Between the many production systems used in the nursery and greenhouse industry, a one-size-fits-all watering solution does not exist, even for a one-gallon pot. Still, there are common concerns when it comes to how water moves, where it lingers, and the impacts for plant health. While research and testing are critical for identifying specific pathogens like Phytophthora, Pythium, and Rhizoctonia, many outbreaks begin with simple environmental conditions that go unnoticed. For growers, recognizing water-related warning signs can be just as important as selecting a fungicide.

Pooled Water

After irrigation, any water that lingers should raise concerns. Pooled water on ground cloth, walkways, or within bench trays is often linked to plant losses in the same area. These wet zones may result from uneven grading, clogged drains, or poor bench design. Water that moves between containers can carry plant disease from plant to plant, so it’s always best to prevent pots from sitting in standing water.

• Regular post-irrigation walkthroughs help identify where water collects and is the first step in troubleshooting.
  • Is there excessive irrigation that indicates long runtimes, watering longer because of poor irrigation distribution uniformity, or additional hand watering?
  • Does this water eventually dry or runoff throughout the day or lingers?

Algae and Moss are Problematic

If you see signs of algae and moss, then you have persistent moisture that spans between irrigations. If growing on greenhouse glazing, moss and algae can limit the amount of light penetrating through to the crop below. Algae and moss serve as a food source or breeding ground for other pests and root-rotting pathogens. In propagation settings, thick algal mats or sheets of moss can be a barrier to seed germination. The best way of managing this issue is to create a dryer environment and to scout for these spots.

• Can the irrigation system be better tuned to reduce the amount of water going onto surfaces?
  • Ensure that water is going on the crop and not on other surfaces or structures. In high humidity areas, this may prevent plants from transpiring and prevent water from evaporating from the soil and from surfaces. Greenhouse exhaust fans may be used to reduce moisture from the air.

Watching Patterns of Decline

When you find yourself suspecting that waterborne plant diseases are present (read up on detailed solutions found in this UCNFA issue), then keep an eye out for patterns. The plant decline of plant pathogens follows the flow of water, and symptoms reflect this water movement pattern. Track where and how irrigation is delivered. Patterns can follow hand watering habits that unintentionally favor diseases such as starting and ending in the same place. Take a step back and evaluate how the water is moving after an irrigation and see if you can connect the dots.

• Following water movement within the bed is a typical for water related pathogens, yet it’s important to consider additional sources such as reusing containers that are not properly sanitized.

Resources:

Managing Algae and Moss Inside Greenhouses: https://www.e-gro.org/pdf/2024-13-26.pdf

Container Nursery Production and Business Management Manual: https://anrcatalog.ucanr.edu/Details.aspx?itemNo=3540

Grant Johnson is an Urban Agriculture Technology Advisor in Orange and Los Angeles Counties; Don Merhaust is an Associate Extension Specialist in UC Riverside.