By Dr. Lynette Morgan
Occasionally problems in a usually trouble free hydroponic system strike most growers, with growth of unwanted algae being the most common, but sometimes the problem can be hidden down in the root zone, where the culprit is not always obvious. Fungus gnat larvae and microscopic nematodes can do considerable damage to a hydroponic crop’s root system without a grower even being aware what is going on. Many inexperienced growers don’t make the connection with the ‘tiny brown flies’ which seem to be fluttering around the system with the sudden death of seedling root systems and collapse of plants, since the adult flies themselves do not attack the plants. However in the plants root zone, hungry larvae, hatched from the eggs laid by the gnat adults are biting into the root systems and causing serious damage.
Prevention of algae, which acts as an ‘enticer’ to fungus gnats, their larvae and nematode infestations is vital, as control options for these problems and pests once present are limited.
All hydroponic growers know what algae looks like – it is often green, but can be brown, reddish or black, it clings to channels, gullies or pumps or oozes over the top of damp media. Algae often has an ‘earthy’ or ‘mouldy’ smell, particularly when large numbers are decomposing in the nutrient. It has the ability to block drippers, emitters, pumps, return channels, and heavy infestations can actually ‘seal’ off the surface of the media or substrate.
The problem with algae, apart from the appearance and smell problems it creates, is not so much that it uses up nutrients from the solution, but as it blooms, dies and decomposes, it removes dissolved oxygen from the hydroponic system. This increases the biological oxygen demand (BOD) on the system and the plant’s roots may suffocate from a lack of O2. Decomposing algae may also release toxins as it breaks down and provides a food source for plant pathogenic fungi which may then multiply to high levels in the system. Algae on plant root systems can suffocate the roots, making the plants prone to attack by opportunist pathogens such as Pythium.
Algae, is a form of plant life, it is a natural consequence of exposing water with nutrients dissolved in it to a light source. Where there is no light, algae can not grow, so the most obvious solution to preventing algae growth is to stop light from reaching the nutrient solution where ever possible. Channels should have light proof covers, return gullies also need covers, large media beds can also be covered with either plastic film or a layer of substrate which is designed to act as a ‘dry mulch’ since algae can not grow on dry surfaces. In aeroponic systems, the root chamber must be light proof and media based pot or container systems can have plastic or rigid collars made which cover the surface of the media. However, even in the best designed system, there is usually somewhere that light will fall on the nutrient – planting holes in NFT, return outlets in channels and tanks are common areas.
Control of algae, once in a hydroponic system can be difficult – most growers tolerate small amounts of algae in the system, provided it does not become excessive and this usually causes no problems. Where algae growth has become thick and widespread, often the best option is to clean up the whole system after crop removal and start again with a clean system.
Some growers add algicide products into the nutrient to kill off algae and there are a number of these products on the market. However, since any product which kills algae, a form of plant life, can also damage young or sensitive root systems, care must be taken with the dose and damage has been known to occur. Algae will also regrow, very quickly after applications of most algaecide products, requiring more and more of the chemical to get good control. In a study carried out in Belgium on Algae control in hydroponic systems, it was found that many of the products tested for control of algae in nutrient solutions (Diazinon, Endosulfan, Propiconazole, Thiram, Ziram, Quinomanid, Irgarol-1051 and Hydrogen peroxide) were either totally ineffective in killing algae, or killed off much of the algae but were also very phytotoxic to the plants and causes considerable root damage. It was also found that dose levels of 50ppm of hydrogen peroxide was requited to control algae, but that this dose was too phytotoxic for young plants, although older plants survived this dose rate. Therefore, careful and selective use of H2O2 could be used on older, more resistant plants, but since H2O2 is a ‘biocide’ rather than an ‘algicide’ there is always a risk of root damage.
Other studies have found ‘organic’ algae control methods such as adding certain ‘grapefruit seed extracts’ to the nutrient will kill algae without harming the plants – this is a method used in drinking water, fish ponds, lakes etc and appears to work well. There could be the potential, in larger hydroponic tanks to use ‘Barley straw rafts’ as a means of algae control as has been proven to work in ponds, lakes and other water ways. However the best method of algae control will still always be prevention of the problem, so excluding light should be the main emphasis in systems with algae problems.
Fungus Gnats and Shore Flies
Fungus gnats are small dark flies, with long legs and distinctive wings, which belong to the Sciaridae and Mycetophilidea families and are sometimes called ‘sciarids’ or ‘sciarid flies’. Adults are usually 0.06 to 0.125 inch long and are weak fliers, often seen sitting near plants and running on growing media or foliage. Females lay tiny eggs in moist media or potting mix. The larvae which hatch from these eggs, cause the damage to the plant root systems, are white or transparent with a black head. Most fungus gnat larvae feed on fungi or dead plants in the media, but some species also feed on living tissue and will attack young seedlings or cuttings. Fungus gnats also spread plant disease pathogens in the media, so good control is important.
There are also shore flies which live in or on algae growth or on very wet, decomposing organic matter and these are common in growing areas where conditions are damp. Shore fly larvae do not tend to feed on root systems, but they are frequently confused with fungus gnats since they often occur together. Fungus gnats are a more serious pest because they can cause so many crop problems by weakening the root system of a large number of plants.
Prevention of gnats includes screening of doors and vents and well as reducing media moisture and organic debris. Allowing the surface of the hydroponic media or substrate to dry out between irrigations will help prevent infestations, as will good drainage of growing beds or ebb and flow systems. Make sure all pruning, organic matter, dead leaves etch which have fallen into the surface of the media are removed regularly. Also make sure that any growing media is not already infected with gnat larvae when you purchase it and keep it enclosed in plastic until use.
The first step in controlling gnats is monitoring for the presence or build up of these pests. The adult flies are attracted to yellow sticky traps – but identification of fungus gnats and shore flies can be difficult as many species look similar. There are three types of control for gnats: chemical, growth regulators and biological agents. Chemical products are often organophosphate based and care needs to be taken when handling and applying these: products containing acephate or malathion are often used for chemical control, also carbaryl which also kills the larvae. Diazinon containing products have also been reported to be effective but they need to be applied to the media as high volume drenches. Pyrethrum or a pyrethroid can give some temporary control. It should be noted, however that most of these products are not registered for use in hydroponic system nutrients and media, so commercial growers always need to check with their department of Agriculture about which products can be legally used.
Of the insect growth regulator products – those containing Azadirachtin (from the Neem tree), kinoprene, diflubenzuron or cyromazine can be effective when applied to the growing media at regular intervals. Another biological agent is Gnatrol, a spore forming bacterium produced commercially, which can be applied to control the larvae in the media. Make sure the bacterial product you use is specific for gnats, not the ones used for caterpillar control which are ineffective against fungus gnats.
There are some insect predators now being used with much success to control gnats. There are two main gnat predators available commercially – one is the predatory soil mite, the other are species of nematodes. Hypoaspis miles is the species of soil mite which feeds on small soil inhabiting insects and is primarily a predator of fungus gnat larvae in the media. These predators will eat 1 to 5 larvae per day and can survive as a scavenger by feeding on algae and plant debris. Hypoaspis are for preventive control only, before fungus gnat populations are high, and can be purchased in a peat mixture for application.
Nematodes, also called ‘eelworms’ or ‘needle worms’ are microscopic worms, which can only be seen under a microscope, making identification difficult for most growers. Not all nematodes cause plant problems, many are beneficial and feed on other tiny insects, clean up decaying organic matter in the root zone and some are even natural predators of plant pests such as fungus gnat larvae. Of the nearly 15,000 types of nematodes so far discovered only 2,500 are parasitic on crop plants. Many hydroponic growers assume that nematodes are soil based and therefore don’t affect soilless systems, however nematode damage of hydroponic crops can be common where the system has been contaminated with soil, through seedlings and when nematodes might be present in the water supply. Water from wells, streams, dams, anywhere the water has been in contact with soil and not sterilised before use, can contain nematodes and become a source of infection. However the most common source of nematode infestation in hydroponic systems is where soil has been used to raise the seedlings, or soil raised plants, seedlings or cuttings have been introduced to the hydroponic system, carrying with them an infestation of nematodes. Nematodes can also be spread from soil to hydroponic systems via wind, equipment, animals and humans. However, fungus gnats and fungus gnat larvae can also carry nematodes and spread these from infected plants to healthy plants.
The symptoms of nematode infestations include damage to the root system in the form of root ‘knots’ swellings, galls, stunting, root death and overall low vigour of the plant.
Some of the most common nematodes affecting hydroponic crops are the ‘Lesion Nematode’ which causes reddish brown root lesions, stunted plants and leaf yellowing. The ‘Needle Nematode’ whose symptoms include inhibition of root elongation and swelling of the regions just behind the root tips resulting in a forked, and shortened root system. The ‘Root-Knot Nematode’ which causes round to spindle shaped swellings (galls) on roots, with plant stunting and wilting and the ‘Spiral Nematode’ which causes lesions on the roots and a stunted root system. Nematodes can cause further damage to hydroponic plants since they can transmit a number of plant viruses. Nematodes which invade hydroponic systems can be divided into two groups – those which invade plant tissue and spend part of their life cycle isolated from the media or nutrient and those which live in the media/substrate and get their nutrition from feeding on the roots. Root systems damaged by nematodes are then very prone to infection by fungi and bacteria which can enter the nematode injured root tissues and cause other diseases.
Control of nematodes can be difficult – in soil, fumigation, crop rotation and growing of resistant varieties are the main ways of controlling the pests. In hydroponics, a severe nematode infestation will mean the system needs to be stopped and cleaned out with infected plants and media removed and destroyed to prevent reinfestation. Sterilisation agents such as H2O2, or chlorine can be used to clean the system. Nutrient solutions need to be replaced and clean, sterile media and nematode free seedlings replanted. Identifying how nematodes entered the system in the first place is also important to prevent re-infestation.
Nematodes, fungus gnat larvae and algae are all hydroponic ‘parasites’, using the nutrients, plant roots and favourable environment provided by growers to multiply and potentially cause a great deal of damage. Luckily, all these potential parasites can be prevented and controlled so that problems don’t become severe and plant growth seriously affected. Recognising these types of nutrient and media problems is important, but so too is preplanning so the pests and algae don’t find conditions in your hydroponic system favourable enough to bother hanging round and settling in.