Greenhouse samples - A common theme

Posted: May 1, 2012

Over the last 3 weeks, the Plant Disease Clinic has received many greenhouse samples with root problems from a wide variety of plant material.

The common theme is over-fertilization and some also have Pythium root rot which is aggravated by over-fertilization. Look for stunted and yellowed plants, stubby roots, dead darkened roots, and dying plants.

The underlying issue is that growers in all these cases are combining the use of slow release fertilizer AND soluble fertilizer. Whereas the conductivity of the saturated paste extract of the potting mix should be between 1 and 2 mS, these samples are running 6-10 mS!! 

Over-Fertilization of Potted Plants Fact Sheet
The problem is that if the slow release was incorporated into the potting mix, leaching the pots will only release more salts. To save the plants, they need to be repotted in fertilizer-free mix and then fertilized with only soluble fertilizer. If the slow release is just on the surface, it can be scraped off and leaching will reduce salts.

Growers can submit samples to the Plant Disease Clinic to check for soluble salts, but if samples are submitted make sure to send sufficient material to do so. One thumb-sized plug is not enough.

Submitted by Gary W. Moorman, Prof. of Plant Pathology