street Sydney NSW 2000 Telephone: (02) 8295 2300 Fax: (02) 8295 2399 E-Mail: [email protected] ©Copyright 2001 Horticulture Australia Limited VG402: Development of an onion downy mildew forecasting system Completion Date: 30 October 1997 Rob O'Brien Principal Plant Pathologist, DPI, Indooroopilly Victor Galea Senior Lecturer, University of Queensland, Gatton Campus Gerry MacManus Plant Pathologist, DPI, Gatton Research Provider: Queensland Fruit and Vegetable Growers Association - Heavy Vegetable Sub Committee HRDC Project No.: VG 402 Principal Investigator: Rob O'Brien, Principal Plant Pathologist Queensland Horticulture Institute, Department of Primary Industries, 80 Meiers Road, Indooroopilly. The investigations covered by this report were aimed at improving the management strategies for onion downy mildew in Lockyer Valley onion crops. The major thrust was to develop a fungicide control schedule related to critical weather patterns to replace the current program of sprays applied at regular intervals. A second aim was to determine the variability of weather parameters (temperature, relative humidity, leaf wetness) through the district and thirdly to try to enhance the systemic activity of fungicides by the use of spray adjuvants. The authors gratefully acknowledge the financial contributions of the Heavy Vegetable Sub- Committee of QFVG and the Horticultural Research and Development Corporation. The Queensland Department of Primary Industries and the University of Queensland, Gatton Campus also contributed to this project through the generous provision of staff and facilities. Queensland Government # Department of Primary Industries Queensland „ „ r~A!r^ Fruit &.Vegetable ** **• *-»*- Growers Any recommendations contained in this publication do not necessarily represent current HRDC policy. No person should act on the basis of the contents of this publication, whether as to matter of fact or opinion or other content, without first obtaining specific, independent professional advice in respect of the matters set out in this publication. CONTENTS Page No. INDUSTRY SUMMARY 3 TECHNICAL SUMMARY 4 1. GENERAL INTRODUCTION 5 2. FIELD TRIALS EVALUATING DISEASE FORECASTING SYSTEMS AND FUNGICIDES 9 1993 FIELD EXPERIMENT 13 1994 FIELD EXPERIMENT 15 1995 FIELD EXPERIMENT 18 1996 FIELD EXPERIMENT 22 2.1 DISCUSSION - FIELD TRIALS 27 3. DISTRICT WEATHER CONDITIONS — VARIABILITY OF IMPORTANT WEATHER PARAMETERS IN THE LOCKYER VALLEY 28 4. GLASSHOUSE AND CONTROLLED ENVIRONMENT EXPERIMENTS 30 4.1 FUNGICIDE DRENCH EXPERIMENTS 33 Experiment 1. Evaluation of phosphorous acid, dimethomorph and metalaxyl fungicides as drench treatments applied pre-infection for control of onion downy mildew , 33 Experiment 2. Evaluation of dimethomorph as a post-infection (72h) drench treatment for control of onion downy mildew 34 Experiment 3. Evaluation of dimethomorph and metalaxyl at six rates of application (0, 1, 2.5, 10, 25, 100 ppm) as a 7 h pre-infection drench treatment for control of onion downy mildew 35 1 4.2 FUNGICIDE SPRAY EXPERIMENTS 37 Experiment 4. Comparison of six different spray adjuvants on the post­ infection activity of Acrobat® MZ on onion downy mildew control in mature plants 37 Experiment 5. Comparison of six different spray adjuvants on the pre- and post-infection activity of Acrobat® MZ on onion downy mildew control in young plants 39 Experiment 6. Enhancement of the post-infection efficacy of Acrobat® MZ in the presence of different adjuvants at three rates of application, in the control of onion downy mildew 40 Experiment 7. Enhancement of post-infection efficacy of Acrobat® MZ in controlling downy mildew on young onion plants by the addition of DC-Tron® 42 Experiment 8. Efficacy of post-infection Acrobat® MZ sprays with varying rates of adjuvant, (Synertrol® Oil), in controlling a mixture of phenylamide resistant isolates of P. destructor in mature onions 43 Experiment 9. Fungicide formulation and its effect on the post- infectional efficacy of dimethomorph in phenylamide-resistant isolates of P. destructor on mature onions 45 Experiment 10. Timeliness of post-infection of Acrobat® MZ sprays to control downy mildew on mature onions 46 Experiment 11. Scanning electron microscope (SEM) study of the effect of spray adjuvants on the epicuticular wax layer of onion leaves 47 4.3 DISCUSSION — GLASSHOUSE EXPERIMENTS 52 5. GENERAL DISCUSSION 54 6. ACKNOWLEDGEMENTS 56 7. REFERENCES 57 2 INDUSTRY SUMMARY Downy mildew is a regular cause of crop loss to Australian onion growers. Standard control measures are to spray the crop regularly with protectant and systemic fungicides. The causal fungus {Peronospora destructor) is, however, very dependent on certain weather conditions before it can reproduce and spread. Thus, a knowledge of critical weather factors (temperature, relative humidity, leaf wetness, rainfall) allows us to pinpoint the danger periods and focus fungicide control options. In previous work, a disease forecasting system called DOWNCAST was tested over one season and showed it was capable of identifying sporulation/infection periods for onion downy mildew in the Lockyer Valley. In this project we set out to determine whether DOWNCAST had practical advantages for onion growers and, if so, what would be required to introduce it as a successful district spray warning service. Our findings were that:- • Field trials in 1995 and 1996 showed a spray program based on DOWNCAST maintained a high level of disease control with fewer fungicide applications than the standard program. • For practical use, DOWNCAST should be used to time the application of systemic fungicides. A regular schedule of protectant fungicides should be maintained. • Acrobat MZ with an oil based adjuvant was the most suitable fungicide. The oil adjuvant allowed better penetration of the systemic component, dimethomorph, into the leaf. It consistently eradicated 48 hour infections. • Fungicides of the phenylamide group such as Ridomil MZ and Fruvit were unsuitable due to fungicide resistance problems. Phosphorous acid was ineffective. • At least three weather data collection sites would be needed through the district to cater for variations in weather at different localities. Introduction of DOWNCAST as a prediction service for the Lockyer Valley is feasible and offers a way of more accurate fungicide scheduling. The major benefits would be improved control, with no additional fungicide use and minimising fungicide resistance threats by reducing the use of systemic fungicides. 3 TECHNICAL SUMMARY Onion downy mildew (Peronospora destructor) is an important disease in the Lockyer Valley (Qld) onion crops. Its occurrence is, however, spasmodic and severe outbreaks depend on favourable weather periods. During 1993-1996 disease forecasting systems based on spore trapping and the DOWNCAST system were assessed. Spore trapping was an unreliable method, but in two seasons, a program of irregular fungicide applications made according to the DOWNCAST parameters was as effective as the standard calendar based program. The number of fungicide applications was reduced by five in 1995 and three in 1996. For commercial use, DOWNCAST is seen as a method of timing systemic fungicides such as dimethomorph (Acrobat MZ) and the phenylamides (Ridomil MZ, Fruvit). In the Lockyer Valley the use of a regular protectant program should be maintained to minimise fungicide resistance problems and provide some assurance of disease control if wet conditions delay the DOWNCAST-timed applications. Spray warning systems such as DOWNCAST indicate when conditions are favourable for the fungus to complete a sporulation/infection event. Fungicide applications made in response to this must, of necessity, have an eradicant action. In the Lockyer Valley, fungicide resistance to the phenyl amide fungicides is widespread and these fungicides are unsuitable for use in a spray warning system. The curative ability of dimethomorph is poor when applied alone or with non-ionic spray adjuvants. By using oil-based adjuvants (organic or mineral), we showed dimethomorph reliably eradicated 48 h infections. This is due to the modification of the epicuticular wax platelets allowing increased uptake of dimethomorph. Dimethomorph (Acrobat MZ plus oil adjuvant) is recommended as the fungicide to be used with DOWNCAST and should be applied within 48 h of the sporulation/infection event. A spray warning system based on DOWNCAST has two main benefits for Lockyer Valley onion