First Report of Pythium myriotylum as a Causal Agent of Crown and Root Rot in Soilless Green Bean Cultivation in Italy

Abstract

HomePlant DiseaseVol. 102, No. 3First Report of Pythium myriotylum as a Causal Agent of Crown and Root Rot in Soilless Green Bean Cultivation in Italy PreviousNext DISEASE NOTES OPENOpen Access licenseFirst Report of Pythium myriotylum as a Causal Agent of Crown and Root Rot in Soilless Green Bean Cultivation in ItalyS. Vitale, L. Luongo, E. Marinelli, and A. BelisarioS. VitaleSearch for more papers by this author, L. LuongoSearch for more papers by this author, E. MarinelliSearch for more papers by this author, and A. Belisario†Corresponding author: A. Belisario; E-mail: E-mail Address: [email protected]http://orcid.org/0000-0001-8912-7638Search for more papers by this authorAffiliationsAuthors and Affiliations S. Vitale L. Luongo E. Marinelli A. Belisario † , Consiglio per la Ricerca in Agricoltura e l’analisi dell’economia agraria, Centro di ricerca per la Difesa e la Certificazione (CREA-DC), 00156, Roma (Italy). Published Online:23 Jan 2018https://doi.org/10.1094/PDIS-07-17-0942-PDNAboutSectionsSupplemental ToolsAdd to favoritesDownload CitationsTrack Citations ShareShare onFacebookTwitterLinked InRedditEmailWechat Green beans are the unripe young fruit of common bean (Phaseolus vulgaris). Italy is one of the larger European producers, together with Spain, of green beans with 140,000 t of fresh vegetable (FAOSTAT 2014). To increase green bean production and quality, its cultivation includes soilless growing. In the summer 2016, during a survey in a commercial greenhouse farm in central Italy (Latium region), over 200 out of 1,000 green bean plants (P. vulgaris cv. Valdarno) showed symptoms of crown and root rot followed by wilting. Plants were cultivated in a pot with sterile perlite and coconut fiber with no recycled-water irrigation system. The infected crowns and roots appeared water soaked with a cinnamon brown color, and dead plants occurred in distinct and well-delimited areas. Microbial isolations were performed from 10 diseased plants previously abundantly washed under running tap water. Fragments cut from the margin of subepidermal stem lesions were plated on P5ARP (Jeffers and Martin 1986). A Pythium species was consistently isolated, and three single-hyphal cultures out of 25 colonies were transferred onto potato dextrose agar (PDA). Sporangia induced by soil filtrate were terminal or intercalary, filamentous, consisting of undifferentiated and inflated lobulate or digitate elements of variable length, mostly 7 to 17 µm in diameter. Oogonia mostly globose were terminal or intercalary from 20 to 35 µm in diameter (average 29 µm). Oospores were mostly aplerotic. On the basis of morphological features, the isolates were identified as Pythium myriotylum Drechsler (van der Plaats-Niterink 1981). The identity was confirmed by cytochrome oxidase subunit 1 (COI) and by internal transcribed spacer (ITS) sequence comparison with 100 and 99% homology with HQ708745 and HQ643701, respectively. Both sequences were from a P. myriotylum Israeli (CBS 25470) isolate obtained from peanuts. The sequences of the three isolates AB290, AB291, and AB292 were deposited in GenBank with numbers LT882483, LT882484, and LT882485 and numbers MF374745, MF374746, and MF374747 for ITS and COI, respectively. Pathogenicity tests were conducted in the greenhouse on P. vulgaris cv. Valdarno plants at the 4 to 5 leaf stage. Five plants for each isolate were inoculated with a 3-mm mycelial plug cut from the margins of actively growing 6-day-old PDA cultures. Plugs were inserted at the collar level and wrapped with Parafilm. Two plants for each isolate were used as controls and treated as described above except that sterile PDA plugs replaced the inoculum. Inoculated plants were kept for 2 weeks under controlled conditions in a greenhouse at 25 ± 2°C. After that period all inoculated plants showed stem necrotic streaks extended downward to the crown and roots. Most of the plants eventually died, developing symptoms similar to those observed on plants naturally infected, thus fulfilling Koch’s postulates. Control plants remained symptomless. Colonies isolated from the lesions morphologically resembled the inoculated isolates. Pythium myriotylum is the causal agent of root rot in numerous economically important crops including peanuts, tomato, wheat, oats, cucumber, soya bean, tobacco, and maize (Wang et al. 2003). However, only one report described this pathogen on green bean in Europe, namely, in Spain (Serrano et al. 2007). To our knowledge, this is the first report of P. myriotylum on green bean plants in Italy. A more careful water management in soilless potting mix would avoid the emergence of Pythium root rot in this crop’s production system.