Characteristics, genetic diversity and pathogenic variability of Fusarium oxysporum f. sp. niveum in watermelon (Citrullus lanatus var. lanatus (Thunb.) Matsum. & Nakai)
Watermelon (Citrullus lanatus L.) is one of the most economically important horticultural fruit crops. It belongs to the Cucurbitaceae family. Presently a major hindrance to this crop production is the soil-borne disease of Fusarium wilt caused by Fusarium oxysporum f. sp. niveum (Fon). This disease...
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| Format: | Thesis |
| Language: | English |
| Published: |
2022
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| Subjects: | |
| Online Access: | http://psasir.upm.edu.my/id/eprint/113226/1/113226.pdf |
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| Summary: | Watermelon (Citrullus lanatus L.) is one of the most economically important horticultural fruit crops. It belongs to the Cucurbitaceae family. Presently a major hindrance to this crop production is the soil-borne disease of Fusarium wilt caused by Fusarium oxysporum f. sp. niveum (Fon). This disease causes yield losses to growers approximately 30-80% or even more. There is limited information on the detection, genetic diversity, and pathogenic variability of the Fon pathogen in Malaysia and Bangladesh. Hence, this study was conducted with the following objectives: to survey and characterize Fusarium oxysporum isolates collected from different watermelon growing areas in Peninsular Malaysia and Bangladesh based on morphological and molecular methods; to determine the genetic diversity of F. oxysporum f. sp. niveum through simple sequence repeat (SSR) markers; and to determine the pathogenic variability and hosts reaction of watermelon commercial cultivars against F. oxysporum f. sp. niveum. A survey of Fusarium wilt disease of watermelon was done in major watermelon-producing areas in Peninsular Malaysia and in Bangladesh from August, 2019 to December, 2020, using a diagonal sampling method. Based on the cultural and morphological characteristics were used to identify the fungus. The Fon forma speciales-specific primers (Fn-1/Fn-2 and Fon-1/Fon-2) were used to identify the fungus based on the molecular method. The molecular characterization of Fusarium species was determined by PCR amplification of different gene regions (tef1-α, IGS, and mtSSU). The analysis of molecular variance (AMOVA) for genetic diversity was done using SSR marker. A pathogenicity test was carried out for 65 Fon isolates to determine their aggressiveness and 12 watermelon commercial cultivars were tested to identify adequate resistance to Fon. The findings of field disease surveys revealed that field disease incidence was 5-45% based on random sampling. The highest disease incidence (45%) was in Pahang, whereas, Terengganu had the lowest disease incidence (5%). The mycelia of the collected isolates were delicate, sparse to numerous, white to pinkish-white, and violet to dark violet pigmentation. Microconidia were small, oval-ellipsoid, straight to curve, non-septate and macroconidia were fusoid-subulate, scarce to abundant, mostly three-septate, hooked apex and pedicellate base. Whereas, chlamydospores were abundant, formed terminally or intercalary, and found singly or in pairs, chains, or clusters. These typical cultural and morphological characteristics of 65 isolates were identified as F. oxysporum. The Fon forma speciales-specific primers (Fn-1/Fn-2 and Fon-1/Fon-2) were amplified amplicons of 320 bp and 174 bp in 100% and 91% of the isolates, respectively. The result of phylogenetic analyses of individual and combined gene sequences of tef1-α, IGS, and mtSSU revealed that 65 isolates belonging to F. oxysporum f. sp. niveum. Multigene phylogenetic analysis using tef1-α, IGS, mtSSU data set of all isolates were clustered into four main clades, indicating that Fon is polyphyletic. The analysis of molecular variance (AMOVA) showed a total variation of 67% within the population and a 33% total variation among the populations. The results of this investigation indicated that there is a maximum genetic diversity existing within the Fon populations. Based on the pathogenicity test, the 65 Fon isolates were characterized into 4 groups, viz., highly aggressive 15 isolates, moderately aggressive 7 isolates, weakly or low aggressive 29 isolates, and non-pathogenic 14 isolates. Among the 12 watermelon commercial cultivars, Black Giant and Big Family were identified as highly resistant cultivars with disease severity index (DSI) of 5.55-18.15% and 18.11-30.00%, respectively. These results indicated that these two cultivars have the potential to cultivate as resistant varieties against Fon. |
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