The number and type of diseases that can affect greenhouse and field-grown crops are numerous. Any one of them could spread quickly if not identified at the first symptoms and wipe out hundreds, thousands, even millions of dollars worth of crops. These diseases are overwhelmingly caused by bacteria, viruses, and/or fungi. Many have similar symptoms; some have distinct ones. It is important to not only identify the common plant disease by their symptoms, know how to treat it when it occurs, and more importantly, learn how to create a growing environment that is less likely to harbor and spread diseases.
Unlike insect outbreaks, needing only a predator and vulnerable crop to cause devastation, plant diseases require three conditions that must be met for them to spread. Known as the “disease triangle,” these three components are: a susceptible host, pathogen able to infect the host, and an environment favorable for the development of disease. By managing even one of these variables, plant disease can be avoided, though that is much easier said than done as many pathogens travel through the air and can be splashed onto a plant with water.
Air is one of the common vectors of disease transmission. Pathogens, especially fungal spores, can travel through the air. While nearly impossible to totally prevent in field-grown crops, even greenhouses have a difficult time excluding them. Spores from such common diseases as powdery mildew, Botrytis, Alternaria, and rust can come into a greenhouse through vents, fan openings, an open door, even inadvertently by people coming into the greenhouse.
Irrigation water can be imported into a greenhouse or field unknowingly. Growers who get their water from lakes, ponds, or streams are particularly susceptible. Even wells can become contaminated with these pathogens if they leach from the soil or enter through run-off. This scenario is especially dangerous to crops grown in any system using recycled water.
Other plants can be sources of pathogens. Many pathogens need living plant tissue to survive. These can lay dormant or not cause visible symptoms to a host plant until the environment is right for them to thrive. Keeping “pet” plants or overwintering annuals in the same area where crops are grown is not recommended. This is why it is good practice to ensure a greenhouse has at least some periods where it is completely devoid of plant material and can be thoroughly sanitized.
Weeds are also a source of pathogens. Not just the weeds that may pop up through a crack in the floor, but ones growing outside in proximity to a grow room. They can release spores or airborne materials that travel and enter the greenhouse through the aforementioned openings. Any living plant material could be essentially a reservoir of pathogens (insect pests too) that can harm your valuable crops.
Management of greenhouse spaces, or other growing areas including fields, should be performed with a strict eye towards reducing the chances for pathogens to be introduced. Money and other resources devoted to this will not be wasted as the cost in lost crops, or in control measures will generally far outstrip what was spent in prevention. Good prevention will take on multiple forms on multiple fronts. This includes keeping things clean, ensuring all equipment is in good working order, and that plants are closely monitored. Anything to interfere with the completion of the disease triangle will be time and money well spent.
Common Diseases of Greenhouse Plants:
Diseases commonly found in greenhouses can be caused by bacteria, viruses, or fungi. Deficiencies caused by lack of certain nutrients may present symptoms like a disease, but those instances can usually be treated quickly and effectively by supplying the affected plants with the needed nutrients. While it is impossible to prevent the introduction of 100 percent of disease causing agents, 100 percent of the time, it is possible to create an environment and enforce certain practices to make the appearance of diseases less likely.
Bacterial diseases cause damage by colonizing plant tissue, interfering with a plant’s ability to grow and function normally. They build up their population quickly and are often not spotted until symptoms are already evident. They have the ability to double their populations as quickly as every hour if nothing is done to impede that growth. If that bacteria enter a plant’s vascular system, it can be quickly transported to many if not all parts of the plant. Affected plants then wilt because bacteria is essentially plugging the water-conducting cells.
Symptoms of bacterial diseases include leaf spot, cankers, soft rot, wilt, and galls. It can be difficult to distinguish symptoms of a bacterial disease from those of fungal diseases, making diagnosing some plant diseases very challenging. With some types of bacteria, such as Erwinia chrysanthemi, the intercellular material (pectin) is attacked and dissolved, causing soft rot. Pseudomonas cichorii bacteria cause dry lesions on their hosts because they are unable to generate the enzyme needed to dissolve pectin.
Viruses spread within the host plant very similarly to bacteria. Like bacteria, viruses impede normal cellular functions of the plant host. They too can spread very quickly before being detected. Their symptoms however, are very different from those of bacterial or fungal diseases.
Symptoms of viral diseases are usually evinced on the leaves. They can be crinkled, distorted, or otherwise misshapen. Other viruses cause unusual coloration or patterns in the leaf, such as ringspots or line patterns. With mosaic type viruses, affected plants may have both healthy tissue and affected tissue on the same leaf. Still others may cause chlortice streaking or yellowing of the leaf veins.
Flowers can also display signs of viral diseases. They may show the same streaking, or abnormal colors as are evidenced on affected leaves. They may also be deformed.
Viruses can have very narrow, specific hosts or be much more generalized, affecting thousands of different plant species. For example, the cymbidium mosaic virus is a disease-causing pathogen that only affects orchids. On the other hand, cucumber mosaic virus affects more than just cucumbers, spreading to many different types of greenhouse crops not even in the cucumber family.
Fungal diseases pop up easily in greenhouses, because they can form wherever there is excess moisture. Spores can be carried in on air currents or splashed onto leaves from infected water. In addition to the symptoms described above for bacterial diseases, fungal diseases can present themselves with other symptoms.
One of the more prominent fungal diseases is powdery mildew. This fungal disease is known for white, powder-like spots, which can enlarge and join into one larger blotch. Small yellow spots and sometimes seen first. Plants that are affected may have trouble photosynthesizing, the plant can become weaker overall, and fruit can ripen prematurely or become stunted.
Downy mildew is another common fungal disease of crops. It is less common in greenhouse crops as cooler temperatures favor its development. Both sides of the leaves are affected with downy mildew, unlike powdery mildew where it usually only affects the top sides of leaves. Yellow to brown spots form and the leaves curl and wilt in extreme cases of downy mildew.
Rust is another common fungal disease of plants. It causes yellow, orange, or dead spots on leaves, starting with the older bottom ones. Stems and flowers can also be affected if not controlled. Botrytis is another fungal disease. It can present as a gray mold, leaf blight, stem canker, or literally any other part of the plant.
Control Options for Greenhouse Diseases
Options for controlling greenhouse diseases are varied. As with any pest, prevention is the best way to avoid issues, but is often where best practices may be overlooked. Preventative measures include cultural practices (predominantly sanitation), cultivar selection and mechanical solutions (mostly air movement). Treatment options range from organic to chemical.
Cultural Disease Control Options
Observing proper practices in the greenhouse will pay huge dividends. Unfortunately this is where many growers cut corners because it is difficult to quantify the extra effort and expense poured when diseases don’t occur. It is however the one thing that growers have the most control over when it comes to plant susceptibility to disease-carrying pathogens.
If there is only one thing that growers do to prevent disease it should be to disinfect all tools, growing surfaces, floors, walls, and anything that is going to remotely come into contact with or be near any plant. There is no such thing as a growing area being “too clean”. Special attention should be paid to those growing areas with dirt or wood floors. Concrete floors are the easiest to keep clean, but fabric and gravel is a better alternative to floors made of absorbent material that can hide pathogens.
Plant debris should be collected as soon as it appears. When trimming, pruning, repotting, etc. all of this material should be collected and removed from the growing area. Even if it does not currently harbor pathogens, it can become a host for opportunistic diseases and spread to your healthy plants. Many pathogens can remain dormant for extended periods of time, hiding out in plant or soil debris in greenhouse corners and floors. Weeds should be pulled as soon as they appear. Cracks in greenhouse floors and the margins between the floors and side walls often host weeds. These can also provide safe harbor for pathogens and should be eradicated as soon as they are discovered.
It may be a small thing, but hose ends should never touch the ground or table surface. It should be set down in water that may have pathogens in it, then the next time the hose is turned on it can blast your plants with those diseases, right along with their irrigation water. Make sure hoses get hung up in between usage. This has the added benefit of reducing a major trip hazard in the greenhouse. Drip irrigation or overhead watering not only reduces the amount of water used in the growing area but eliminates or at least reduces the amount of hoses that are used.
Personal sanitation cannot be overlooked. Hand washing is important while handling plants as pathogens can easily be transferred unknowingly while handling plants.This is especially true of smokers who can transfer the tobacco mosaic virus which is found in many tobacco products onto other plants in the greenhouse that may be susceptible to it. Using disposable gloves and changing them frequently is an additional good practice to reduce disease spread.
Debris from shoes should also be knocked off before entering a greenhouse. Even better biosecurity would be to have footbaths at each entrance of the greenhouse or grow area to kill some of the potential pathogens that might be traveling along on people’s shoes. This disinfect should be changed daily and the mats themselves should be sanitized as well routinely. Requiring the wearing of boot or shoe covers while in the grow area is an alternative to the foot bath, or for added security could be an additional step.
Sanitation is often the step that breaks the chain of disease transmission. Several common and devastating viruses, bacterial, and fungal diseases can be stopped by practicing good sanitation in the greenhouse and growing areas. Mosaic and mottle viruses, bacterial wilts, cankers, and leaf spots have all been proven to have their spread interrupted by good sanitation in the growing areas. While sanitation is far and away the biggest step a grower can take to avoid plant disease, there are a couple more cultural practices that can help to mitigate disease spread.
When possible, grow crops of different species in different greenhouses or otherwise segregate them. This can prevent the spread of some diseases between and among susceptible hosts. Growing vegetable crops apart from ornamental crops can help to reduce some amount of powdery mildew spread as well as some viruses that can affect multiple hosts. When introducing plants grown elsewhere into a grow area, segregate them if possible for a period of time to determine if they show signs of disease that could spread to your “domestic” plants.
Selecting species or cultivars that are known to be disease resistant as well as obtaining seeds verified to be “clean” should be standard practice to most growers to help prevent disease. Choosing a medium that both retains adequate moisture but is also well drained is critical to help prevent the development of a disease-susceptible environment.
Finally, one of the most basic cultural practices to stop or slow the spread of disease is to space out plants. It is easy to think in terms of dollars and sense that the more plants per square foot, the more potential profit. This is only true to a point. Once airflow is restricted then disease can set in and there is potential to lose some or all of the crop to various pathogens.
Mechanical Disease Control
One of the most important tools in controlling greenhouse spread of disease is maintaining an environment that is conducive to plant growth but impedes the development of disease. This is easier said than done, since many diseases thrive in conditions that favor plant growth. Controlling the air flow, temperature, and humidity of the greenhouse can help to reduce incidence of plant disease.
Installing both horizontal air flow fans (HAF) and exhaust fans will help to keep air circulation and temperature where they need to be. Exhaust fans help to blow out some of the warm air in the growing area while HAF fans help to regulate the temperature to keep leaf temperatures closer to the air temperature. This is important because if leaf temperatures are cooler than the ambient air, then condensation can occur. This can support the emergence of pathogens.
Air movement also helps to replace depleted carbon dioxide in a growing area. Fan capacity in growing rooms should be sufficient to move air at 50-100 ft per minute. Two cubic feet of air per minute should be moved per each square foot of growing area. Fans should be located approximately 30 to 50 feet apart (9 to15 meters) down the length of the growing area with the end ones approximately 10-15 feet (3 to 5 meters) away from the end walls. THese should be mounted above head height (for safety) but not so high as to render their benefit to the plants negligible.
Ideal temperature in a grow area is most often achieved through a combination of thermostats and ventilation. This can be as complicated and high-tech or as simple and manual a setup as each grower wants or can afford. Temperatures that support plant growth should be maintained between 75 and 90 F (24-32 C). Outside of these ranges it can be easier for diseases to develop.
High humidity in growing areas is one of the major contributors to mildews and other diseases, including Botrytis. Humidity levels to help avoid some of these can be controlled by a few different methods. HAF fans are critical for keeping standing moisture off of plant surfaces which raise the humidity around them. Carefully timed ventilation to control humidity is important as well. During the heating season, greenhouse vents can be just cracked barely open or exhaust fans can be run on a low setting to allow some cool air to enter while warm air exits. This can be done for periods of up to 15 minutes at a time if humidity levels need to be controlled during colder months. Dehumidifiers can be run as well and are very efficient especially for smaller growing spaces. Watering earlier in the day or when there is enough time for all excess moisture to evaporate before nightfall is ideal to help keep humidity levels down.
The use of benches for propagation and cultivation can also reduce the spread of disease. Benches allow for better airflow around each plant, and prevent lower branches and leaves from touching the ground. They also prevent water from splashing onto the plants from the ground that may have bacteria or spores. Metal or plastic benches are better than wood since wood can absorb pathogens and is more difficult to fully sanitize.
Organic Control of Plant Diseases
There are dozens (or more) examples of available fungicides on the market labeled as acceptable for use in organic systems. These are easily discerned and most are just as effective if not more so than their chemical counterparts. Many of these work by drying out the pathogens or otherwise disrupting their ability to spread. Some are composed of “good” bacteria and fungi that either outcompete harmful pathogens or block their ability to function. There are however many other organic and non-chemical controls of plant diseases than just spraying affected plants with organic pesticides.
Besides starting out by selecting disease resistant cultivars, selected plants can be chosen based on their disease indexing. This practice does not eliminate the chance of a plant developing future disease but it does ensure that they are not sold already having any. Disease indexing in plants means a grower has been propagating or taking cuttings from only plants that are verified to not have any bacterial, fungal, or viral pathogens that could be passed along. At the point of sale, these plants are free of any disease-causing pathogen. This is not a fail-safe method as not all pathogens are tested for, but the incidence of disease in indexed plants is far lower than that of the general population of any particular plant species as compared to its indexed counterpart.
When all non-chemical efforts have been exhausted and there are still signs of disease-causing pathogens at work, it is time to consider chemical disease controls.
Chemical Disease Control
Numerous chemical fungicides (a catch-all term for fungicides, bactericides, and virucides), are available and on the market to prevent or eradicate diseases of greenhouse plants. For some growers, they are the first response. For other growers who practice a more sustainable approach, they are the last resort. In either case, they can be very effective if used according to the instructions on the label in the volume prescribed and for the particular diseases enumerated.
As with insect pests, some pathogens can develop resistance to the use of some fungicides. It is therefore recommended to use a range of different fungicides, with differing formulations and distinct modes of action. Fungicides can be wettable powders, flowable, in suspension, etc. and be protectants to coat the plant or systemic to be absorbed.
Preventative applications of chemical fungicides, on tools, tables, walls, and floors between crops goes a long way towards eliminating the opportunities for diseases to affect the next crop of plants. Bleach, quaternary ammonium, hydrogen peroxide, chlorine dioxide, and ethanol are active ingredients in a number of available disinfectants designed for greenhouse and growing spaces.
Common Diseases of Field-grown Plants
Crops grown outside are prone to fungal, bacterial, and viral diseases. While some diseases may be more or less common in field-grown versus greenhouse-grown crops, the same panoply of diseases can plague them. The most common cause of disease in the field tends to be fungi.
Fungal diseases are easily spread outside by wind, rain, insects, even carelessness of people. Spores can be unknowingly transferred on clothing or shoes, tools, even irrigation water. Fungi is diverse and can grow on a variety of hosts in a variety of habitats. Symptoms of its damage to hosts can be equally diverse. Some fungi attack roots, while other types affect flowers, leaves, stems, or stalks. Common fungi like Botrytis often appear in overgrown plants or those that are grown too close together.
Bacterial diseases can be spread by the same vectors as fungal diseases. Their symptoms are often easier to identify though. Pathogenic bacteria can cause wilt, leaf spots, blights, soft rot, scabs, cankers, galls, and other cancer-like overgrowths. Bacteria grow within the spaces of cell walls and produce enzymes that break down various plant structures. Bacteria like Agrobacterium species will genetically modify its hosts to create cancer-like symptoms. Bacteria need to find an open wound or weak point on plants in order to enter and begin their destructive paths.
Viral diseases are much more difficult to prevent than fungal or bacterial diseases. They may already be embedded in seeds, the soil, or carried by insects such as the Western flower thrip. It is often much more difficult to diagnose as well. Viral symptoms on plants often appear similar or the same as nutrient deficiencies or toxicities, chemical damage, or even insect feeding. Unlike bacteria, viruses are in the plant cells themselves, not the spaces in between them. There are no chemical controls available to treat plant viruses. Once a crop has contracted them, the only option is to destroy the crop. Prevention and insect control of virus-carrying pests are the only options available to combat viral disease in field-grown plants.
Control options:
Controlling disease in field-grown plants is not much different than doing so on plants grown indoors. Some of the methods are different, but largely it is the same diseases that can affect crops no matter where they are grown. As long as the conditions are proper for their onset, plant diseases can pop up anywhere.
Cultural Controls
How crops are grown and with what tools and media will directly impact the ability of diseases to set in. Clean tools, clean pots, and clean soil will go a long way towards keeping plant diseases at bay. Even requiring all employees to maintain sanitation during their interactions with plants will keep the incidence of disease lower.
Hand-washing stations with clean water and soap will help with the potential for transferring plant diseases between and among plants. This is especially true for any employees allowed to smoke. Tobacco mosaic virus is transferable from the hands of a smoker onto any number of crops that are in the solanaceous family. This includes tomatoes, peppers, eggplants, and hundreds of other vegetable and ornamental crops. Tools should also be disinfected thoroughly between uses and even during usage between crop types or large amounts of any one type of crop.
If containers are going to be reused, they must be cleaned well. Soap and water should be used to rinse out any solid matter clinging to the pots and then they should be exposed to a bleach or disinfection solution for up to 30 minutes prior to reusing.
Soil should not be reused between crops unless it is sterilized. Plant pathogens can be abundant in soils, even laying dormant until the conditions favor emerging. Many commercially available mixes are treated to kill pathogens. These products should be stored closed, away from other soils and only touched with clean tools and placed into clean pots.
Contaminated tools are not the only way diseases can spread between plants grown in the field. Infected soils can be unknowingly transplanted from one part of the field to another on the shoes of workers and the feet of animals. Hoses and irrigation equipment that gets moved around can also be the culprit of transferring plant diseases.
Fields areas should be cleaned between crops. As pathogens can lay dormant for lengthy periods of time in plant debris, it is important to make sure there is none that overwinters or has a chance to work itself into the soil for a future crop to contract. All crop debris from the field after harvests must be removed and ideally destroyed before that field should be used again for cultivation.
Mechanical Disease Controls
The options for mechanical disease controls for field-grown diseases are far fewer than for greenhouse or indoor grown crops. While it is possible to increase the factors needed to heighten the incidence of diseases through wind barriers and irrigation, the opposite is not true.
For areas where too much moisture is an issue, withholding further irrigation is possible, but that does not remove what is there. Where practical, tilling between rows can aid some in helping soil to dry out, but not where it is needed most.
Mechanical controls for field-grown crops are best done as a preventative strategy. Tilling amendments into the soil that aid in drainage can be an effective approach in areas known to be prone to slow drainage or standing water. The contours of the land can also be shaped before planting to aid in drainage.
If air flow is impeded by natural growth of surrounding windblocks, intentional or not, then trees can be thinned or cut down. The same is true for tall brush that may also be blocking some amount or air flow. If buildings or other structures block air flow, the remedy is indeed mechanical, but costly and mostly impractical. Cultural controls and application of products are generally far more effective measures against diseases in field-grown crops.
Organic Disease Controls
Many of the same biological controls and organic fungicides that are available for indoor grown plants are just as useful for outdoor plants. Quantities have to be adjusted for the additional field space, but the modes of action are the same. There are options for spraying just the plants themselves, or options for treating the soil.
Biofungicides that contain living organisms can suppress plant diseases. They work by outcompeting, poisoning, or directly predating on the disease-causing pathogen. These typically need to be used preventatively and need to be nurtured themselves to make sure they are in sufficient populations to suppress and eliminate diseases. The types available vary based on types of pathogens targeted, soil types and pH, level of disease pressure and the plant types as well.
Fungicides formulated for organic crop production will always be labeled as such. One standard to look for is the OMRI label. The Organic Materials Review Institute (OMRI) evaluates formulations to make sure that there are no artificial ingredients or ingredients that would be non-compliant with organic production. Though organic fungicides are not necessarily safer than their chemical counterparts, they are usually less phytotoxic to plants, and have lower re-entry intervals (REIs).
Steam treating or sterilizing soil is another non-chemical approach to mitigating diseases in field-grown crops. There are disinfectants available on the market for soil disinfecting, but steam is preferred for many reasons. It is faster, more effective, and safer than chemical fumigation. It also has the added benefit of killing weeds too. It cannot however be done when crops are in place as it will kill crops. It is a treatment to be done before planting or when a pathogen has affected a crop to the point where it is lost.
To effectively kill disease-causing pathogens with steam, the soil must be moist, but not over-saturated. It must reach a temperature of between 160-180 F (71-82C) for half an hour. It is important to not over-treat. Beneficial organisms can be killed as well in the process and toxic amounts of ammonium nitrogen and manganese can be created if this method is used in excess. It is best to leave the soil alone after steam treatment for several weeks to allow microorganism populations to rebuild. Keeping the steam temperature to closer to 160 F (71 C) will reduce the potential for negative effects.
Chemical Control of Plant Diseases
Just as with organic fungicides and other approaches to killing plant pathogens, many of the same options exist for field grown crops and their diseases. Foliar sprays, systemic granules, and soil drenches are all viable methods of chemical disease control. Broad-spectrum fungicides can be used preventatively to stave off common root diseases. When conditions are likely to favor disease formation, such as extended periods of rain, fungicides should be applied proactively as well. When symptoms of disease appear on leaves, then a foliar spray should be used.
When applying a chemical soil drench, it is advisable to supply additional water to force the fungicide deeper into the root zone. When applying foliar fungicides, it is important to make sure that total coverage is achieved, both on the top and bottom sides of the leaves, unless it is a translaminar fungicide.
