First report of Lasiodiplodia brasiliensis causing root rot in melon plants in northeastern Brazil.

Abstract

Melon (Cucumis melo L.) is the second most exported fruit in Brazil with an annual production of 27.5 million tons (FAO 2023). From September 2019 through February 2020, 50-day-old melon plants started showing root rot symptoms (dark-brow necrotic zones in their roots that extended to the collar zone) in northeastern Brazil, 30% of the plants in the fields were affected by the disease. The fields are in clay soil where melon, in monoculture, is produced all year long with three cycles of the culture per year. A total of 132 samples from "Yellow" and "Cantaloupe" cultivars were collected from four melon fields (4°59'45.3"S, 37°33'39.7"W; 4°57'10.2"S, 37°31'37.1"W; 5°38'17.9"S, 37°56'27.7"W; and 5°00'25.5"S, 37°23'55.3"W). Small pieces of diseased tissues were surface disinfested in 70% ethanol for 30 sec, in 2% sodium hypochlorite for 1 min, washed in sterilized distilled water, plated on a PDA Petri dishes with tetracycline (0.05g/L), and incubated for seven days at 28 ± 2 ºC. Nine representative isolates were selected for downstream analysis. Colonies were white and later became dark gray, pycnidia and conidia were produced after 30 days ofncubation at 25°C under near-UV light in water-agar medium. Conidia were hyaline when immature and dark brown when mature, ranging from cylindrical subovoid to ellipsoidal and septate to non-septate, and with an average size of 12.54 to 21.97 µm. The colonies were morphologically identified as Lasiodiplodia sp. (Phillips et al. 2013). Total DNA from the isolates was extracted and the ITS, TUB, and TEF-1α genes (Jayawardena et al. 2019) were partially amplified by PCR, Sanger sequenced, and deposited in Genbank: ITS (OM102511 to OM102520), TUB (OR062087 to OR062094 and OR062095), and TEF-1α (OP536826 to OP536835). Blastn analysis of the partial sequences ITS (519bp), TUB (388bp), and TEF-1α (315bp) showed 100% nucleotide similarity of the isolates with sequences of L. brasiliensis and L. theobromae from the GenBank. A phylogenetic tree was constructed using the Maximum Parsimony Analysis method. All nine isolates were grouped into the L. brasiliensis clade with 71% bootstrap support, confirming the isolates's identity. Pathogenicity assays were conducted in a greenhouse using the wooden toothpick inoculation method (Nogueira et al. 2019). "Goldex" Yellow melon seedlings were used in a completely randomized experimental design, with 10 treatments (9 isolates + Mock) and six replicates, with one plant per pot. Plants were inoculated 15 days after sowing, and disease severity was evaluated 50 days after inoculation. All nine isolates caused symptoms in the assessed melon plants. The fungus was reisolated from the lesions and looked morphologically identical to the inoculated fungus, fulfilling Koch's postulates. The pathogenicity test was repeated and yielded similar results. All samples in this study were provided by melon growers who were concerned about the high incidence of root rot disease in their plantations. More research needs to be conducted to determine the epidemiology and the extension of the economic impact caused by this pathogen to melons to develop strategies for disease control to properly assist the growers's concerns. This pathogen has been reported to cause disease in other crops in Brazil, e.g., watermelon (Alves et al. 2023) and apples (Martins et al. 2018). However, to the best of our knowledge, this is the first report of L. brasiliensis causing root rot in melons in Brazil.