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Disease Cycle and Epidemiology

Because of its wide host range, numerous weeds can serve as reservoirs for CMV and contribute to virus spread to crops at the beginning of the season. Perennial, biennial, and winter annual weeds harboring CMV in roots, tubers and underground organs throughout the winter include common milkweed (Asclepias syriaca), yellow rocket (Barbarea vulgaris), marsh yellowcress (Rorippa islandica), and yellow toadflax (also called butter-and-eggs, Linaria vulgaris). These were shown to be important sources for infection of lettuce in upstate New York. Seven additional weeds, including common chickweed (Stellaria media) were shown to be important overwintering sources in Britain for lettuce. It is important to note that infected weeds are often symptomless.

Crop hosts may also serve as significant sources of CMV for other crops. A good example of this is how the perennial crop alfalfa can serve as the reservoir of CMV for subsequent infection of snap beans. Although alfalfa is symptomless, it is a formidable reservoir of CMV, and is currently a major production problem for snap beans grown in Wisconsin, Pennsylvania and New York.

Seed as a source of CMV has been reported in common chickweed and in 19 other plant species. In chickweed (Caryophyllaceae), the rate of transmission was as high as 40% in plants grown from infected seed. Other plant families with seedborne CMV (including crop plants) are Amaranthaceae (syn. Chenopodiaceae) (spinach), Brassicaceae, Fabaceae (syn. Leguminosae) (bean, chickpea, cowpea, lentil, lupin, subterranean clover) and Labiatae. This seed transmission feature has proven to be important for the annual occurrence of CMV-infected legumes in commercial fields, and for the potential infection of greenhouse crops growing close to chickweed plants surviving in protected areas near greenhouses. The seedborne characteristic increases the probability that the virus will survive in nature.

More than 80 aphid species (Insecta: Hemiptera: Aphidoidea), including Myzus persicae and Aphis gossypii, are capable of transmitting the virus in a nonpersistent, stylet-borne manner (Figure 10). The nonpersistent relationship implies that the insect acquires the virus in short probes (generally less than 60 sec.) and retains the virus for short periods of time (a few minutes) and loses the virus due to normal feeding activities. This means that the insect would have to reacquire the virus again (become viruliferous) to be able to transmit again. In the case of snap bean, the most important aphid vectors are the soybean aphid (Aphis glycines, recently introduced into the US), the yellow clover aphid (Therioaphis trifolii) and less importantly, the pea aphid (Acyrthosiphon pisum). The virus is rapidly acquired by all instars, generally within one minute of feeding, but their ability to transmit the virus declines and is lost within several hours. Transmission efficiency varies with the aphid species, virus strains, host plant species, environmental conditions and time of the year. Experimentally, CMV is easily transmitted mechanically, but efficiency is increased by addition of a reducing agent such as 10mM sodium sulfite to the buffer when preparing inoculum. Experimental mechanical transmission involves grinding infected tissue in a buffer (typically in a mortar with a pestle); many viruses can remain infectious during this process by using a simple buffer to resist pH change and protect the virus particle. Some viruses, like CMV and Tomato spotted wilt virus, are less stable and more vulnerable to being degraded during this process. Addition of a reducing agent can help to stabilize the virus, thereby enhancing its ability to remain infectious for transmission. Since CMV is not a stable virus like Tobacco mosaic virus, it is not likely to be transmitted by workers touching infected plants during normal field operations.

Figure 10. Winged green peach aphid. Click the image to enlarge, or click here for a sample video clip of non-persistent transmission of a virus (the video shows PVY, not CMV).

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