Mosaic Virus (Several species)
There are a large number of mosaic viruses, each having its own particular host (or group of hosts). Notable victims include corn, melon, pea, spinach, squash, celery and cucumber. Mosaic viruses usually shows themselves as a mosaic of mottled light and dark green patterns on the leaves (they often look like they are variegated). Depending upon the age, variety and health of plant, conditions and strain of virus, they may not have a significant effect on the plant (or perhaps slowing growth slightly) or they may cause stunting and severely reduce the harvest.
Mosaic viruses are often seed borne (especially from home saved seed, so be careful where you get it from). Even a single infected seed can be a problem, as the virus can then be transmitted from the infected plant to those around it (aphids are a common vector) and can eventually spread right through a planting. Plants grown from infected seed are usually stunted and yield poorly, but plants infected by aphids may still produce well (though the seed will be infected and shouldn’t be saved for replanting).
If any plants start to show the characteristic sign of mottled leaves, remove them immediately to reduce source of infection. Mosaic virus can overwinter on crop debris so it is important to clean up after harvest. You also need to control sucking insects that spread the disease. If mosaic is prevalent in your area you could use row covers to keep aphids and other sucking insects off of your plants (though it’s usually easier to plant resistant varieties where they exist). Viruses can also be spread by people, so don’t touch wet plants and wash your hands frequently with soap and water (especially after touching infected plants). You can minimize the effect of viruses by keeping the plants in good health, so make sure they get all of the sun, nutrients and water they require.
Mosaic is sometimes introduced into the garden on vegetable seedlings, so it’s best to grow your own, using disease-free seeds (beans and cucurbits are so easy to grow from seed that buying seedlings is a waste of money anyway).
Images: Howard F. Schwartz, Colorado State University, Bugwood.org