First Report of Chalara elegans on Roots of Black Elderberry.

Elderberry (Sambucus nigra L.) is organically grown in Switzerland primarily for the flowers, which are used for the production of candies. In June of 2006 in a commercial orchard in Knonau, Switzerland, leaves prematurely turned yellow and dropped off. Other symptoms included weak growth of new twigs in the spring and wilting of leaves during the hotter temperatures of summer. Chlamydospores of Chalara elegans Nag Raj & Kendrick (synonym Thielaviopsis basicola (Berk. & Broome) Ferr.) (1) were found on fine roots of 4-year-old trees. In 2004, similar aboveground symptoms had been observed in two elderberry orchards in Emmental, Switzerland, but no causal organism was detected at that time. All three orchards were planted during 2002 with plants obtained from the same nursery. The pathogenicity of C. elegans on black elderberry was confirmed at the Agroscope Changins Wädenswil ACW Research Center in Conthey. Black elderberry seedlings were produced from seeds collected from a healthy plant in the fall of 2007. Seeds were placed on wet filter paper in petri dishes and incubated at 1°C for 6 months. Germinated seeds were transplanted in seed trays filled with steam-sterilized peat substrate and maintained in a greenhouse at 16 to 22°C. In the spring of 2008, a strain of C. elegans was recovered from carrot on potato dextrose agar (PDA) for artificial inoculation of elderberry. Mycelium edges of the isolate were transferred on two pieces of PDA to a clay/V8 medium. The medium consisted of 50 ml of expanded clay granules and 20 ml of clarified V8 juice in 250-ml Erlenmeyer flasks that had been autoclaved for 30 min. C. elegans cultures were incubated in darkness at 24°C for 1 month. Sandy loam soil with a pH of 7.4 (soil/water; 1:1 (wt/vol)) was autoclaved twice for 45 min with a 1-day interval. Plastic pots (0.5 liter) were first filled with 300 ml of sterile soil. Twenty milliliters of C. elegans inoculum were then added and covered with 100 ml of sterile soil. One black elderberry seedling with two true leaves fully expanded (3 to 5 cm high) was transplanted into each of five pots with inoculated clay granules and noninoculated clay granules. Plants were maintained in a greenhouse at 18 to 25°C and watered with tap water. Inoculated and control pots were kept in two separate trays to avoid cross contamination by drainage water. After 2 months, inoculated plants were approximately half the size of control plants. Approximately 75% of the inoculated roots were brown because of the occurrence of chlamydospores of C. elegans, which was reisolated on PDA. To my knowledge, this is the first report of C. elegans on roots of black elderberry. References: (1) K. H. Domsch et al. Compendium of Soil Fungi. Vol. 1. IHW-Verlag, Eching, Germany, 1993.