Kalmia latifolia L., common name Mountain Laurel, is an evergreen shrub that is becoming increasingly popular in gardens. It is also grown as a potted plant for its round flowers that range from light pink to white and occur in clusters in late spring. During July 2011, 3-year-old plants of K. latifolia ‘Olympic Fire’ showed extensive chlorosis and root rot on several commercial nurseries close to Maggiore Lake (Verbano-Cusio-Ossola Province) in northwestern Italy. Twigs wilted and died and leaves dropped, although in some cases, wilted leaves persisted on stems. The whole root system was affected with vascular tissues on the lower stem exhibiting brown discoloration, followed by plant death. The disease was severe and widespread, affecting 5% of approximately 3,500 plants. Tissue fragments of 1 mm2 were excised from the margins of the lesions and plated. A Phytophthora-like organism was consistently isolated on an oomycete-selective medium (BNPRA + HMI at 25 μg/ml) (4) after disinfesting root pieces for 1 min in a 1% NaOCl solution. The pathogen was identified based on morphological and physiological features as Phytophthora cinnamomi (2). Oogonia didn't form in single culture. On V8 agar, the microorganism was characterized by coenocytic coralloid hyphae, with spherical lateral and terminal swellings 23 to 46 (average 34) μm in diameter (n = 50), single or in clusters, and produced spherical, terminal chlamydospores 35 to 47 (average 40) μm in diameter (n = 50). No sporangia were produced after growing pure cultures in sterilized soil extracts nor were they produced on V8 agar. The internal transcribed spacer (ITS) region of the rDNA of a single isolate was amplified using the primers ITS1/ITS4 and sequenced. BLAST analysis (1) of the 898-bp segment showed a 99% homology with the sequence of P. cinnamomi (GU799638). The nucleotide sequence has been assigned the GenBank Accession No. JQ951607. Pathogenicity of one isolate obtained from infected plants was confirmed by inoculating 18-month-old plants of K. latifolia ‘Olympic Fire’. The isolate was grown for 50 days in a mixture of 70:30 wheat/hemp kernels and then mixed into a substrate containing sphagnum peat moss/pumice/pine bark/clay (50:20:20:10 v/v) at a rate of 3 g/l. One plant per 2-l pot was transplanted into the substrate and constituted the experimental unit. Five plants were inoculated. Noninoculated plants represented the control treatment and the trial was repeated once. All plants were kept in a greenhouse at 24 to 27°C. Two of five plants inoculated developed symptoms of chlorosis, wilting, and root rot after 70 days and remaining plants after about 80 days. P. cinnamomi was reisolated consistently from inoculated plants but not from controls that remained symptomless. To our knowledge, this is the first report of P. cinnamomi on K. latifolia in Italy and in Europe. The disease has been reported in the United States (3). The economic importance of the disease is uncertain because of the limited number of nurseries that grow this crop in Italy, although its importance could increase as the popularity of K. latifolia increases.
References: (1) S. F. Altschul et al. Nucleic Acids Res. 25:3389, 1997 (2) D. C. Erwin and O. K. Ribeiro. Phytophthora Diseases Worldwide. American Phytopathological Society, St Paul, MN, 1996. (3) L. F. Grand. North Carolina Agric. Res. Serv. Techn. Bull. 240, 1985. (4) H. Masago et al. Phytopathology 67:425, 1977.