Beware of using manure or compost from an unknown source to fertilize your vegetable garden. It could be contaminated with long-lasting herbicides (weed-killers). I first noticed this problem in my garden in 2009, and I just figured out the cause of the problem this year.

I grow tomato plants from seed every year. I give about 100 plants to my father, who has an enormous garden and donates his surplus tomatoes, plus the proceeds from selling his tomatoes, to the local food bank. My husband and I also grow a few of the plants in our own backyard. In 2009, some of the plants in our backyard were hit by a weird disease. After we planted the seedlings in the garden, the new growth came in wilted and stunted. I'd never seen anything like it before, and I've seen a lot of tomato plants over the years. At first, we suspected that it might be a fungal disease that was supposedly going around that year. However, the seeds I used were supposedly resistant to all of the common tomato diseases. All of the varieties we grew had problems in our backyard. Yet the plants that I gave to my father were completely unaffected. We'd just fertilized our tomato patch with some well-rotted horse manure that a friend had given to us. The people at the local nursery said that the problem was overfertilization. We had no problem in 2010 or 2011.

This year, we planted a lot more tomato plants. We planted some in our plot at our new community garden and some in our backyard. The ones in the community garden have been badly hit by the weird disease, but the plants in our backyard are fine, as are my father's plants. This time, I was sure that the problem had to be the manure. 

When I ran an Internet search on tomatoes and manure, I found out that our problem is almost certainly due to weed-killers that had been used on the fields where the horses' hay had been grown. Some long-lasting weed-killers contaminate the hay, go right through the horse, and end up in the manure. These weed-killers can last for a long enough time in the manure to destroy the vegetables in your garden. According to this notice from North Carolina State University,   "some of these herbicides have a half-life of 300 days or more."   It suggested that we may have to wait several years before planting "sensitive crops" in contaminated soil. In other words, we might not be able to grow anything but sweet corn in our community garden plot for the next several years.

Our community garden has strict rules against using any weed-killers or other toxic chemicals. Even so, our garden seems to have been accidentally contaminated by a weed-killer that is likely to kill "susceptible plants," such as "tomatoes, potatoes, lettuce, spinach, carrots, peas, beans, dahlias, and some roses." Some commercial organic farms have been badly hit by this problem.    

As long as those long-lasting weed-killers are on the market, I will have to avoid using any manure or compost or mulch from any unknown source. Perhaps the best thing for gardeners to do is use a green manure , which means planting a fast-growing crop and then tilling it into the soil. Of course, this means that our local government is going to have a hard time getting rid of its composted yard waste, and our friends who have horses are going to have trouble disposing of their manure.