by Elizabeth A. Tomlin
Root rots are a worrisome concern for plant growers. These diseases plague greenhouses.
Dr. Janna L. Beckerman, Professor and Extension Plant Pathologist, Department of Botany and Plant Pathology, Purdue University addressed this issue in the webinar Root Rot Management for Annual and Perennial Crops, sponsored by BASF.
Dr. Beckerman said although soil-borne fungi and water molds cause some of the most widespread and serious diseases in the greenhouse, they tend to be the most under-diagnosed and misdiagnosed, and frequently growers don’t look for them until all other diseases have been ruled out.
Accurately diagnosing the type of root rot — or root rots, as they may occur simultaneously — is key to controlling the spread of the disease.
“Sustainable disease management has a foundation in proper diagnosis,” stressed Beckerman. “Diagnosis is essential for effective management of plant health problems.”
Beckerman noted plant pathogens are constantly changing and the local, regional and international movements of seed and plant material are constantly challenging researchers with new diseases and disease mutations.
Although seedlings and cuttings are commonly affected, symptoms of root rot may also be observed in most annual, biennial and herbaceous perennial plants.
“Root rot symptoms, regardless of the pathogen, are surprisingly similar,” acknowledged Beckerman.
Pythium, a fungus-like organism, which displays symptoms similar to fungal diseases, is distinctly different from fungi.
“The first symptoms of Pythium infections include stunting; however, careful examination of root tips early in the infection will show only dead tips. With Pythium root rots, roots appear water-soaked, and the root cortex easily sloughs off, leaving a strand of vascular tissue. This is not a conclusive symptom, but one to note. On the stems of cuttings, a soft, watery rot may develop. Key signs include the cells of the plant root containing round, thick-walled oospores and/or round zoosporangium. Accurately diagnosing this disease is essential because fungicides labeled to control other root rot pathogens, such as Thielaviopsis, Fusarium, or Rhizoctonia, will not be effective against Pythium root rot.”
Also discussed was Black Root Rot (Thielaviopsis), a soil inhabiting fungus affecting woody and herbaceous plants including begonia, cyclamen, geranium, gerbera, holly, pansy and poinsettia. Branch dieback is one symptom.
This fungus lives in field soil, but has also been found in commercial peat moss.
Symptoms may include softening, darkening and discoloration of tissues around the stem base. Beckerman said the disease can be challenging and stressed the need to have cultural practices in order.
Rhizoctonia is another pathogenic fungus soil-borne fungus known to cause root rots. Once the fungus begins growing in plants, infected areas rapidly decay. Brown lesions form near and just below the soil line. Lesions enlarge, forming cankers. Leaves touching the soil become infected. This fungus will web to neighboring leaves and will rot leaves quickly. Symptoms also include wilting of the plants in midday. Older plants are more resistant to this disease.
“Prevention is key.” Beckerman stressed.
Soil pasteurization of soil and peat moss will control these diseases. Soils should be steamed to 180 degrees for 30 minutes.
“Pythium species are often found contaminating commercially available soilless potting mixes.”
What water you are using also needs to be considered as it, too, will spread root rot disease.
“When using pond or stream water for irrigation, place the intake pipe well above the bottom of the pond so that it does not draw in sediment,” advises Beckerman. “Also, make sure the intake pipe isn’t near the surface, either. If Pythium contamination is a problem, slow sand filtration is an effective method for removing Pythium — and other plant pathogens — from recycled water.”
Other water treatment options include ultraviolet radiation, ozonation and chlorination.
Overwatering sets your plants up for disease by providing the conditions pathogens will thrive in.
“In addition to overwatering, using poorly draining media, or placing pots or flats in standing water, will also affect drainage and predispose plants to Pythium infection.”
Beckerman points out that excess watering creates conditions conducive for shore flies and fungus gnats that will feed on roots, damaging them and providing a site of entry for Pythium. “These insects are also effective vectors of the pathogen, spreading the disease throughout the greenhouse or growing area.”
Over fertilizing will also increase Pythium infection rates. “The cause of this damage is two-fold. First, excess nitrogen suppresses the plant’s natural defense response. Second, the accumulation of salts in the growing medium damages root tips, providing an easy means for Pythium to infect.”
Once you have had problems with Pythium it is essential to be proactive to prevent outbreaks.
“Biological control agents such as Trichoderma harzianum or Gliocladium virens do provide some protection when disease pressures are low,” said Beckerman. “However, overwatering or excessive fertilization will reduce their efficacy to the point that severe outbreaks of Pythium can occur despite the use of biological controls. If severe outbreaks have occurred in the past, consider incorporating a granular fungicide in your potting mix in lieu of a biological control agent, and reevaluate your cultural practices that may result in excess water or fertilizer.”
There is no cure once plants are infected. However, you may prevent disease spread by promptly removing infected plants and all soil mix surrounding their roots. And, although sanitation is essential for all greenhouses, once disease sets in sanitation is a must to protect other plants from contamination.
“Poor sanitation — including the careless use of dirty tools or containers, and proximity to previously infected plants or media — can readily contaminate sterilized soil or soilless mixes. Surface clean and disinfect all bench surfaces, tools, trays, containers, and equipment that will contact the potting mix.”
Beckerman said cleaning with bleach alone will not reach all bacteria. “You must use a detergent.” Scrubbing is advised.
There are very effective fungicides available for use in managing root rots. However, Beckerman said, “Using the correct fungicide is essential!” Therefore she reiterates, “accurately diagnosis the type of root rot.”
Rotation of fungicides is also recommended to avoid developing resistance.
Contact your local Cooperative Extension for help with diagnosing root rot and for information on what fungicides are currently recommended.
“Use high-quality cuttings, and immediately remove any cuttings or plants that show symptoms of disease,” said Beckerman.
Prevention is key in root rot management
by Elizabeth A. Tomlin