Nigrospora oryzae Causing Panicle Branch Rot Disease on Oryza sativa (rice)

The panicle branch, which is the key node for transport of photosynthesis products from source to sink, is vulnerable to many diseases caused by fungal pathogens, such as Magnaporthe oryzae, Cochliobolus miyabeanus. Among these diseases, rice blast is the most important one which causes devastating losses in many regions. In 2019 and 2020, panicle branch rot of rice with a symptom which could be mistaken with rice blast was observed in a paddy field, where is not traditional epidemical region of rice blast, in Fuyang, Zhejiang province. In 2020, similar symptom was also observed in Hubei and Anhui Province. In a paddy field in Fuyang, the symptom appeared on more than 30% investigated panicles. Diseased panicle exhibited brown to black lesions on primary or secondary branches as well as pedicels, however the grain and the neck of spike could not be infected which is the most obvious difference with rice blast. Obviously, the disease can't destroy the entire function of branch and blank grain was rarely observed, so its damage is not comparable with neck blast. Normally, it caused incomplete grain filing commonly leading to 5% - 25% grain weight loss. During the booting stage of rice, local solar irradiation time and temperature were fewer and lower than common years which may be responsible for losses caused by this disease. After surface sterilized, lesion parts cut from infected branches from 25 panicle samples were cultured on 2% water agar at 28℃ for 24-28 h, and fungi were isolated and purified by mycelial tip transferring. Among 31 isolates, 26 showed similar cultural characters. The wool-like mycelia were luxuriant and grew rapidly on PDA spreading the whole 9 cm petri dish in less than a week at 28 ℃. The mycelia were white to ashen at beginning and gradually turned black from center of the plate after 5 days culture at 28 ℃. Hyphae were smooth, branched, septate, hyaline or pale brown. Conidia were single-celled, black, spherical to subspherical, and 10.2 to 14.6 × 12.2 to 15.7 μm (n=50) in dimension and born on tip of hyaline and ampulliform conidiophores. The fungus showed similar morphological characteristics with Nigrospora oryzae (1). ITS sequences of 6 representative strains of the fungus were amplified, sequenced with primer pair, ITS1/ITS4 (2), and submitted into GenBank with an accession number, MW228165. Phylogenetic analysis was conducted with sequences of reference strains (3). The result showed that the fungus obtained in this study was fallen into the same group with N.oryzae. In view of above both morphological and molecular analysis, the strains were finally identified as N. oryzae. Pathogenicity tests were conducted in triplicate with rice panicles in initial heading stage. Fifty panicles were wounded on branches with needles and inoculated by spreading the conidia suspension (10μl, 1 × 106 conidia ml-1) on the wounds. The panicles used as control were treated in same way with 10μl of sterile water. The inoculated and control plants were kept in dark, 25 ℃ and relative humidity of more than 85% for 24 h in culture chamber. Symptoms appeared on 44 of 50 inoculated panicles which were basically similar with those observed in paddy field, while negative controls remained symptomless. The fungi re-isolated from inoculated panicles were also confirmed as N. oryzae by both morphological and molecular analysis. To the best of our knowledge, this is the first report of N. oryzae causing panicle branch rot disease on Oryza sativa (rice). This disease not only cause yield losses and lower milling quality, but could also be mistaken as rice blast incurring unnecessary fungicides spray.