US: Rhizoctonia infection in high density fern production
by Paul Thomas and Jean Williams-Woodward
The fungal pathogen, Rhizoctonia solani, causes damping-off, root rot, crown rot, and web blight on numerous herbaceous and woody ornamental plants. The fungus has an extremely wide host range, and can affect plants from plug production to landscape use. The disease appears on fern foliage as a dark grey webbing that spreads rapidly between fern pinnae. Most often it starts in the dense crown and radiates along middle aged or older fronds. Rhizoctonia infection is favored by moist, humid, and warmer conditions. It is present year-round inside greenhouses; however, it is most common in outdoor plantings or production during the summer months. Dense canopy coverage increases disease incidence because the dense canopy creates a good environment for the hyphae to grow and spread along the soil surface beneath the leaves. Always check the interior of and under the canopy for signs of Rhizoctonia hyphae. Hanging baskets in particular need to be checked routinely as the disease can spread quickly.
A Boston fern with a well-established infestation of Rhizoctonia. Notice how the disease spread along the frond outward towards the tip.
Critical to the Fern hanging basket issue is that Rhizoctonia survives in fallen leaf debris; within infected roots and rooting medium debris on benches, floors, tools, and used containers; and as hardened survival structures called sclerotia. Sclerotia are hyphae that is tightly wrapped around itself to form a small, hardened mass. Sclerotia allow Rhizoctonia to survive for years within an area. Rhizoctonia does not produce any spores. Hyphae or sclerotia can be splashed into the plant canopy or can be introduced on any soil-contaminated tools, containers, stakes, pot labels, irrigation emitters, or worker’s hands.
Pronounced webbing can be seen in this image. This is the hyphae of Rhizoctonia that easily traverses small spaces between pinnae.
Once fern leaves start falling onto other crops, such as newly transplanted bedding plants, or vegetative cuttings, the disease spreads rapidly. Staring with seed trays, damping-off can occur pre- and post-emergence. Pre-emergent damping off results when Rhizoctonia infects the germinating radical preventing the plant from ever growing. Post-emergent damping off occurs when the lower stems of the seedling is infected causing death and collapse of the plant. This can often occur in plug trays, especially when the seedling foliage canopy closes creating a warm, moist pocket beneath the leaves. Plant death due to damping-off spreads in a radial pattern as Rhizoctonia grows outward from the initial infection point.
Note the necrotic zones on infected pinnae near the main vein as the infection takes over the entire frond.
Across most greenhouse crops, roots infected with Rhizoctonia are darkly discolored and shriveled. Infected plants wilt, yellow and die. As mentioned previously, Rhizoctonia does not produce any spores. Growth and spread is by hyphae (thread-like material) that can look like brown, spiderweb-like webbing along the soil surface and grow upward into the foliage causing leaf blighting and an aerial web blight. Infected leaves become spotted and necrotic. Infected, dying leaves often drop from the plant, but are matted together and held to stems by fungal hyphae. If infected plants are jiggled slightly, the infected leaves may “dangle” from the stems because the hyphae is holding it in place. The thread - like hyphae is often seen between infected tissues. Rhizoctonia hyphae grows quickly and may spread from between closely spaced plants when the canopies touch.
Management
When greenhouse Extension folks come across these situation, we immediately talk about the disease triangle. In order for a pathogen to take hold on your crop, your plants have to be susceptible as a host, the pathogen need to be present, and the ENVIRONMENT needs to be conducive to the growth of the pathogen! Financial reality may demand more intense cropping of greenhouse crops such as ferns. In order to reduce the likelihood of the disease, growers need to pay attention to 1). Increase air flow – as the ferns grow, and fronds start meshing and intertwining, air flow can be restricted.
Running HAF fans 24/7 is often recommended. Fern growers will wince as fans blowing on nearby ferns can damage fronds. The way to deal with this is to water early in the morning, and while the ferns are wet, run the HAF fans and vent the greenhouse thoroughly until leaves have dried off. 2). Pre-plan a preventative fungicide spray program for the ferns. If your economics dictate intense populations, a regular spray program can significantly slow disease spread and should be considered necessary. 3). Scout intensely. Ferns happily sitting above a grower’s head tend to be ignored until a fern frond dangling down turns brown. There’s no way around it, you must take a few baskets down each time you scout to look at the entire plant, especially in the crowded center of the basket. You can be smart about this and take baskets from areas that are particularly dense, or areas where there seems to be larger numbers of fern pinnae on the ground.
Management of Rhizoctonia also begins by following (maintaining) good sanitation practices. Rhizoctonia can survive and spread in and on anything containing contaminated soil. Rhizoctonia is known to survive and spread in dust and soil debris swept from floors. Never add the sweepings from floors or benches back into fresh or steam-sterilized rooting medium. This will only contaminate the medium. Benches and floors should be cleaned of debris and disinfested between crops. Plug trays and containers need to be cleaned of soil debris and disinfested before considering re-use.
Intense fern production must have lanes that allow air flow and sunlight to reach each basket. Adding more would be inviting infection.
Evaluating costs
When not controlled early, economic losses to Rhizoctonia can be large. If you evaluate the costs involved with losses to Rhizoctonia , you may want to simultaneously consider cost-benefits to increasing plant spacing to reduce plant-to-plant contact and adding a thought-out fungicide program to reduce disease spread.
Inoculum dropping from infected baskets once in contact with seedlings will spread in the traditional round, expanding circle. Sanitation prior to this level of infection is crucial.
Carefully timed applications are needed to reduce Rhizoctonia growth and disease development. Fungicides containing fludioxonil, flutolanil, azoxystrobin, pyraclostrobin, fluoxastrobin, metaconzole, myclobutanil, triflumizole, trifloxystrobin, polyoxin D zinc salt, iprodione, and thiophanate methyl can reduce Rhizoctonia infection and spread.
Reducing fertility can improve growth and reduce salt stressed foliage being infected by the pathogen. Note the new growth after cutting back on fertility.
Concurrent with poor environment can be high fertility levels that stress fern foliage. Do not “push” fern production with high ammonium-based fertilizers. Ferns are more responsive to temperature and moderate light levels. Salt stress can be an open door to Rhizoctonia establishment.
Preventative fungicide applications are more effective than curative applications. Read all product labels carefully as some products cannot be used on certain plant species. Follow all label direction for rate and frequency of application.
Source: e-GRO