First report of white thread blight caused by Ceratobasidium niltonsouzanum on yerba mate in Argentina

VARGAS-GIL, SILVINA; BERNARDI-LIMA NELSON; KRYVENKI MARIO; CONFORTO, C; DANNAE SERRI; KRAMER, R.; ROCA, MONICA; SILVINA VARGAS GIL

PLANT DISEASE

AMER PHYTOPATHOLOGICAL SOC

Año: 2020

0191-2917

Yerba mate (Ilex paraguariensis A.St.-Hil.) is a native species of thesubtropical and temperate regions of South America. In the Misionesprovince, in northeastern Argentina, yerba mate is an important agriculturalcrop grown on approximately 150,000 ha. In March 2019, diseased leavesand stems from four fields near Comandante Andresito city were receivedfor diagnosis at the Instituto de Patolog´ıa Vegetal (IPAVE) and InstitutoNacional de Tecnolog´ıa Agropecuaria, Argentina. Disease symptoms wereaccompanied by white mycelium threads in the young branches. Thethreads extended longitudinally, along the surface of the stem, which laterdeveloped into leaf blight, with leaves hanging on white threads of fungalhyphae. Symptoms were the same in samples from all four fields (100%prevalence), and disease incidence varied between 50 and 80%. Isolationswere performed by transferring small sections of diseased plant tissue,disinfested in 1.5% sodium hypochlorite for 1 min, washed with steriledistilled water, transferred to Petri dishes containing potato dextrose agar(PDA), and incubated at 25°C in the dark. Over 10 isolations from fiveplants, eight Rhizoctonia-like colonies were isolated. Pure cultures were obtained by transferring hyphal tips to plates containing PDA. Four isolates(IPAVE 0075, 0076, 0077, and 0078) were selected as representativesfor characterization and deposited at the Culture Collection of IPAVE(C´ordoba, Argentina). On PDA, the colonies initially had white aerialmycelia, and concentric rings were visible on the reverse side. The coloniesturned brown after approximately 30 days. Hyphae were branched with 90°angulation, a typical character of members of the form genus Rhizoctonia.For molecular identification, the internal transcribed spacer region wasamplified by polymerase chain reaction (White et al. 1990) and sequenced.Phylogenetic analyses using Bayesian inference grouped the isolates fromyerba mate with the type species of Ceratobasidium niltonsouzanum (Meloet al. 2018) with a high posterior probability (0.98). Pathogenicity testingwas conducted using toothpicks colonized with the pathogen (Souza et al.2009). Inoculum was produced on PDA medium by allowing the isolates tocolonize sterilized toothpicks placed on PDA and incubating for 7 days at25°C under a 12-h photoperiod. Yerba mate plants (24 months old) wereinoculated by inserting the colonized toothpicks into the base of the leafpetiole. Plants were kept in the greenhouse at 25°C under high humidity for30 days and observed daily (Souza et al. 2009). Four control plants weremock inoculated using only sterilized toothpicks. White thread blightsymptoms were observed on all inoculated plants after 20 days, whereascontrol plants remained symptomless. C. niltonsouzanum was reisolatedfrom symptomatic tissue (leaves and petiole) of the inoculated plants, witha frequency of 100%, thereby validating Koch?s postulates. The experimentwas performed twice. C. niltonsouzanum has been reported as a pathogenon numerous crops including Coffea arabica, Camellia sinensis, Azadirachtaindica, and Eugenia uniflora in Brazil (de Melo et al. 2018). Thiswork improves knowledge surrounding the etiology of the diseaseassociated with yerba mate in Argentina, which is essential informationfor the identification of appropriate management practices. To ourknowledge, this is the first report of C. niltonsouzanum causing whitethread blight of yerba mate in Argentina and worldwide.