First report of Fusarium oxysporum Species Complex causing root and rhizome rot of Canna edulis in Colombia

Achira, Canna edulis Ker, a plant native to South America, is the source of a starch used for food and industrial purposes. Since 2016, Colombian growers of the main cropping regions, Cundinamarca (CU), Nariño (NA), and Huila (HU) are experiencing yield losses due to rhizome rots. Surveys of the affected areas evidenced wilting and collapsed plants, with oxidized rhizomes and affected root masses. Disease incidence per field was around 10%, but diseased plants were found in all 44 visited farms. To study this problem, wilting plants were collected, and symptomatic tissues, pseudo-stems, roots, and rhizomes, were cut and disinfested in 1.5% hypochlorite, rinsed in sterile water, and plated onto PDA amended with 0.01% tetracycline. A total of 121 isolates were recovered; of these, 77 Fusarium-like isolates stood out, given their recovery frequency (64.7%) and cross-region distribution. To morphologically study the isolates, carnation leaf agar cultures of NA01, NA16, NA48, CU08-1 and HU02, were made. Isolates had hyaline, mostly aseptated microconidia, oval in shape, developing in false heads with short monophialides. Macroconidia were hyaline and falcate, straight to slightly curved, 2 to 4 septate, with apical cells curved and basal cells foot shaped. For NA01 the average size and width of the microconidia was 4.3 x 3.2 μm (n=80), while macroconidia averaged 18.9 × 5.7 μm (n =80); NA16 was slightly bigger (6.5 x 3 and 22.9 x 5.5 um respectively). This morphology resembles Fusarium oxysporum (Fox) (Leslie et al. 2006). Identity confirmation was obtained by Sanger sequencing of the rRNA internal transcribed spacer (ITS) and the translation elongation factor 1α (TEF1α) loci using protocols of White et al. 1994, and O'Donnell et al. 1998. Blast comparisons against NCBI databases, showed a very high identity (>99.5%) to MN528565.1 (ITS), and KU985430.1 (TEF 1α), both, F. oxysporum sequences. The identity of NA01 and CU08 was further confirmed by sequencing the DNA-directed RNA polymerase II (RPB1) locus (O'Donnell et al. 2015), observing more than 99% identity to CP052885.1 (RPB1) a F. oxysporum strain. BLAST check against the Fusarium MLSD database confirmed the identity. The obtained sequences were deposited in NCBI as MN963788, MN963793, MN963801, MN963782, MN963786 (ITS); OK143597, OK141601, OK143596 MW594202, OK169575 (TEF1α); and ON297670 and MZ670431 RPB1). To confirm causality, pathogenicity assays were conducted using NA01, NA48 and CU08. To this end, 25, 35 day-olds sprouted rhizomes, from each of the "purple", "green" and "white" varieties, were inoculated by drench with 30 ml of conidium suspension (1x106 conidia/ml) (Schmale 2003). Control rhizomes (25 per variety) were treated with sterile distilled water. Greenhouse conditions were 25 °C, 40% RH, and photoperiod 12h. Disease symptoms were detected 10 days after inoculation and evolved to resemble those from the field. While symptom and severity of infection varied with the isolate and host combination used, pathogen re-isolation and identification was successful fulfilling Koch´s postulates. Control plants remained healthy. The data shows that F. oxysporum species complex is the causal agent of this achira root and rhizome rot. To our knowledge, this is the first report of this problem in Colombia and clarifies local reports of Fusarium sp. causing disease in this crop (Caicedo et al. 2003). The disease affects the food security of local communities and strategies for control are being developed.

First report of Rhizopus oryzae causing rhizome rot on ginger in China

Ginger (Zingiber officinale Rosc.) is a herbal plant, widely grown in China for its medicinal and culinary purposes. In July 2020, a new rhizome rot disease was observed on ginger in Laiwu, Shandong Province, China. The disease symptoms were observed on both above-ground and underground plant parts. The above ground stems and leaves becoming withered and yellow, and water-soaked symptoms were observed on the collar region. The diseased rhizomes were poorly developed with brown lesion and eventually they would rot, without offensive odors. Disease incidence was estimated at approximately 5% across the survey area. To isolate the pathogen, tissues from 30 rhizomes were cut from the border between diseased and healthy tissue, surface sterilized in 75% alcohol for 15 s, soaked in 0.1% mercuric chloride for 1 min, washed with sterile distilled water three times, and plated on potato dextrose agar (PDA) at 25°C for 2-3 days. Twenty nine fungal isolates with similar morphological characteristics were obtained and pure cultures were obtained using single spore isolation. The colony of AQJ-1, a representative isolate, on PDA was cottony, fluffy, white, and beige coloration on the reverse side at first, and subsequently many black sporangia were produced. The sporangia were black, sub-globose, and 45.2-181.7 μm (n = 50) in diameter. The sporangiospores were unequal, globose or sub-globose, about 3.2-8.7 × 4.6-12.3μm (n = 50) in diameter. For the molecular characterization, genomic DNA was extracted by modified CTAB method (Niu et al., 2008). Internal transcribed spacer (ITS) region and translation elongation factor 1-alpha (EF-1α) gene were amplified using the primer pairs ITS1/ITS4 (White et al., 1990) and MEF10/MEF4 (Abe et al., 2007), respectively. The ITS and EF-1α sequences of isolate AQJ-1 were submitted to GenBank (MN606288 and MN735220, respectively). The BLASTn analysis of the sequences showed 99%-100% similarity to the sequences of R. oryzae strain CBS 120.12 (MH854609, AB281529, respectively). Therefore, based on morphological and molecular characteristics, isolate AQJ-1 was identified as R. oryzae. For pathogenicity tests, thirty ginger seedlings (Laiwu Big Ginger) were grown for 30 days in plastic pots and removed from the pots and the rhizomes washed in running tap water. The rhizomes of fifteen ginger seedlings were attached to a 7 mm agar disk from a plate containing 2-day-old mycelium, and the other fifteen seedlings were attached to agar disk without mycelium as control. Then the inoculated and control seedlings were planted in pots and were kept in separate chambers in a greenhouse at 25±2 °C. After 14 days, the same symptoms of rhizome rot were observed in all inoculated plants as previously described, and no symptoms were observed on the control plants. The pathogen was re-isolated from symptomatic tissues, and was identified as R. oryzae, which full-filled the Koch's postulates. To our knowledge, this is the first report of R. oryzae causing rhizome rot on ginger in China. This disease may pose a potential threat to ginger production in China.

First report of rhizome rot of banana caused by Klebsiella variicola in India

Rhizome rot or soft rot disease is one of the major problems in banana (Musa spp.) cultivation, as it causes germination failure and death of early stage plants. A roving survey conducted during 2017 to 2019 in the major banana growing states of India indicated a 5-30% incidence of rhizome rot in commercial cultivars. The symptoms observed were yellowing of leaves, necrotic drying with or without heart rot, and yellow or brown water soaked spots with dark brown margins in the rhizomes. Decay of tissues, cavity formation and brown ooze with foul smell, and toppling were also observed. To isolate bacteria, dissected diseased tissues were surface sterilized and plated on Crystal Violet Pectate (CVP) medium. Of 60 samples plated on CVP medium, three samples collected from cvs. NeyPoovan-AB (Karur, Tamil Nadu, 10°56'36.8"N;78°24'12.5"E), Grand Naine-AAA (Tiruchirappalli, Tamil Nadu, 10°47'26.1"N;78°34'14.8"E) and Thellachakkarakeli-AAA (East-Godavari, Andhra Pradesh, 16°51'32.1"N;81°46'08.4"E), did not yield any bacteria; however, when plated on nutrient agar, they produced whitish to dull white, mucoid, raised, round and translucent colonies, and three isolates were named as NPK-3-48, GTC-5 and 1-1B-3, respectively. Because these colonies were distinct from colonies obtained on CVP medium (which were analyzed and confirmed separately as Pectobaterium sp.) (Gokul et al. 2019), they were further characterized. Amplification of 16S rDNA genes of NPK-3-48, GTC-5 and 1-1B-3 isolates using universal primers (27F 5' - AGAGTTTGATCCTGGCTCAG - 3'; 1492 R 5' - GGTTACCTTGTTACGACTT - 3') and rpoB gene (Rosenblueth et al. 2004) was carried; the amplicons were sequenced and deposited in NCBI (Accessions MW036529-MW036531; MW497572-MW497574). Phylogenetic analysis of rpoB clearly showed that the isolates NPK-3-48, GTC-5, 1-1B-3 are Klebsiella variicola (Rosenblueth et al. 2004) Besides, biochemical tests also indicated that all three isolates were Gram negative, catalase positive, oxidase negative and able to utilize glucose, maltose and citrate (Ajayasree and Borkar 2018). Therefore, the above said morphological, molecular and biochemical analyses carried out indicated that NPK-3-48, GTC-5, 1-1B-3 are of K. variicola. Earlier, K. variicola causing soft rot has been reported on banana in China (Fan et al. 2016), plantain soft rot in Haiti (Fulton et al. 2020) and carrot soft rot in India (Chandrashekar et al. 2018). For pathogenicity tests, these three isolates were grown in nutrient broth for 48 h at 37±1°C and the cells were harvested by centrifugation. Five milliliters of the culture suspension (2×108 CFUmL-1) taken in a syringe was injected into rhizomes of three month old tissue cultured Grand Naine plants. Each bacterial isolate was injected into eight banana plants at soil level. Appropriate controls were maintained. Inoculated plants were maintained in a glasshouse at 32±2°C and after 30-35 days, rhizome rot symptoms appeared in all the three bacterial isolates inoculated plants but in none of the control plants. The Koch's postulates were proved by re-isolation and identification.To the best of our knowledge, this is the first report of K. variicola causing rhizome rot disease of banana in India.