First Report of Fusarium semitectum as the Agent of Twig Cankers on Persian (English) Walnut in Italy
Authors: Belisario A. , Luongo L. , Vitale S. , Santori A. 
Published: May 2010

English (Persian) walnut (Juglans regia) is the most widely cultivated walnut species. During the last 10 years, the increment of walnut cultivation in Italy has been accompanied by changes in cultural management. Changes were addressed to develop highly specialized cultivations with intensive pruning, fertilization, irrigation, and chemical treatments. For these reasons, more consideration has been given to the sanitary situation, in particular since 1998 when brown apical necrosis (BAN) was first observed (1). BAN is a fungal complex disease causing fruit drop, in which several Fusarium spp. are involved, among which Fusarium semitectum represents one of the major causal agents (2). From 2005 onward, investigations on sources of inoculum for BAN led to observing the presence of twig cankers on walnut trees cv. Lara located in northern Italy (Po valley). Cankers observed in late spring to summer were usually small (1 to 3 cm long) and mainly occurred on the new growth strongly incited by intensive pruning. Pale orange sporodochia were evident on lesions. Isolations were made from the margins of lesions, and small fragments of tissues (approximately 3 mm) were plated onto potato dextrose agar (PDA) after surface disinfection with 1% NaOCl. Whitish, light brown colonies were consistently obtained. On PDA, the production of fusoid, 1- to 3-celled mesoconidia was abundant. This characteristic was combined with the presence of two-spored polyphialides with a “rabbit-ear” appearance. Three to five septate macroconidia (38 × 4 μm) were produced in sporodochia on carnation leaf agar (CLA). On the basis of morphological characteristics (3), the fungus was identified as F. semitectum Berk. & Ravenel (synonyms F. incarnatum and F. pallidoroseum). Sequence comparison of internal transcribed spacer (ITS) and translation elongation factor 1-alpha (TEF1-α) was used to support the identification. A 99% identity for ITS was obtained with Accession No. AY633745 from Vietnam, while for TEF 1-α, a comparison was not available in GenBank. The sequences of one isolate (ISPaVe1946) were deposited in GenBank (Accession No. FN430680 for ITS and No. FN430737 for TEF 1-α). Pathogenicity tests were conducted outdoors on 1-year-old shoots of J. regia potted plants using ISPaVe1946 single-spored isolate. Mycelial plugs of 5-mm diameter, cut from the margin of PDA actively growing cultures, were placed under the bark and protected with Parafilm to prevent desiccation. Six inoculation points were performed. Controls were inoculated with plain PDA plugs. Within 2 months after inoculation, cankers developed in all inoculated points and were similar to those observed in nature. Controls showed no symptoms. Koch's postulates were fulfilled and the pathogen was constantly reisolated from lesions. To our knowledge, this is the first report of F. semitectum as the causal agent of twig cankers on walnut in Italy. This pathogen was already reported as an agent of canker on walnut in Argentina (4).

References: (1) A. Belisario et al. Inf. Agrario 21:51, 1999. (2) A. Belisario et al. Plant Dis. 86:599, 2002. (3) J. F. Leslie et al. The Fusarium Laboratory Manual. Blackwell Publishing Press Ltd, Oxford, UK, 2006. (4) S. Seta et al. Plant Pathol. 53:248, 2004.