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Disease Management

Development of genetic resistance is the simplest and most effective method of controlling virus diseases and is especially appropriate for CMV. Success has been noted, especially for cucumber and spinach, but is variable among most other crops. The history of CMV resistance in cucumber (Cucumis sativus) dates back to 1927 when the oriental varieties ‘Chinese Long’ and ‘Tokyo Long Green’ were introduced to the US. After extensive study, it was concluded that homozygosity of three partially dominant genes was needed to convey a high level of resistance for cucumber. This resistance formed the background for all modern day slicing and pickling cucumber varieties and has been effective for many decades, perhaps because resistance is based upon several genes.

Progress in finding resistance in other cucurbit species has not been as successful. Even though resistance has been found in other Cucurbit species, genetic incompatibility among species remains a formidable barrier for transferring this resistance to Cucurbita pepo crops, such as summer squash. The precocious gene, which masks color breaking in yellow summer squash for CMV (and WMV), is present in many yellow straightneck and crookneck varieties, and several have transgenic resistance or intermediate resistance for CMV. Insertion of the precocious gene is by traditional cross breeding whereas transgenic resistance involves the insertion of one or more foreign genes from a different organism. A handful of zucchini squash varieties have intermediate resistance for CMV and a few have transgenic resistance.

Genetic resistance for CMV in melon (Cucumis melo) is derived from oriental melons, and depending on the source, resistance is controlled by two or three complementary recessive genes. Some of these factors for resistance are strain and/or temperature dependent, with plants developing symptoms at temperatures below 20°C. No commercial muskmelon varieties with CMV resistance are available. Fortunately, most varieties of watermelon (Citrullus lanatus) are resistant to the most prevalent strains of CMV, with the exception of a specific strain that can infect plants systemically.

CMV resistance in spinach is controlled by a single dominant gene in the variety Virginia Savoy, and this has been incorporated into many current spinach varieties. This resistance, however, is not complete and may break down at temperatures greater than 28°C.

Virus resistance in lettuce to CMV was identified in Lactuca saligna PI 26153 from Portugal, a distantly related species of lettuce (L. sativa). However, this resistance is strain specific. An accession from L. serriola proved to be tolerant to three strains of CMV studied, but this tolerance has not been transferred to any current commercial variety.

Extensive effort was deployed to develop pepper resistance for CMV, and although tolerance has been described in some varieties, such as ‘Perennial’, it was not utilized in any commercial variety. More recently the pepper variety ‘Peacework’ was developed with a high level of resistance.

Sources of resistance and tolerance to CMV have been reported in wild tomato relatives but not in Solanum lycopersicum. No tomato varieties with CMV resistance have been released.

Eradication of weed hosts is often a difficult task because of the extensive host range of this virus. However, elimination of several of the key perennial or biennial weeds located near the crop may reduce severe virus pressure, and has successfully been used to control CMV.

In addition, isolation of crop fields by growing taller, nonsusceptible barrier crops such as corn, may delay initial infections. Application of insecticides and mineral oil sprays has been used to control CMV, as well as other nonpersistently transmitted viruses, but mineral oil sprays are most effective for crops with high plant populations per acre (e.g. pepper and tomato) and less so for crops with low plant populations (e.g. vining crops like cantaloupes or watermelons). The easiest explanation for this phenomenon is that when using a protective spray like mineral oil, and trying to delay infection for a matter of weeks, the probability of obtaining an infection in a low acreage crop is much greater than in a crop with higher plant populations. Insecticide applications are very effective for aphid control, but do not act quickly enough to prevent insect transmission because of the nonpersistent manner of transmission.

One method that has been used successfully to prevent infection of a susceptible crop like cantaloupe has been to use a Remay floating row cover to exclude migrant aphids during the early weeks of crop growth. Crops that are “protected” during the main period of aphid migration and transmission can remain free of virus infection, often to the end of the season. The covers are removed after this period of greatest vulnerability has passed so that bees can successfully pollinate the crop.

Alternative methods of reducing CMV losses have also been reported. Cross-protection, the inoculation of plants with mild, protective strains of CMV or the use of satRNAs (see Pathogen Biology), have been described in China. Transgenic tomato genotypes, expressing either the CMV coat protein or a satRNA, protected plants from CMV infection or the severe effects of some satellite RNA-associated CMV infections. The application of resistance-inducing bacterial treatments that may also enhanced plant growth provided protection against CMV, and may be integrated into other pest management schemes. There are numerous commercial products available that consist of single, or more often, multiple species of bacteria that colonize plant roots or root zones. These bacteria induce or enhance a plant’s ability to resist infection and, in some cases, have been effective against CMV.

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