First report of root rot caused by (Pythium dissotocum) on hydroponically grown collard greens (Brassica oleracea var. acephala)

Collards (Brassica oleracea var. acephala) are grown throughout the United States. Hydroponic greens are more common now due to technological advances lowering the cost and increasing the efficacy of production. In January 2021, a 325 m2 indoor hydroponic farm opened to provide fresh produce for a school in Los Angeles County, CA. Three week old collard seedlings were purchased from a local nursery, rinsed of their rooting media, and transplanted into deep water culture beds (1.2 m x 2.5 m x 0.3 m). Two weeks later, symptoms including plant stunting, chlorosis, leaf curling and wilting, and brown necrotic roots appeared. By and by 80-100% of usable plants were lost to disease. Symptomatic roots were plated on corn meal agar (CMA) amended with 2 ml of 25% lactic acid and CMA amended with pimaricin, ampicillin, rifampicin, and pentachloronitrobenzene (PARP) (Kannwischer et al. 1978). After 2 days a single colony type emerged on PARP but no growth occurred on acidified CMA. Representative isolates were transferred to CMA and to filtered (0.02 µm) soil extract solution with boiled grass blades (Martin 1992), both of which were incubated at 22 C and ambient light conditions. On CMA, isolates produced coenocytic mycelium with minimal aerial hyphae. After 24 h in soil extract, isolates developed filamentous sporangia, elongated discharge tubes with slightly inflated tips, and zoospores. Oospores were not observed. Pathogenicity was confirmed by soaking the roots of five day old collard seedlings in beakers containing zoospores (1 x 102 zoospores/ml) in filtered soil extract. Four isolates were tested on 15 seedlings each. After 24 h at 22 C in ambient light conditions, plants were transferred to new beakers with roots placed on filter paper at the bottom and saturated with sterile distilled water. Three days after this transfer, leaves on all plants turned chlorotic and roots developed brown lesions from which morphologically identical colonies were isolated. Control plants, soaked in filtered soil extract, developed no root or foliar symptoms. To molecularly identify the collard isolates, DNA was extracted from mycelial original and re-isolated isolates and was amplified by PCR using mitochondrial primers for the cytochrome oxidase I (COI) gene (Robideau et al. 2011) and the cytochrome oxidase II (COX2) gene (Martin 2000). The only species that matched both loci from the original and re-isolated isolates with a high percent identity was Pythium dissotocum. The COI locus from the original isolate (MZ027311) matched P. dissotocum with 99% identity and with 332/334 base pairs matching the isolate with Sequence ID MT981134.1. From the re-isolated isolate (MZ027313), the COIequence perfectly matched 657/657 base pairs of P. dissotocum (Sequence ID MT981147.1). The COX2 locus from the original isolate (MZ027312) matched P. dissotocum (Sequence ID MG719859.1) with a 99% identity and 517/518 matching base pairs and the re-isolated isolate (MZ027314) perfectly matched P. dissotocum (Sequence ID MG719859.1) with 515/515 matching base pairs. Based on these molecular and morphological data, the isolates were identified as Pythium dissotocum. To our knowledge, this is the first report of P. dissotocum causing root rot on collards. At this same facility, P. dissotocum was also confirmed as the cause of declining bean (Phaseolus vulgaris) plants. As hydroponics will be necessary to feed a growing population – especially in urban areas -- and because leafy greens are a main crop of the hydroponics industry, we anticipate this issue may become common. Hydroponic systems are highly conducive to the persistence of Oomycetes and a record of infection and plan of action will be necessary to preserve crop health.

Studies of Aquatic Fungi. XXIV. Aquatic Fungi in the Water of Melting Snow

The work was undertaken to investigate the mycoflora in the water of melting snow. Samples of water were collected in March 1987-1988 for hydrochemical analysis (3 sites) and studies of the fungus content (9 sites). Forty-nine species of fungi were found in this waters. The following fungi unknown from Poland were found: <i>Skirgiella septigena, Monoblepharis macraodra, M. polymorpha, M. fascicutlta, M. insignis, Achlya apiculata, Apodachlya punctata, Pythium dissotocum, Hansenula holstii, H. saturnus, Actiaospora megalospora<i> and <i>Heliscus lugdunensis</i>.

First Report of Pythium dissotocum Affecting Cilantro in Hydroponic Systems in Puerto Rico

Pythium dissotocum was detected on cilantro in hydroponic systems in Puerto Rico. Isolates from stem lesions were inoculated in cilantro ‘Lonestar’ and in pea ‘Sugar Baby,’ producing soft rot 21 and 15 days after inoculation, respectively. The pathogen was re-isolated from diseased plants and it was confirmed to be Pythium dissotocum by PCR. Accepted for publication 5 December 2012. Published 14 December 2012.

Avaliação patogênica in vitro de Pythium middletonii Sparrow e Pythium dissotocum Drechsler em alface

Infecções radiculares nos cultivos hidropônicos de alface são frequentes e, na maior parte das vezes, causadas por espécies de Pythium, extremamente bem adaptadas ao ambiente aquático. Este estudo buscou avaliar o potencial patogênico in vitro de Pythium middletonii e P. dissotocum, em quatro cultivares de alface: Elisa (lisa), Vera (crespa), Mimosa (mimosa) e Tainá (americana). Sementes de alface, de cada cultivar, foram desinfetadas superficialmente, pré-germinadas por 24 horas, e colocadas na superfície do meio ágar-água. Em seguida, um disco de micélio dos isolados de Pythium, foi disposto no centro de cada placa. Placas contendo apenas as sementes de alface serviram como controle. O delineamento experimental utilizado foi inteiramente casualizado, com cinco repetições, sendo cada uma delas representada por uma placa de Petri. Avaliou-se o comprimento dos hipocótilos, das radículas e a porcentagem das plântulas sobreviventes após dez dias de incubação. O experimento foi conduzido na temperatura ideal de crescimento da alface (20ºC), e nas ideais para o crescimento dos isolados, 23ºC para P. middletonii e 27ºC para P. dissotocum. A 20ºC, P. dissotocum foi mais patogênico que P. middletonii, reduzindo significativamente o comprimento dos hipocótilos e, principalmente, das radículas, de praticamente todas as cultivares. Na temperatura de 27ºC, P. dissotocum foi responsável pela mais baixa porcentagem de plântulas sobreviventes entre as cultivares, sendo mais patogênico em Vera (54% de sobrevivência). Dentre as cultivares analisadas, a Mimosa mostrou tendência de menor suscetibilidade a este patógeno, apresentando a maior porcentagem de plântulas sobreviventes e o maior comprimento das radículas em relação ao controle. P. dissotocum apresentou maior crescimento micelial e foi mais patogênico que P. middletonii em todos os experimentos realizados.

Yellow Stunt, a Tobacco Disease Caused by Pythium dissotocum, in Southern Parts of Brazil

Yellow stunt, an emerging disease of tobacco (Nicotiana tabacum), has become increasingly prevalent in tobacco-growing regions in southern Brazil. Major symptoms are moderate to severe stunting, yellowing of leaves, severe wilting, darkened roots, necrosis of stem tissue directly above the soil line, and plant death. Phytophthora glovera was first proposed in 1999 as the primary causal agent of yellow stunt (1), but since then, there has been no data or completion of Koch's postulates to support this. Fifty-six isolates of fungi and fungus-like organisms were obtained from stem and root samples of tobacco plants with typical symptoms of yellow stunt in the Brazilian States of Rio Grande do Sul, Santa Catarina, and Paraná during the growing seasons of 2004/05 to 2006/07. They were identified to species level by analysis of the morphological characteristics (2) and sequence of rDNA internal transcribed spacer regions 1 and 2 (ITS1 and ITS2) (3). The isolates were identified as Pythium dissotocum (29), Fusarium oxysporum (10), P. graminicola (5), Rhizoctonia solani (4), F. solani (3), P. ultimum (3), P. deliense (1), and P. inflatum (1). The ITS sequences of the 29 isolates identified as P. dissotocum were identical. The nucleotide sequence of one isolate, LFM27/2005, has been deposited in GenBank (GQ495982). Analysis of ITS sequences alone was not sufficient to differentiate this isolate from other species in the Pythium subclade B2, such as P. coloratum, P. lutarium, P. marinum, or P. diclinum. However, the combination of morphological and cultural characteristics (2) and sequence data support our identification of LFM27/2005 and similar isolates as P. dissotocum. Colonies of LFM27/2005 on cornmeal agar had filamentous sporangia and formed slightly inflated, dendroid structures. Zoospores formed at 5°C. Daily growth rate on potato carrot agar was 13 mm at 25°C. The oogonia (22 μm in diameter) were nonornamented and either intercalary or terminal. Antheridia, commonly 1 to 2 per oogonium, were sessile, born on unbranched stalks, and either monoclinous or diclinous. Aplerotic or nearly plerotic oospores measured 20 μm in diameter with a smooth wall 2.5 μm thick. Pathogenicity tests for each pathogen were performed in a greenhouse at ~24°C in pots filled with pine bark substrate infested with inoculum at the time Burley tobacco plants showed five expanded leaves. Each test consisted of five plants and was repeated three times. Inoculum for one to three isolates representative of each pathogen was prepared by growing 2-month-old cultures at 28°C in the dark for 7 days on potato dextrose agar medium overlaid with three sterile oat kernels. Noninfested oat kernels were used for control plants. Forty days after inoculation, only plants inoculated with isolates of P. dissotocum exhibited all symptoms associated with yellow stunt. P. inflatum and R. solani did not induce yellow stunt symptoms and the others induced only wilting and root rot. P. dissotocum was recovered from an inoculated, symptomatic plant, fulfilling Koch's postulates. Its morphology was identical to isolates obtained from original field samples. The results demonstrate the association of isolates of P. dissotocum with tobacco yellow stunt in Brazil. References: (1) H. D. Shew et al. Phytopathology (Abstr.) 89(suppl):S72, 1999. (2) A. J. van der Plaats-Niterink. Stud. Mycol. 21:1, 1981. (3) T. J. White et al. Page 315 in: PCR Protocols: A Guide to Methods and Applications. Academic Press, San Diego, CA, 1990.

A Diagnostic Guide for Fusarium Wilt of Watermelon

Pythium dissotocum was detected on cilantro in hydroponic systems in Puerto Rico. Isolates from stem lesions were inoculated in cilantro ‘Lonestar’ and in pea ‘Sugar Baby,’ producing soft rot 21 and 15 days after inoculation, respectively. The pathogen was re-isolated from diseased plants and it was confirmed to be Pythium dissotocum by PCR. Accepted for publication 26 October 2011. Published 29 November 2011.

Patogenicidade in vitro de Pythium aphanidermatum e Pythium dissotocum em variedades de alface (Lactuca sativa L.)

Pythium spp. são importantes causadoras de infecções radiculares nos cultivos hidropônicos, tendo sido objetivo deste estudo avaliar a patogenicidade de Pythium aphanidermatum e P. dissotocum, em alface. Placas de Petri contendo ágar-água e micélio dos isolados foram incubadas de 15 a 45 ºC, para verificação da temperatura ótima de crescimento, estabelecendo-se 31 ºC para P. aphanidermatum e, 26 ºC para P. dissotocum. Avaliou-se, então, a patogenicidade dos espécimes nas suas temperaturas ótimas e, em 20 ºC, ideal para a alface, colocando-se sementes em ágar-água e um disco com micélio no centro das placas. Placas contendo somente sementes em meio de cultura serviram como controle. Verificou-se o comprimento dos hipocótilos, das radículas e a porcentagem de plântulas sobreviventes, selecionando-se P. aphanidermatum como mais patogênico; as variedades Vera e Tainá como menos suscetíveis e, Elisa, como mais suscetível.