FUNGICIDE RESISTANCE IN CROP PATHOGENS: HOW CAN IT BE MANAGED? 2, revised edition

Cover: Scanning electron micrograph of 7-day-old colony of powdery mildew (Blumeria graminis f.sp. tritici) on a wheat leaf. Insert shows a 2-day-old colony at higher magnification. Although the sensitivity of mildew populations towards certain fungicides has changed considerably over the years, implementation of resistance management strategies has helped to sustain an overall satisfactory degree of control. SUMMARY This publication gives a broad overview of efforts worldwide to combat problems in crop protection that are caused by development of resistance to fungicides. The following major points are emphasised: • Fungicide treatments are, and will remain, essential for maintaining healthy crops and reliable, high-quality yields. They form a key component of integrated crop management, and their effectiveness must be sustained as long as possible. • Pathogen resistance to fungicides is widespread. The performance of many modern fungicides has been affected to some degree. • Resistance problems could be much worse. All types of fungicide are still effective in many situations. Current countermeasures are by no means perfect, but they have proved to be necessary and beneficial. • Resistance builds up through the survival and spread of initially rare mutants, during exposure to fungicide treatment. This development can be discrete (resulting from a single gene mutation) or gradual (considered to be polygenic). Resistance mechanisms vary, but mainly involve modification of the primary site of action of the fungicide within the fungal pathogen. • Resistance risk for a new fungicide can be judged to some degree. High risk indicators include: single site of action in the target fungus; cross-resistance with existing fungicides; facile generation of fit, resistant mutants in the laboratory; use of repetitive or sustained treatments in practice; extensive areas of use; large populations and rapid multiplication of target pathogen; no complementary use of other types of fungicide or non-chemical control measures. • Monitoring is vital, to determine whether resistance is the cause in cases of lack of disease control, and to check whether resistance management strategies are working. It must start early, to gain valuable base-line data before commercial use begins. Results must be interpreted carefully, to avoid misleading conclusions. 3 • The main resistance management strategies currently recommended are: avoid repetitive and sole use; mix or alternate with an appropriate partner fungicide; limit number and timing of treatments; avoid eradicant use; maintain recommended dose rate; integrate with non-chemical methods. Wherever feasible, several strategies should be used together. Some are still based …