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SUMMARY The diverse ecological and epidemiological factors which determine virus infection of vegetable crops are discussed. These include the sources and spread of viruses, together with some agricultural and horticultural practices which have influenced their prevalence. Control measures are described, demonstrating ways of avoiding or minimising infection. Developments are reported with insecticides, insect repellents, anti‐feedants, and fungicides for the control of virus vectors. Some of the successes and future requirements in breeding vegetable cultivars with virus resistance are listed. An Appendix lists the economically most important vegetable virus diseases present in 28 countries or regions with temperate, Mediterranean‐like or subtropical climates. The five most important viruses in field‐grown vegetables are transmitted non‐persistently by aphids (cucumber mosaic, turnip mosaic, potato virus Y, lettuce mosaic and watermelon mosaic 1 (= papaya ringspot virus)). In contrast, with protectively‐grown vegetables, the most important are the mechanically‐transmitted tobamoviruses (tomato and tobacco mosaic, Capsicum mosaic and cucumber green mottle mosaic).

A method for nucleic-acid-based detection of pathogens in plant material has been developed which comprises a simple and rapid method for extracting DNA on the nitrocellulose membranes of lateral-flow devices, loop-mediated isothermal amplification (LAMP) of target DNA using labeled primers, and detection of the generically labeled amplification products by a sandwich immunoassay in a lateral-flow-device format. Each of these steps can be performed without specialist equipment and is suitable for on-site use, and a result can be obtained in just over an hour. A LAMP assay for the detection of plant DNA (cytochrome oxidase gene) can be used in conjunction with pathogen-specific assays to confirm negative results. The use of this method is demonstrated for the detection of Phytophthora ramorum, the causal agent of sudden oak death and dieback/leaf blight in a range of tree, shrub, and herbaceous species, and the recently described pathogen P. kernoviae.

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Phytophthora ramorum is a recently described pathogen causing bleeding cankers, dieback, and leaf blight on trees and shrubs in parts of Europe and North America, where the disease is commonly known as sudden oak death. This article describes the development of a single-round real-time polymerase chain reaction (PCR) assay based on TaqMan chemistry, designed within the internal transcribed spacer 1 region of the nuclear ribosomal (nr)RNA gene for detection of P. ramorum in plant material. Unlike previously described methods for the molecular detection of P. ramorum, this assay involves no post amplification steps or multiple rounds of PCR. The assay was found to have a limit of detection of 10 pg of P. ramorum DNA, and could detect P. ramorum in plant material containing 1% infected material by weight within 36 cycles of PCR. The assay also was used to test DNA from 28 other Phytophthora spp. to establish its specificity for P. ramorum. A quick and simple method was used to extract DNA directly from host plant material, and detection of P. ramorum was carried out in multiplex with an assay for a gene from the host plant in order to demonstrate whether amplifiable DNA had been extracted. Amplifiable DNA was extracted from 84.4% of samples, as demonstrated by amplification of host plant DNA. The real-time protocol was used to test 320 plant samples (from 19 different plant species) from which DNA extraction had been successful, and was shown to give results comparable with a traditional isolation technique for diagnosis of P. ramorum in plant material from common U.K. hosts.