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Cadophora malorum: A New Pathogen of Sunflower Causing Wilting, Yellowing, and Leaf Necrosis in Russia

Martín-Sanz, A., Rueda, S., García-Carneros, A. B., Molinero-Ruiz, L.
Plant disease 2018 v.102 no.4 pp. 823
Cadophora, Helianthus annuus, color, conidia, cortex, culture media, disease incidence, dwarfing, field experimentation, fungi, genotype, greenhouse experimentation, inbred lines, leaves, loci, necrosis, pathogenicity, pathogens, peat, roots, sand, sodium hypochlorite, species identification, stems, tubulin, vascular wilt, wilting, Russia, Ukraine
Russia devotes more than six million hectares to sunflower (Helianthus annuus L.) crop every year. Together with Ukraine, they are the most important sunflower-producing countries worldwide. Since 2014, an increase of wilt diseases has been observed in the country. Plants with symptoms of wilting, leaf yellowing, and necrosis were observed in two commercial fields and one demonstrative field trial in the Orenburg region in August 2016, and in one commercial field in the Saratov region in August 2015. Disease incidence was estimated between 15 and 30% depending on the field, and yield from infected plants was near zero. Four symptomatic plants per location were collected for observation and pathogen isolation. From each sample, the basal part of the stem was surface sterilized for 5 min in 10% sodium hypochlorite solution, rinsed in water, cut longitudinally, and sections of the cortex with brownish appearance indicative of possible fungal colonization were incubated in water-agar medium at 25°C in darkness. Fungal colonies were consistently isolated from all samples. Then four isolates, one from each of the four fields, were grown in potato dextrose agar for morphological characterization. Single spore isolates were used for characterization. Colonies were flat, with felty to fleecy rings, the color varying from white-beige to brown-olivaceous. On the basis of morphology, the fungus was identified as a species of Cadophora (Schol-Schwarz 1970). Species identification was conducted by amplification of the β-tubulin locus using primers BTCadF/BTCadR (Travadon et al. 2015). Sequences of 750 bp of the four isolates were deposited in GenBank (accessions MG458768 to MG458771) and BLAST analyses showed 92% identity with C. malorum (KU532784.1). The pathogenicity was confirmed in a greenhouse experiment (20 to 25°C day and 15 to 20°C night, 14 h light). The four isolates were independently inoculated to 3-week-old plants of HA458 (public) and DP1 (Dupont Pioneer) inbred lines. Plants were inoculated by immersing roots in a suspension of 10⁶ conidia per ml for 30 min. Inoculated plants were individually transplanted to 2.5-liter pots filled with a 2:1 peat/sand mix. Roots of the control treatments were immersed in water. Seven replications (pots) were established for each isolate × genotype combination, according to a complete randomized factorial design. Six weeks after inoculation, symptoms developed in 100% of plants in both sunflower genotypes. Wilting and dwarfism were observed in the inoculated plants and at least half of the nodes of those plants had necrotic leaves. Water-inoculated plants did not show any symptoms. The fungus was reisolated from basal stems of two inoculated plants per isolate, completing Koch’s postulates. To our knowledge, this is the first report of C. malorum causing wilting, yellowing, and leaf necrosis in sunflower in Russia.