Root Rot of Lamb’s Lettuce (Valerianella olitoria) in Northern Italy Caused by Pythium irregulare

Abstract

HomePlant DiseaseVol. 99, No. 11Root Rot of Lamb’s Lettuce (Valerianella olitoria) in Northern Italy Caused by Pythium irregulare PreviousNext DISEASE NOTES OPENOpen Access licenseRoot Rot of Lamb’s Lettuce (Valerianella olitoria) in Northern Italy Caused by Pythium irregulareA. Garibaldi, G. Gilardi, S. F. Ortega, and M. L. GullinoA. GaribaldiSearch for more papers by this author, G. GilardiSearch for more papers by this author, S. F. OrtegaSearch for more papers by this author, and M. L. GullinoSearch for more papers by this authorAffiliationsAuthors and Affiliations A. Garibaldi G. Gilardi S. F. Ortega M. L. Gullino , Centre of Competence for the Agro-Environmental Sector (AGROINNOVA) and DISAFA, Largo Braccini 2, 10095 Grugliasco, Italy. Published Online:15 Sep 2015https://doi.org/10.1094/PDIS-03-15-0365-PDNAboutSections ToolsAdd to favoritesDownload CitationsTrack Citations ShareShare onFacebookTwitterLinked InRedditEmailWechat During the fall of 2013, lamb’s lettuce (Valerianella olitoria L.) plants of the cv. Cala Nasi, grown under a plastic tunnel near Asti (northern Italy) exhibited stunting, extensive chlorosis, and root rot. Plants were irrigated by overhead sprinklers using well water. Symptoms first developed 20 days after sowing at air temperatures ranging from 17 to 25°C, and 30% of the plants were affected. Roots appeared water-soaked and brown, and were characterized by a soft rot. Eventually, affected plants wilted and collapsed. Fifty 1-mm2 fragments were excised from the symptomatic roots of 10 plants, dipped in a solution containing 1% sodium hypochlorite, rinsed in sterilized water, dried on sterilized paper towels, and plated on both potato dextrose agar (PDA) and the medium BNPRA (benomyl 10 mg/liter, nystatin 25 mg/liter, pentachloronitrobenzene 25 mg, pimaricin 10 mg/liter, rifamycin 10 mg/liter, and ampicillin 500 mg/liter), which is semiselective for oomycetes (Masago et al. 1977). After 4 days of incubation under constant fluorescent light at 22 ± 1°C, 80% of the root sections developed oomycete colonies on both media. One representative isolate, grown for 10 days on V8 agar modified medium (200 ml V8 Campbell Soup; 15 g agar; 0.5 g CaCO3; 1 liter distilled water), and observed with a light microscope, showed aseptate hyphae that were 4.1 to 6.5 (mean 4.8) µm wide. Oogonia were globose, smooth, and from 9.6 to 15.7 (average 13.4) µm in diameter. Antheridia were barrel-shaped, while oospores were globose and 13.2 to 20.26 (mean 18.3) µm in diameter. These morphological characters identified the microorganism as a Pythium sp. (Watanabe 2002). The internal transcribed spacer (ITS) region of ribosomal DNA (rDNA) of this isolate was amplified using ITS1/ITS4 primers and sequenced. BLAST analysis (Altschul et al. 1997) of the 710-bp segment exhibited 99% homology with the ITS sequence of Pythium irregulare (GenBank Accession No. JN630483.1). The nucleotide sequence was assigned the Accession No. KR013747. Pathogenicity tests were performed twice by sowing seed of lamb’s lettuce of the cv. Cala Nasi in 2-liter pots in a steam-disinfested organic peat substrate (black peat, pH 6.5 to 6.8, N 110 to 190 mg/liter, P2O5 140 to 230 mg/liter, K2O 170 to 280 mg/liter) moistened to field capacity and infested with wheat and hemp kernels (200 g wheat kernels, 100 g hemp seeds and 320 ml deionized water into a 1000-ml-flask, and sterilized at 121°C for 30 min) colonized with the isolate of P. irregulare at 1 g/liter. Twenty-five plants/pot and four replicates were used (one pot per replicate). The same number of plants were grown in noninfested substrate as a control treatment. Plants were kept in a growth chamber with 12 h of light/day at 23°C and were irrigated daily to maintain the potting medium at field capacity. Symptoms first developed 15 days after sowing. After 30 days, 60% of plants grown in the infested soil were dead. Control plants remained asymptomatic. Pythium irregulare was reisolated consistently from the symptomatic roots of plants grown in infested medium. No P. irregulare was reisolated from the asymptomatic control plants grown in noninfested substrate. To our knowledge, this is the first report of P. irregulare causing root rot on V. olitoria in Italy. The same pathogen has been reported causing root rot of lamb’s lettuce in Germany (Koch and Riesterer 2001). The disease is, at present, limited to the area where it was first observed.