Tuesday, February 21, 2012

Rhizoctonia Web Blight on Boston Fern

The Boston ferns that will adorn our porches and decks in a couple of months are being grown in greenhouses now. Ferns are easy to grow, but they do have a few disease problems. This week, we received a Boston fern from a greenhouse producer who was concerned about leaflets that were blackened and blighted, especially in the interior of the plant. These symptoms are typical of web blight (aerial blight) caused by the fungus Rhizoctonia. We typically think of Rhizoctonia as being a soilborne pathogen of roots, stems, and seedlings, but it causes foliar blights on several hosts, including Boston fern. The presence of Rhizoctonia on the leaves was confirmed microscopically. 
Hyphae of Rhizoctonia (Photo: M.J. Munster)
Web blight begins as irregular brown lesions on the fronds growing in the interior of the plant. 
Web blight symptoms (Photo: M.J. Munster)

Web blight symptoms (Photo: M.J. Munster)
Eventually, these lesions expand, causing the interior and lower fronds to turn black and die. The mycelium of the fungus can be seen as a web-like coating on the foliage.

Web blight is favored by high relative humidity. To control this disease, improve ventilation and make sure irrigation is timed to avoid prolonged leaf wetness. Wider plant spacing and hanging the plants will also help to improve air circulation and promote drying. Make sure to bag and discard diseased plants, and sweep up fallen leaflets to prevent inoculum from spreading to healthy plants. Greenhouse growers should always sanitize surfaces between crops, and be sure to use sterile potting media and sterile pots when planting.

Web blight is primarily a problem in a greenhouse environment. This disease is not a problem when ferns are grown indoors, as U.S. homes have notoriously low relative humidity levels. It can be a problem on outdoor ferns, especially with North Carolina's humid summers.