Water, Water, Everywhere, but Never on the Leaves

By: Brian Hudelson, UW Plant Disease Diagnostic Clinic

April conjures up visions of spring rain showers, eventually leading to May flowers. While Mother Nature waters plants using an overhead sprinkling system (i.e., rain), from plant disease standpoint, this type of watering is less than optimal.

What’s the problem with watering from above? Overhead watering wets leaves creating what plant pathologists call a “leaf wetness period,” a time when a thin layer of water coats the leaf. This layer of water is exactly what most plant diseases-causing fungi require for their spores to germinate and infect. The longer the leaf wetness period, the more likely leaf disease will be a problem.

What do I do to prevent watering issues? You can’t prevent rain, but during dry periods when you need to water, don’t simulate rain by using a sprinkler. Instead, use a soaker or drip hose that directs water into the soil rather than onto leaves. Water at low pressure so that any sprays from these hoses are minimized. When it does rain, promote rapid drying of leaves by spacing plants as far apart as possible in new plantings, and thinning existing beds to increase spacing. Wider plant spacing increases air flow, promotes more rapid drying of leaves (when leaves do get wet), and shortens leaf wetness periods, making it more difficult for plant pathogens to get a foothold. Don’t overdo it with watering. Excessive soil moisture eventually increases humidity around plants, which slows leaf drying and lengthens leaf wetness periods.

By taking just a little extra care in how you water, you can have a big impact on the health of the plants in your garden.

For more information on leaf diseases, check elsewhere on this website and also visit the UW-Madison Plant Disease Diagnostics website at https://pddc.wisc.edu.

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