Maize (Zea mays L.) is cultivated in 8.6 million ha in India with a total production of 22.67 million tonnes and avg. yield of 27.52 tonnes/ha in 2014 (http://faostat3.fao.org/download/Q/QC/E). In September 2012, symptoms of Fusarium stalk rot were observed on maize cultivar PAC 740 (Advanta Ltd., Hyderabad, India) grown in a field in Tinsukia, Assam, northeast India (27.5° N; 95.37° E; elev. 116 m). Symptomatic plants were collected for study. External symptoms included softening and reddish coloration of the stalks near the first three internodes from the bottom. Internally, the pith appeared to be soft, disintegrating, and light brown to reddish. Lodging was observed in two plants out of a total of seven exhibiting visible external symptoms. The leaves and stalks of the lodged plants were dry and tan. Small pieces of infected stems were surface sterilized in a 5% aqueous solution of NaClO for 1 min, rinsed 3 times with sterile distilled water and dried in a laminar air flow cabinet. Then, 1 to 2 pieces per plate were transferred to potato dextrose agar (PDA) media supplemented with 0.5 g/liter ampicillin and incubated at 25°C for 5 to 7 days. The isolates were single-spore subcultured on PDA and incubated at 25°C for two weeks (Leslie and Summerell 2006). The mycelia were initially white and gradually developed a blackish pigmentation; the microconidia were single-celled, oval to club-shaped, 4.4 to 11.55 × 1.1 to 3.3 µm (avg. 6.6 × 1.65 µm) arranged in catenation from monophialides. The morphological characteristics were consistent with those of Fusarium verticillioides (Sacc.) Nirenberg (Leslie and Summerell 2006). After microscopic confirmation, the ITS1-5.8S-ITS2 regions of the rDNA were amplified using the primers ITS1 and ITS4 (White et al. 1990) and sequenced (GenBank Accession No. KF031434). The sequence was compared with those deposited in GenBank and the isolate showed 99% similarity with Gibberella moniliformis strain SA3 (EU151482) (Visentin et al. 2009). Pathogenicity tests were conducted on maize plants grown in 25 × 28-cm (diameter × height) pots filled with 4 kg sterilized soil. Five 75-day-old PAC 740 plants were injected with 2 ml microconidial suspension (107 conidia/ml) at the second internodes above the soil line (Munkvold et al. 1997) and incubated in a controlled environment chamber at 25°C and 80 to 85% humidity with a 14-h photoperiod. Conidial suspension was harvested from a 10-day-old culture in potato dextrose broth. Ten to 12 days post inoculation (dpi), inoculated plants developed characteristic external symptoms, like softening of the stalks followed by lodging at 21 to 22 dpi, similar to those observed on naturally infected maize plants in the field. The plants were removed from the pots and the pathogen was reisolated from split stalks and identified by morphological characteristics as F. verticillioides. Research on maize cultivation is still fragmentary in Assam with few districts actively involved in cultivation, but due to high commercial value, maize cultivation is gaining wider interest in the area. Considering the impact of stalk rot on maize yields, further research is needed to develop and implement measures for disease control. To our knowledge, this is the first report of F. verticillioides causing stalk rot of Z. mays in Assam or the northeastern India. © 2016 The American Phytopathological Society.
