First Report of Binucleate Rhizoctonia AG-F Causing Stalk and Root Rot of Corn (Zea mays) in Turkey

Maize or sweet corn (Zea mays L.) is one of the most widely grown and important cereal crops in terms of its area coverage and contribution to total food grains production in the world. In Turkey, approximately 640,000 ha of land is under maize cultivation producing 5.9 million metric tons of grain. In Turkey, Şanlıurfa is the third largest province for corn cultivation after Adana and Mardin. In the summer of 2018, plants with suppressed growth owing to a root and stalk rot were observed in a corn field (37°27′57.8″N, 39°26′28.9″E) with an area of 8 ha located in Karakeçi neighborhood of Siverek (northern Şanlıurfa). Approximately 8.8% of the plants were affected. Observed symptoms started with chlorosis of the leaves followed by brown to dark-brown discoloration or rapid death of the upper leaves or whole plant. When the stalk and crown tissues were split longitudinally, we observed a brown to black dry rot or necrosis throughout the central pith and internal tissues of the stalk and crown, which resulted in a hollow stem. Affected plants showed varying degrees of necrosis on roots and crowns, and rotted roots. Plants with stalk rot easily lodged, or bent in severe cases. Isolations were performed from the margin of symptomatic stalk piths, crowns, and secondary and tap roots on potato dextrose agar (PDA) amended with streptomycin sulfate (100 ppm). Fungal colonies recovered were yellowish-gray with a powdery texture. The fungal isolates were preliminarily identified as Rhizoctonia sp. on the basis of colony morphology and typical hyphal branching pattern (i.e., hyphal ramification angles of ∼90°, basal constriction, and a septum next to the lateral hyphae) (Sneh et al. 1991). Nuclear staining with a 1% Safranin O and 3% KOH (Bandoni 1979) and examination under light microscopy at ×400 magnification confirmed that hyphal cells of all isolates had two nuclei per cell (binucleate), belonging to the teleomorphic genus Ceratobasidium. No sclerotia were formed after 20 days on PDA. The genomic DNA of two isolates was extracted, and the partial internal transcribed spacer (ITS) 1, complete 5.8S ribosomal RNA gene, and partial ITS2 region of the nuclear rDNA was amplified by using the primers ITS6/ITS4. The nucleotide sequences obtained (GenBank accession nos. MK442087 and MK442088) had 100 and 99% identity, respectively, with many sequences of binucleate Rhizoctonia AG-F subgroup in NCBI database, including KT265714 and KT265715 isolated from ginger and taro in China (Dong et al. 2017), thus confirming the morphological identification. Pathogenicity tests were performed on 3-week-old healthy potted seedlings of Z. mays ‘Pioneer 2105’. Twenty plants were inoculated by placing three 1-cm-diameter mycelial plugs from 5-day-old PDA cultures of a representative isolate near the crown of plants. Fifteen plants inoculated with only PDA plugs served as controls. The plants were maintained in a growth chamber at 25 ± 1°C and 95% relative humidity with a 16-h/8-h (light/dark) photoperiod. All inoculated plants showed extensive necrosis on the roots and crown, resulting in growth reduction. The symptoms observed 15 days after inoculation were similar to those found in the commercial field. Control plants remained healthy. Binucleate Rhizoctonia was reisolated from symptomatic tissues of inoculated plants, completing Koch’s postulates. To our knowledge, this is the first report of binucleate Rhizoctonia AG-F causing stalk, crown, and root rot on Z. mays in the world. The finding of this pathogen on corn in Turkey needs to be considered when designing disease management programs for corn production.