Fungicide resistance in cucurbit downy mildew – methodological, biological and population aspects

Cucurbit downy mildew, caused by Pseudoperonospora cubensis, is among the most devastating diseases of cucurbitaceous plants. In spite of improved cultural practices and breeding for resistant cultivars, chemical control is still a very important tool to manage the disease. During the last several decades, many fungicides from various chemical classes have been developed. The occurrence of strains of P. cubensis resistant/tolerant to some fungicides encouraged research of this phenomenon. The first part of this article summarises the many different methodological approaches such as field trials, in vitro testing on active plant tissues or molecular diagnoses developed for the detection of resistant/tolerant strains of P. cubensis, as well as methods to collect and maintain pathogen isolates. The second part outlines the commonly used fungicides to control P. cubensis and their features like systemicity, biological and biochemical mode of action and translocation behaviour within plants. The last part deals with geographical aspects such as first appearance of resistance problems, distribution of resistance, temporal development of resistance under selection pressure by a fungicide, fitness of resistant subpopulations in competition with sensitive ones in the absence of a fungicide, as well as genetic and molecular sources of resistance.