Autumn Woes: Root Rots

Root damage due to disease interferes with nutrient uptake, resulting in nutrient deficiency symptoms.

Root damage due to disease interferes with nutrient uptake, resulting in nutrient deficiency symptoms.

In New Zealand, Pythium and Phytophthora are waterborne plant pathogens that can be devastating in the hydroponics, seedling and nursery industries when drought-breaking rains arrive, usually in the autumn. Industry consultant JOHN WHITE describes his professional experience and highlights his preferred water disinfection method.

We had a long dry summer and early autumn, followed by a good rainfall to break the drought. A week or so later the lettuce in my small NFT system suddenly wilted. Examination of the roots showed why: they were showing all the early signs of root disease caused by Pythium or Phytophthora. In this incident, every single plant in the system had root disease. This is not always the case: sometimes the attack is more insidious with a few plants showing slow hard growth, and sometimes nutrient deficiency symptoms, before spreading through the whole crop.

These two pathogens are major problems not only in hydroponics but also in the nursery industry where seedlings and many trees and shrubs of saleable size can be suddenly damaged. These fungi are known as water moulds, as they spread in water and hence are well adapted to NFT and other liquid hydroponics.

They can also be introduced to any hydroponic growing system (including soilless media and rockwool-based systems) in the water supply.

Root rot of the butterhead lettuce a few days after wilting, note the disease free white new roots emerging from the rockwool propagation plug. Most of the roots in the solution in the gully are  light brown, while the few white roots have brown tips. This is typical for Pythium or Phytophthora infections in most crops.

Root rot of the butterhead lettuce a few days after wilting, note the disease free white new roots emerging from the rockwool propagation plug. Most of the roots in the solution in the gully are light brown, while the few white roots have brown tips. This is typical for Pythium or Phytophthora infections in most crops.

They are common pathogens in both cultivated soils and bushland and have a habit of releasing vast quantities of spores and zoospores when they flourish following rain after being semi-dormant through a drought.

If surface water is collected for irrigation, then it can be heavily loaded with spores and swimming zoospores, and using this water is a major risk to all hydroponic growers. Many clients of our consulting practice, growing lettuce, tomatoes and other crops, who draw their water from dams and rivers without any disinfection treatment of the water, have this problem after dry summers. Their water may well have low levels of water mould contamination all through the year, but the sudden exposure of what was often old and tired tomato crops nearing the end of their season to a very high level of contamination resulted in severe disease.

The problem does not arise with water taken from deep bores or treated by reverse osmosis, but it can occur when town water supplies are taken from rivers or other surface sources. Water treatment to drinking water standards does eliminate the risk of human pathogens, but does not guarantee freedom from these plant pathogens.

New Zealand nurserymen have been recommended to bioassay their water supplies, including town water, on a regular monthly basis using lupin seedling baits, but this is not appropriate for hydroponics, where it is essential for both surface water and town water to be disinfected.

My preferred disinfection method is by ultraviolet (UV) filtration. Very good UV disinfection systems are available for larger scale growers, but cost $20,000 or more. Small-scale growers can use UV filtration with capital cost in the $1000 to $3000 range. These are certain domestic UV filters, which can be operated at lower flow rates to provide a higher UV dose to the water.

Pythium and Phytophthora need exposure to 100 mJ/cm2 UV to be killed, but drinking water only needs 36 mJ/cm2 to eliminate human pathogens. The manufacturer can calculate what lower than normal flow rate is needed to achieve the needed high dose. These simple units work well with town water, which has low turbidity and is relatively pure; other water supplies will need pre-filtration to ensure that the water has a high UV transmission.

The UV light output from the bulbs in these domestic units decreases over time and a regular bulb replacement program is essential.

Various off-label fungicide treatments as well as phosphorus acid have been recommended from time to time to control the problem, but my experience is that such treatments very rarely result in a cure; usually the crop staggers on with reduced performance instead of dying more quickly. It is often best to accept the loss and quickly terminate the crop, thoroughly disinfect the whole growing system and start a new crop.

About the author
Dr John White is a horticultural consultant and author. He provides specialist extension and research services to the greenhouse industry in Auatralia and New Zealand. Email: johnwhite@ghvi.co.nz; Website: www.ghvi.co.nz/NFT/Disease_in_Hydroponics.html   Ω

June 2015 / Issue 156


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