There are two common virus' that gourd growers are concerned with, Mosaic Virus and Powdery Mildew. Mosaic virus will create a yellowing or brownish yellow spotting, or mosaic spots on your leaves, thus it's name. This same appearance can be caused by Sevin and other insecticides and people are often concerned they may have this virus, not realizing it was the poison they applied that is causing the problem. Mosaic is the worst problem of the two, as it has a residual effect. If you get mosaic virus this year, you will probably have it again next year. Powdery mildew is seasonal, it can damage and kill your vines this year, but unless circumstances promote it's growth next year, it won't have a lasting effect.
Mosaic Virus: There is a lot you can do to help to prevent the spread of mosaic virus, the first being to stay away from growing other plants that are susceptible to mosaic virus, the most common being tobacco. Tobacco is so capable of carrying mosaic virus, that if you smoke in your garden and the tobacco in your cigarette is contaminated with the virus, your gourd vines could become contaminated.
Testing for Mosaic: A simple field test, although it is not 100% positive, is to take a piece of the vine and break it then hold the two pieces together for a few seconds to allow the plants moisture to develop in the damaged area, then slowly pull them apart in a straight apart motion. Mosaic virus will cause the vines fluids to be a little slimy, and it will string out. If you do not get stringing, then you likely do not have mosaic virus.
Treating mosaic virus: If you have confirmed mosaic virus in your gourd patch, if you catch it soon enough, pull the vines that you feel may be contaminated in order to hopefully prevent it's spreading. If you have used Sevin or other chemical pesticides on your gourd vines, do not be quick to pull vines, this may simply be a latent affect of the poison on your vines and leaves.
If you have absolutely confirmed the presence of Mosaic virus, (although it is rather rare these days, it does still exist), your gardening tools must be sterilized with bleach, and you should grow corn or other non mosaic friendly crops for at least 2-3 years. Once Mosaic is in your soil, it will contaminate future crops unless allowed to die out. Your gardening tools can also spread the virus, as well as contaminated tobacco in your cigarettes or smoking or chewing tobacco if you are a tobacco user.
Powdery Mildew: Powdery mildew is very common with gourd vines, but there are some things you can do to help reduce the chance of it attacking your crops. Powdery mildew is more common in the humid states, and it seems to prefer trapped and stagnant air. Cucurbits seem to be more susceptible to powdery mildew than lagenaria's, (cucurbits are your yellow flowering ornamental gourds, cucumbers, etc., Lagenarias are the hard shelled gourds with white flowers). If you grow both crops in close proximity, normally toward the late summer you may see powdery mildew developing on the cucurbits first, then see it spreading to your lagenaria gourd vines.
One method to help ward off the onset of powdery mildew is to grow your vines on a trellis or fencing so they are of the ground, and can have air circulation through and around them. If you do get powdery mildew, we've been told that spraying with 2% milk will eliminate the mildew. We've never had a problem with powdery mildew, and therefore have not had to test this treatment. Powdery mildew developing late in the season as the temps change and you are heading into the Fall is normal. I think this is due to the vines realizing the end of season and are weakening.