Report of Leaf Blight on Washington Lupine (Lupinus polyphyllus) Caused by Rhizoctonia solani AG 4 in Italy.

Abstract

Lupinus polyphyllus (Leguminosae), Washington lupine, is a perennial herbaceous plant. In March 2008, in a campus greenhouse at the University of Torino, Grugliasco (northern Italy), a leaf blight was observed on 20% of potted 30-day-old plants. Semicircular, water-soaked lesions developed on leaves just above the soil line at the leaf-petiole junction and later along the leaf margins. Lesions expanded for several days along the midvein until the entire leaf was destroyed. Blighted leaves turned brown, withered, clung to the shoots, and matted on the surrounding foliage. Severely infected plants died. Plants were grown in a sphagnum peat/perlite/clay (70:20:10) substrate at temperatures between 18 and 25°C and relative humidity of 60 to 80%. Diseased tissue was disinfested for 10 s in 1% NaOCl, rinsed with sterile water, and plated on potato dextrose agar (PDA) amended with 25 mg/liter of streptomycin sulfate. A fungus with the morphological characteristics of Rhizoctonia solani (4) was consistently and readily recovered, then transferred and maintained in pure culture. Ten-day-old mycelium grown on PDA at 20 ± 1°C appeared light brown, rather compact, and exhibited radial growth. The isolates of R. solani successfully anastomosed with tester isolate AG 4 (AG 4 RT 31, obtained from tobacco plants). The hyphal diameter at the point of anastomosis was reduced, the anastomosis point was obvious, and cell death of adjacent cells was observed. Results were consistent with other reports on anastomosis reactions (3). Pairings were also made with tester isolates AG 1, 2.1, 2.2, 3, 6, 7, 11, and BI with no anastomoses observed between the recovered and tester isolates. The internal transcribed spacer (ITS) region of rDNA was amplified using primers ITS4/ITS6 and sequenced. BLASTn analysis (1) of the 660-bp fragment showed 100% homology with the sequence of R. solani. The nucleotide sequence has been assigned GenBank Accession No. FJ486272. For pathogenicity tests, the inoculum of R. solani was prepared by growing the pathogen on PDA for 10 days. Plants of 30-day-old L. polyphyllus were grown in 10-liter containers (10 plants per container) on a steam disinfested sphagnum peat/perlite/clay (70:20:10) medium. Inoculum, consisting of an aqueous suspension of mycelium disks prepared from PDA cultures (5 g of mycelium per plant), was placed at the collar of plants. Plants inoculated with water and PDA fragments alone served as control treatments. Three replicates were used. Plants were maintained in a greenhouse at temperatures between 18 and 23°C. First symptoms, similar to those observed in the nursery, developed 10 days after the artificial inoculation. R. solani was consistently reisolated from infected leaves and stems. Control plants remained healthy. The pathogenicity test was repeated twice. The susceptibility of L. polyphyllus to R. solani was reported in Poland (2). This is, to our knowledge, the first report of leaf blight of L. polyphyllus caused by R. solani in Italy. The importance of the disease is at the moment limited.