Community gardeners share a lot of things: information, seedlings, resources, but unfortunately also pests and diseases, which can spread easily from plot to plot. At Germantown the gardeners have had a difficult time with mosaic viruses, especially in squash and cucumbers. So (self-interestedly among other reasons!) I've done a little research into the problem.
|Squash leaf from SGCC in July 2012|
Cucumber mosaic virus (CMV) is mainly spread by aphid feeding, as well as by infected transplants and by tools and gardeners' hands. The best control method is to select resistant varieties (you'll find these in seed catalogs marked with CMV). It is also very important to control weeds, because many weed species either harbor and support aphids or are a host for CMV. If your plants show CMV symptoms, remove and discard them immediately (don't put them in the compost pile!) and disinfect your tools and hands before touching any other plant.
Squash mosaic virus (SqMV) is spread by cucumber beetles, which are a difficult pest to control, but can be discouraged early in the season by using row cover over seedlings. It can also be seedborne, so if you actually managed to save seed from last year's affected plants, don't use it. There are no actual resistant varieties, though a few yellow squashes show the effects of the virus less than others. Again, if you see symptoms on plants, discard them immediately.
More information on aphids and cucumber beetles and their control can be found at the GIEI vegetable pests page. Aphid control by predatory insects can be encouraged by planting attractant flowers and avoiding pesticides.
Information on mosaic viruses from these sources: