Turini said some plant diseases can persist in the soil for decades, citing the example of white rot in garlic.

“There are 16,000 acres in the Central Valley that are infested,” he said.

In other instances, the disease cycle can be broken, but growers need to take into account what they are planting as part of a crop rotation.

For example, Turini said, the tomato spotted wilt virus has other hosts that include lettuce and radicchio “that can be a bridge.”

Some diseases, including a fusarium fungus “can sustain itself without causing diseases on the roots of other plants,” he said. “You may have to stay away from certain crops.”

Turini warned against using transplants from outside the region, notably from the low desert of California. He said that common pathogens can be introduced through seeds and transplants.

Timing of planting can reduce risks of certain diseases because of variability in soil temperatures, he added. And variety selection is important because some choices bring greater genetic resistance.

Cynthia Ortegon, regional service representative for California Certified Organic Farmers in the Central Valley, talked of steps to becoming certified.

She also talked of the need to use no prohibited substances under national organic farming rules three years prior to harvest and having distinct defined boundaries and buffer zones.

Ortegon said organic growers need to keep records of what they apply to their land and when they use materials “Keep all labels and receipts,” she said. Using the wrong materials can mean being put out of organic production for three years until recertification is done, she warned.