David Deardorff has heard endless pleas for help from gardeners.

As a longtime horticulturist and plant pathologist, he helps both experienced and novice growers figure out what’s wrong with their plants.

Lately, he’s turned his attention from tomatoes and roses to an annual plant shrouded in mystery and controversy: marijuana.

Deardorff and Kathryn Wadsworth, his wife and co-author, produced what’s believed to be the first mainstream problem-solving gardening book for cannabis growers, “What’s Wrong With My Marijuana Plant?” (Ten Speed Press, 192 pages, $19.99). It’s the fifth in the couple’s award-winning, photo-packed series of “What Wrong With My Plant?” books.

With legalization in California and other states, their new book has rapidly become a best-seller. Based in Washington where Deardorff taught for many years, the couple travel the country, giving lectures to pot growers on how to produce better, healthier plants without chemical pesticides, fungicides or fertilizers. Recently, they were in Berkeley to talk to California growers.

Marijuana is now legal in some form in 30 states plus the District of Columbia. Because cannabis is still outlawed on the federal level, universities have shied away from research of cannabis as a crop. The usual sources of how-to information for gardeners and farmers aren’t available, Deardorff said.

“Anyone who gets federal funding won’t touch it,” Deardorff said. “I’ve been teaching master gardeners (about plant pathology) for 20 years, but they can’t officially buy our book as a resource. It’s like talking about wine grapes during prohibition.”

Deardorff, 75, and Wadsworth, 68, are devoted organic gardeners. All five of their gardening books are dedicated to organic methods without the use of chemical pesticides or fertilizers.

“We’ve observed numerous instances in online marijuana forums where a grower has a problem, posts a photo, and asks for help,” he explained. “Many of the responses from other growers are misleading at best and totally wrong at worst. There is an enormous amount of misinformation on the web that is a disservice to this burgeoning industry.”

The couple do not grow cannabis. Deardorff said he experimented with growing it briefly in 1971 and 1973. Most of the information for their book came from longtime medical marijuana growers as well as university research overseas.

“All marijuana is pretty easy to grow; that’s why they call it ‘weed,’” he said. “It’s a vigorous plant and generally very healthy.”

The couple compared marijuana to roses or citrus in terms of its needs, growing conditions and common maladies. Like any plant, cannabis can develop problems, either from improper growing conditions or pests and disease. Cannabis grown indoors (a mandate in many cities, including Sacramento) tends to have more issues than if grown outside.