In deep water

By Kirsty Wright, plant pathologist at Stockbridge Technology Centre (STC) October 2018

In deep water hydroponic systems Phytophthora cryptogea, and other oomycete root rot pathogens, can cause severe deterioration of crop roots, leading to reduced vigour and yield.

Lettuce is increasingly being produced in these systems, but UK hydroponic lettuce growers currently have no access to fungicides (conventional or biopesticide) that are approved for application into hydroponic growing solution for direct control of root rot disease.

Recognising the industry need for plant protection products in deep water hydroponics, the Crop Health and Protection Centre commissioned a replicated testing facility for this system in 2017, consisting of 60 independent tanks, sited at Stockbridge Technology Centre (STC). With technical input from the British Leafy Salads Association (BLSA), each tank was purpose made to provide a model deep water hydroponics unit, with each tank holding 330 L of solution and 30 individual plants, floating on rafts. Water movement was ensured through aeration, with solution temperature also controlled through a heater in each tank.

As soon as this facility was completed, I ran a SCEPTREplus project through it investigating the efficacy of nine experimental products to control root rot in lettuce, caused by P. cryptogea, when applied directly to the nutrient solution. Five of these products were conventional pesticides and four were biopesticides. Two of the products were completely novel, while the other seven were already approved for foliar application to lettuce or similar crops.

The trial ran over two months with regular assessments taken of root length and colour, and yield data gathered at the end of the trial. Having artificially infested each tank at the start of the trial, it was relatively straight-forward to identify effective treatments when comparing against the negative control (where tanks had been infested, but not treated with any product), as anyone attending the BLSA event at STC back in mid-March could confirm!

After crunching the numbers at the end of the trial, two conventional fungicides came out on top as giving excellent control of disease symptoms, and one other gave some control. No biopesticides were effective at reducing the symptoms of root rot, although one did seem to reduce the number of zoospores present in the nutrient solution. This product might be useful if concerns over phytotoxicity could be addressed, for example by use at a lower dose. The final report will be available soon and will hopefully help to inform product developments for the deep water hydroponics sector moving forward.