Impact of watering techniques on disease and other root zone problems

Watering not only affects crop growth and how evenly crops dry out, but is also a major factor contributing to certain root diseases, algae growth on the growing medium surface and shore flies. As the old adage goes, “The person who holds the hose, holds the profits.” So how can watering help reduce problems in your crops?

Root rot pathogens
Most plant pathogens need moisture in order to infect and attack plants. The water mold pathogens, Pythium and Phytophthora need a wet growing medium with free water for their spores to germinate and infect plant roots.  Allowing the growing medium surface to dry out until it turns a tan color before each watering greatly reduces the possibility these pathogens will be able to thrive in the pot.
How crops are watered influence the amount of Pythium and Phytophthora infection seen in a crop. Crops that are watered shallowly often have wet growing media surfaces. This creates a more ideal environment for Pythium and Phytophthora. Also plants that dry out unevenly will see more disease problems. Since most plants are watered to the dry plants, the wet plants receive water too frequently and therefore have ideal conditions for Pythium and Phytophthora growth.

Algae
Algae can be found growing anywhere where there is light, nitrogen and water. When growing greenhouse crops, the first two cannot be changed without affecting plant quality. However the moisture can. Algae will grow on wet growing media surfaces. It starts out as a thin green layer and can get thicker if not addressed. It eventually gets thick enough, turns black and makes the surface of the growing medium appear to be wet. These plants do not get watered, the growing medium dries out, the algae layer dries up, dies and curls up away from the container edges. Algae do not harm plants, but they are unsightly and attract shore flies.
As with the plant pathogens, it is best to allow the media surface to dry out to a tan color. This will greatly reduce or eliminate any algal growth on the growing medium surface. Again watch the watering, especially shallow, frequent watering.

Shore flies
Shore flies lay eggs on the growing medium surface that hatch if the surface stays wet. The larvae that emerge eat algae growing on the growing medium surface and can occasionally feed on roots and vector plant pathogens. These larvae need high moisture to survive. If the growing medium dries out as suggested above, the larvae dry out and perish. Also, shore fly populations can be diminished by drying out their algae food source. Although fungus gnats are less sensitive to the moisture levels in the growing medium, a dry growing medium does reduce their populations.