Common Root Diseases in the Greenhouse

Plant root diseases present constant challenges for greenhouse growers, as they require a combination of a susceptible host, a conducive environment, and an appropriate pathogen. The two most common causes of root diseases are over-fertilization and pathogenic fungi. Understanding these potential threats and the environmental conditions that can affect plant health is crucial for preventing the development of diseases. Recognizing the symptoms of root disease is essential in order to take prompt action and prevent crop loss.

Plants affected by root diseases often exhibit various characteristics, including stunted growth, discolored or wilted leaves, leaf loss, soft and “mushy” stems, brown or black roots, or even death. Regular monitoring of plants for any noticeable symptoms is critical for identifying potential issues early on and implementing appropriate measures to mitigate the damage.

The progression of root diseases generally occurs in stages. Initially, a healthy plant must be made susceptible to root disease, which can be caused by factors such as overfertilization, overwatering, insect damage, inadequate drainage, or prolonged stress. If the source of stress is not identified or corrected, the plant weakens and becomes vulnerable to infection by a plant pathogen or even death.

Once the plant becomes infected, its roots start to deteriorate, changing from white and firm to brown and mushy. The plant begins to wilt, lose leaves, and the roots may appear waterlogged as they can no longer take up water. In some cases, root-colonizing pathogenic fungi can move into the stem, causing canker or “black leg.” These factors ultimately lead to the eventual death of the plant. It’s important to note that the visual symptoms exhibited by a plant can vary depending on the specific plant pathogen involved.

Common Root Disease Organisms Known to Affect Greenhouse Plants

1. Pythium spp.: Pythium is an oomycete, commonly known as a “water mold,” that thrives in wet conditions. It is one of the most common root disease pathogens in greenhouses. There are over 200 different species of Pythium, although not all are pathogenic to plants. Many of them are natural inhabitants of the soil that have adapted to survive in harsh conditions. Once introduced into a greenhouse, Pythium can be challenging to eliminate and may pose a long-term threat. It is often introduced through infected plugs, plant material, or irrigation water. Excessive nutrient levels, high electrical conductivity (EC), ammonium toxicity, and wet growing media favor Pythium attacks. Symptoms include stunted growth, brown roots, wilted or discolored leaves, and black leg.
   
2. Phytophthora spp.: Phytophthora is another oomycete that, like Pythium, thrives in excess moisture and nitrogen fertility. However, Phytophthora species are generally more aggressive and more likely to be host-specific. They are less commonly found in greenhouses but can cause significant damage when introduced. Prevention is crucial as general-purpose fungicides may not effectively suppress Phytophthora, and its spores can survive for extended periods. Greenhouses can be contaminated with Phytophthora through infected plant material or irrigation water from nearby sources such as lakes or retention ponds. This pathogen is responsible for root/crown rot, blackened cankers, and foliar blights.
   
3. Rhizoctonia spp.: Rhizoctonia is a soilborne fungus that primarily affects cuttings and young greenhouse transplants. It prefers warm and moist media but can tolerate drier conditions. It typically resides in the top layer of soil where there is a high percentage of organic matter. Brown or reddish lesions develop on plant structures at or just below the surface of the growing medium. If favorable conditions persist, the fungus progresses to form black cankers. Rhizoctonia is often introduced into greenhouses through soil contamination, such as soil residue brought in from outside on unclean hands. Insects such as fungus gnats and shore flies can also introduce and spread the fungus within a crop under suitable conditions. Symptoms of Rhizoctonia infection include damping-off, root/crown rot, web blight, stem canker, and a dry or shriveled appearance.
   
4. Thielaviopsis spp. (black root rot): Thielaviopsis is a soilborne fungus that attacks the roots of weakened plants. When infected, the roots of affected plants turn dark black due to the presence of the pathogen’s reproductive structures. Unlike Rhizoctonia, which prefers warm and dry soils, Thielaviopsis thrives in cool and wet soils with a pH above 6. This fungus can sometimes be mistakenly identified as an iron nutrient deficiency due to the chlorosis it causes while attacking plants. However, increasing fertilization will only result in further harm to the plant. Thielaviopsis is transmitted to greenhouses through direct contact with spores from soil-contaminated objects or through dust, insects, water, or infected plant material. Plants grown in growing media that do not dry out quickly enough in low-lighting conditions are more susceptible to Thielaviopsis attacks. Symptoms include stunted growth, wilting leaves, chlorosis, black roots, and plant death.
   
5. Fusarium spp.: Fusarium is a widespread, soilborne fungus that greenhouse growers may encounter. It thrives in moderate to high temperatures with high moisture or humidity and poor drainage. Fusarium primarily targets plant foliage and roots. The initial symptoms appear as red lesions on the stems of plants above the growing medium, followed by root rot. As the disease progresses, plant growth becomes stunted, and plants may experience stem discoloration, wilting, and ultimately death. Fusarium can be introduced into greenhouses through infected seed, infected transplants, contaminated equipment, or soil residue from outside. Once inside, Fusarium can easily spread from plant to plant through root-to-root contact or via spores splashed around during watering. Signs of Fusarium infection include crown/root rot, yellowing of plants, wilting, bulb rot, and root rot.

By understanding the characteristics, transmission methods, and symptoms of these common plant root disease organisms, greenhouse growers can be better prepared to identify and manage potential issues. Implementing preventive measures, such as maintaining proper drainage, avoiding over-fertilization, and ensuring clean practices in handling plant material and soil, can significantly reduce the risk of root diseases and promote healthier plant growth in the greenhouse environment. For more detailed information, you can refer to the research conducted by the US Department of Agriculture.