First Report of Fusarium Wilt Disease on Watermelon Caused by Fusarium oxysporum f. sp. niveum in Malaysia

Abstract

Fusarium wilt disease incited by Fusarium oxysporum f. sp. niveum (FON) is the utmost devastating soil-inhabiting fungal pathogen limiting watermelon (Citrullus lanatus) production in Malaysia and globally. The field disease survey of fusarium wilt was carried out during December 2019 and November 2020, in three major production areas (3 farmer fields per location) in Peninsular Malaysia namely, Mersing, Serdang and Kuantan and disease incidence of 30 and 45%, was recorded for each year, respectively. Infected watermelon plants showed symptoms such as vascular discoloration, brown necrotic lesions to the soil line or the crown, one-sided wilt of a plant, or a runner or the whole plant. Infected root and stem tissues, 1-2 cm pieces were surface sterilized with 0.6% NaOCl for 1 minute followed by double washing with sterile water. The disinfected tissues were air-dried and transferred onto semi-selective Komada’s medium (Komada 1975) and incubated for 5 days. The fungal colonies produced were placed on potato dextrose agar (PDA) to attain a pure culture and incubated at 25±2℃ for 15 days. The pure fungal colony was flat, round and light purple in color. Macroconidia were straight to slightly curved, 18.56-42.22 µm in length, 2.69-4.08 µm width, predominantly 3 septate and formed in sporodochia. Microconidia measured 6.16-10.86 µm in length and 2.49-3.83 µm in width, kidney-shaped, aseptate and were formed on short monophialides in false-heads. Chlamydospores were single or in pairs with smooth or rough walls, found both terminally or intercalary. To confirm their pathogenicity, two-week-old watermelon seedlings (cv. NEW BEAUTY) were dipped into spore suspension (1 ˟ 106 spores/ml) of representative isolates of JO20 (Mersing), UPM4 (Serdang) and KU41 (Kuantan) for 30 second and then moved into 10 cm diameter plastic pots containing 300 g sterilized soil mix. Disease symptoms were assessed weekly for one month. Control seedlings were immersed in sterile distilled water before transplanting. The inoculated seedlings showed typical Fusarium wilt symptoms like yellowing, stunted growth, and wilting, which is similar to the farmer field infected plants. However, the seedlings inoculated by sterile distilled water remained asymptomatic. The pathogen was successfully re-isolated from the infected seedlings onto Komada’s medium, fulfilling the Koch’s postulate. For the PCR amplification, primers EF-1 and EF-2 were used to amplify the tef1-α region. A Blastn analysis of the tef1-α sequences of the isolates JO20 (accession nos. MW315902), UPM4 (MW839560) and KU41 (MW839562) showed 100% similarity; with e-value of zero, to the reference sequences of F. oxysporum isolate FJAT-31690 (MN507110) and F. oxysporum f. sp. niveum isolate FON2 790-2 (MN057702). In Fusarium MLST database, isolates JO20, UPM4 and KU41 revealed 100% identity with the reference isolate of NRRL 22518 (accession no. FJ985265). Though isolate FJ985265 belongs to the f. sp. melonis, earlier findings had revealed Fusarium oxysporum f. sp. are naturally polyphyletic and making clusters with diverse groups of the Fusarium oxysporum species complex (O’Donnell et al. 2015). The isolates JO20, UPM4 and KU41 were identified as F. oxysporum f. sp. niveum based on the aligned sequences of tef1-α and molecular phylogenetic exploration by the maximum likelihood method. To the best of our knowledge, this is the first report of F. oxysporum f. sp. niveum as a causative pathogen of Fusarium wilt disease of watermelon in Malaysia. Malaysia enables to export watermelon all-year-round in different countries like Singapore, Hong-Kong, The United Arab Emirates (UAE), and Netherlands. The outburst of this destructive soil-borne fungal pathogen could cause hindrance to watermelon cultivation in Malaysia. Thus, growers need to choice multiple management tactics such as resistant varieties, cultural practices (soil amendments and solarization), grafting, cover crops and fungicide application to control this new pathogen.

Identification and Characterization of Fusarium Species Causing Watermelon Fruit Rot in Northern Thailand

“…According to the results, both fungal species can be regarded as causal agents of fruit rot disease in watermelons. Our results are supported by previous studies that reported that Fusarium species are the cause of various disease symptoms in watermelons in tropical and subtropical regions around the world [ 73 , 74 , 75 , 76 ]. Prior to this study, F. solani , F. oxysporum , F. verticillioides , and F. chlamydosporum were considered to be the causal agents of fruit rot in watermelons in Nigeria [ 16 , 24 , 77 ].…”

“…For example, F. equiseti and F. oxysporum f. sp. niveum have been observed to cause Fusarium wilt disease in fruits grown in Korea [ 76 ] and Malaysia [ 74 ], respectively. On the other hand, Fusarium brachygibbosum and F. oxysporum have been shown to lead to vine decline symptoms in the United States [ 73 ] and root rot in China [ 75 ], respectively.…”

Passion fruit plants alter the soil microbial community with continuous cropping and improve plant disease resistance by recruiting beneficial microorganisms

“…Fusarium produces culture filtrates toxic to nematodes [ 44 ] and can potentially protect plants by suppressing the growth of pathogens and depriving niches [ 45 ]. Additionally, it is also a well-known soil-borne phytopathogenic species that can cause severe crop losses [ 21 , 46 , 47 ].…”

Conservation and Expansion of Transcriptional Factor Repertoire in the Fusarium oxysporum Species Complex

“…The fungal species complex of Fusarium oxysporum (FOSC) has been used as a model to study cross-kingdom fungal pathogenesis. Members within FOSC can cause devastating fusarium wilt diseases among economically important crops [ 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 , 9 , 10 , 11 , 12 ] and is listed among the top five most important plant pathogens [ 8 ]. With strong host specificity, plant pathogenic F. oxysporum strains are further grouped as formae speciales [ 13 ].…”