UK graduate student Jane Patterson doesn't mind if it rains on her laboratory tomato plants. In fact, she's actually causing it to rain on her plants. Patterson is using a rain-making machine to test the amount fungicide left on plants after a shower.

"The rain simulator has a nozzle at the top attached to a lever arm that goes back and forth, and we set tomato plants underneath it," said Patterson. "We then vary the amount of water falling onto plants to see the effect of different rain intensity levels. We also include a time effect to see if duration of the rainfall has an effect on the amount of residue left on the plants as well."

Before plants are taken to the rain simulator, they're sprayed with a commonly-used fungicide. Small pieces of leaf are then cut from each plant and assayed to determine precisely how much fungicide is present. Later, the plants are tested a second time to see how much fungicide remained on them after being rained on.

"We're interested in how well the plants are protected," said Sue Nokes, UK agricultural engineer and Patterson's faculty adviser. "We think it's possible that farmers could apply less fungicide when it rains less frequently, but right now we don't have a method for calculating that. Our goal is to develop a computer model that predicts how much fungicide is left on the plant after certain levels of rainfall."

According to Nokes, growers currently have two methods for applying fungicides. One is the calendar method where every seven-to-ten days they spray. The other is a disease-predicting computer model call TOMCAST which looks at temperature and leaf wetness in the field.

"But we're going one step further," Nokes said. "We're measuring residue actually left on the plant, because if the plant is still protected after certain levels of rainfall then that's what the farmer needs to know."

The rainfall simulator gives researchers something that natural rainfall cannot provide - precise control over duration and intensity.

"You need the right size droplet approaching the plant at the right speed in order to get accurate results from an experiment," Nokes said. "The simulator we're using is a research scale simulator with one nozzle. It's an indoor type, smaller than ones used outdoors for larger areas."

According to Patterson, who's earning a master's degree in agricultural engineering, rainfall is the biggest weather factor that affects loss of fungicides from plants.

"We've been conducting this research for several months. What we want is to calibrate our computer model in order to predict how much residue is left on the plants," said Patterson. "The result might be producers applying less fungicide during the growing season."