Mosaic virus is caused by a variety of viruses which attack all members of the curcurbit family, but especially thrive on summer squash, cucumber and muskmelon plants. It is spread by a variety of methods and so is a serious disease for plants of the curcurbit family, including cucumbers, gourds, muskmelons, winter squash, summer squash, watermelons and pumpkins.
Mosaic virus damage first appears in the form of green leaves which look as if they are mottle or distorted. Often these leaves will also be curled upward, or appear as if their growth has been stunted. Typically these leaves will have yellowish spot on them, adding to their mottled appearance. If cucumber fruits are affected they will vary in color from light green to dark green mottled areas and some which pale to white. Affected areas of the curcurbit family plants may also be covered with warts or alternately the skins may be have faded and be very white and smooth.
Mosaic virus overwinters on a variety of plants including debris from curcurbit family plants which was not cleared from the garden, as well as catnip, pokeweed, motherwort, milkweed and wild cucumber plants. Aphids and cucumber beetles spread the disease as they feed going from infected plant to healthy plant. The prevalence of these insects once they have infested a garden can be damaging on it's own, not to mention when these insects are spreading mosaic virus. The earlier in the season the disease is spread, the more plants will have severe damage from mosaic virus. Although mosaic virus can eventually kill off the curcurbit family plants, the main affect of this virus on the crop is that plants fruits will taste bitter, and therefore be inedible. However, it is good to know that plants which are infected after the fruit is already half grown typically do not turn out bitter.
No chemical control for mosaic virus, and plants need to be removed and destroyed promptly if they are infected with this viral disease. To control the spread of the disease by cucumber beetles and aphids, you will need to control these insect populations with a diazinon containing insecticide repeating the application as much as necessary in seven day intervals. Although there are mosaic virus resistant cucumber varieties, so far no resistant varieties of muskmelon and summer squash are available to plant.