Parasitic Pathogens: Infectious Diseases in Hydroponics
Problems with parasitic pathogens in your indoor garden? You’re not alone. Unfortunately, parasitic or infectious diseases are not an uncommon occurrence in the growroom. Fortunately, there are many weapons in the battle against these plagues, and all that’s needed from growers is the knowledge of how to use them.
Even plants cocooned in the well-protected environment of an indoor garden are susceptible to the forces of evil called parasitic pathogens from time to time. We may not be able to completely exclude their presence, but there are many weapons in the battle against these plagues, and all that’s needed is the knowledge of how to use them.
Parasitic or infectious diseases are those caused by a living enemy—a pathogen that may be a fungus, bacterium or virus. On the other hand, non-parasitic or non-infectious diseases are caused by non-living agents such as nutrient deficiencies, cold, heat, toxins, pollutants, salts, drought, mechanical injury and physiological disorders, including tip burn and blossom-end rot.
But just to compound the problem, many non-infectious disease problems can look remarkably like a pathogen problem, leaving growers fighting an enemy that doesn’t exist and not tackling the real cause of the problem.
Pathogenic or Parasitic Diseases
Just as we can’t see the pathogens that bring on our dreaded colds each winter, plant diseases are just as good at avoiding detection. Plant pathogens are all around us: fungal spores are common in wind-blown dust, on soil, in water, on clothing, hands and equipment, not to mention on incoming plant material and even seeds.
Fungi, bacteria and viruses can even be carried by insect pests and injected directly into plant tissues during feeding. While it’s virtually impossible to exclude these from a growing environment, parasitic diseases require three essential factors before they infect plants: a susceptible host plant, a live parasitic agent and an environment favorable to disease development.
This is known as the disease triangle. The key to preventing outbreaks in an indoor garden or greenhouse is to eliminate one of these factors and the problem is averted before it’s even begun.
Disease Prevention in a Hydroponics Garden
Host Plant Susceptibility – While disease spores may be present in and around a hydroponic system, if the plants are strong and healthy, they may be less susceptible to an attack. An overcrowded garden is extremely inviting for invading pathogens. Overcrowded plants are often weakened by a lack of access to light, and humidity can build inside and under dense canopies, where air movement is restricted.
Host plants are also more prone to infection if they are physically damaged, as this provides an open wound for direct entry into the plant. Pruning cuts, leaf removal and torn roots during transplanting all provide a moist surface for fungal spores and bacteria. Over-watered plants with saturated root systems are particularly prone to pythium and other root rot diseases. Levels of dissolved oxygen in solution culture or NFT systems should be maintained at optimal levels to help prevent root rot issues.
One further way of reducing host susceptibility to pathogens is with genetic resistance and this is one of the most effective methods of disease prevention. Most plants are prone to just a few significant pathogens—powdery mildew in cucumbers and melons and wilt pathogens in tomatoes being common examples.
Many modern cultivars have some degree of disease resistance bred into them and these are well worth taking advantage of as it means higher yields and less need for spray applications. Tomato hybrids usually have resistance to a number of common issues, including fusarium, verticillium wilts, a number of viruses, certain leaf spot pathogens and others.
With cucumbers and melons, selecting varieties resistant to powdery mildew will avoid many problems with these plants. If a particular pathogen has become an ongoing battle, checking out what genetic resistance is available in hybrid cultivars is worth the effort.
Environmental Factors – The growing environment is often the main contributing factor to a disease outbreak. Many fungal and bacterial diseases, although not all, need either high humidity or water on the plant for them to successfully invade tissues. This is why ventilation, with air movement up, under and across foliage, and good moisture removal, are so important within enclosed growing areas.
Keeping things clean is another way of thwarting pathogens before the infection process can begin. Used growing media and old plant debris can harbor disease pathogens for a considerable length of time.
Keeping the system clean, removing prunings and old or sick plants helps reduce the level of pathogen inoculum in the growing area. Avoiding heavy algae buildup and controlling fungus gnats, which carry plant diseases such as pythium, is also important in hydroponic systems, as is choosing a high-quality water supply that is not contaminated with soil or other organic matter.
Signs & Symptoms of Diseased Plants
Once a pathogen is inside the plant, there are three main ways the plant will respond during this initial phase of infection: development of galls, swellings or leaf curl; the stunting, yellowing or incomplete development of leaves, fruit and roots; or plant tissue die-off, such as blight, leaf spots, wilting and rots, including root browning and disintegration with certain pathogens.
Sometimes this initial phase of infection inside the plant is not that visible and growers may miss the early signs. Fungal diseases, once established, will then go through their life cycle and start to reproduce by producing spores. It is at this stage that many of the most common and easily recognizable diseases are detected and identified.
White, powdery spores over the upper leaf surface indicate powdery mildew, while fine, grayish-white, powdery patches on the undersides of lower leaves are produced by downy mildew. Larger, brown or gray, furry-looking spores on fruit, stems and leaves, particularly on plants such as tomato, basil and strawberry, are caused by botrytis (gray mold). Fungal rust disease appears as rust red or brown spore patches.
Some fungal pathogens, such as those that cause root rot, don’t produce visible spores that make identification easy. Often the first sign of a root disease infection is wilting during the warmest part of the day, as damaged root cells are unable to take up enough water and nutrients to maintain the plant.
Roots will look thin and brown, and will eventually rot away; seedlings may topple over from the base (called damping off); plants will become stunted; and the disease can spread quickly through an entire system.
While fungal diseases are far more common in indoor gardens, bacterial pathogens can still cause problems and are more difficult to identify. The lack of leaf wetness and rain in a protected cropping area generally restricts the spread of bacterial pathogens, so keeping plants dry is a good form of defence.
Bacteria don’t produce the highly visible spores that help identify many fungal diseases; instead, bacterial infection is characterized by a few distinct symptoms including leaf spots with a slightly greasy or water-soaked appearance or bacterial dieback, which may ooze with gummy fluid or become black and slimy.
The most commonly encountered bacterial diseases are leaf spots, soft rot/Erwinia, and bacterial wilt/canker. Determining if a disease outbreak is fungal, bacterial or viral is essential for effective control. Most fungicides will not control a bacterial disease outbreak and viruses cannot be controlled at all once inside the plant.
Copper sprays are often highly effective on isolated outbreaks of bacterial disease, but the cause of the infection—often dampness—should also be identified and prevented.
Virus diseases are more complex than fungal or bacterial infections. They can occur through infected seeds, either present on the outside or inside the seed coat, or hitchhike in on insect vectors and are spread from one infected plant to another through sap cross-contamination.
Viruses are highly infectious and while most come in on insects or new plant materials, they are often spread from plant to plant by the grower during pruning or harvesting. While there are many thousands of plant viruses, the main symptoms are strange, deformed, twisted, curled and thickened growth; mottling or streaking of the foliage; and overall plant stunting.
There is no cure and infected plants need to be removed from the growing area as soon as possible to prevent further spread. Many commonly grown plants such as tomatoes, capsicums and cucumbers have inbred genetic resistance to the most common viruses such as tobacco mosaic.
Dealing With a Disease Outbreak in Your Hydroponics System
Once a disease outbreak has occurred and is correctly identified, there are several ways to resolve the situation. Organic options for disease control may include manual removal of infected foliage or plants; natural sprays; botanical or microbial compounds; silicon applications for certain species; surfactants for root rot pathogens; UV, ozone or slow sand filtration treatment of nutrient solutions/water supplies; or even home remedies such as sodium bicarbonate for mildew control.
Other growers prefer to select from a wide range of commercial fungicide or bactericide spray products, attempting fast and direct control. Both of these approaches can lead to other issues. For example, chemical sprays for fungus problems can cause resistance to build up within populations of disease pathogens, making the product ineffective after just a few applications.
A spray program that rotates different chemical classes for fungicides may be required to gain consistent control of a disease and prevent disease resistance. Organic approaches generally take longer to work and may not always be effective, depending on the environment, disease and how severe the outbreak is.
Plant Defense Systems
Plants are not without their own defense strategies when it comes to preventing disease infection. In fact, the majority of fungal spores and bacterial pathogens present in a growing environment never actually cause a significant disease outbreak. A complex series of anti-microbial compounds termed phytoalexins are produced within the plant when it senses it’s under attack by pathogens.
Phytoalexins are compounds that accumulate around the area of the invasion and create a number of different strategies to fight, wall off and generally limit microbial growth to prevent the disease from spreading further. While phytoalexins are naturally produced by plants, there are a number of compounds known to be inducers, which when applied to a crop can have the ability to boost phytoalexin levels.
Some of the more commonly known phytoalexin inducers used in crop production include salicylic acid, chitin and chitosan, phosphorous acid and phosphonate fungicides.
These stimulate the plant’s defense systems and have been incorporated into a range of commercially available plant-protection products, but the effectiveness of the process of phytoalexin stimulation can be somewhat variable between different plant species and dependant on the pathogen and environment.
Since most of the phytoalexin inducer compounds are safe and easy to use, they are certainly worth considering as an option in hydroponic systems and indoor gardens.
Most parasitic pathogens are not deadly if correctly identified and dealt with early, but attention to disease prevention through environmental controls, genetic resistance, plant screening and hygiene go a long way towards stopping any pathogen in its tracks.
Written by Lynette Morgan | Author, Partner at SUNTEC International Hydroponic Consultants
Dr. Lynette Morgan holds a B. Hort. Tech. degree and a PhD in hydroponic greenhouse production from Massey University, New Zealand. A partner with SUNTEC International Hydroponic Consultants, Lynette is involved in remote and on-site consultancy services for new and existing commercial greenhouse growers worldwide as well as research trials and product development for manufacturers of hydroponic products. Lynette has authored five hydroponic technical books and is working on her sixth.