Characterization, pathogenicity and fungicide response of Exserohilum rostratum causing leaf spot on rice in Thailand

Exserohilum rostratum has been recorded as an emerging pathogen causing rice leaf spot in many countries, and it is also implicated as one of the factors contributing to rice seed discoloration, which affects grain quality. However, there is currently insufficient understanding of the biological aspects of foliar symptoms and pathogen development during the interaction between rice and E. rostratum, particularly under varying temperatures. Additionally, there is limited information available regarding the sensitivity of E. rostratum to fungicidal treatment. Therefore, this study aims to address these gaps and gain insights into these areas. Herein, 29 selected fungal isolates recovered from rice leaves with tiny-spot symptoms from different paddies in Thailand were morphologically and molecularly characterized (based on the sequences of three loci: rDNA-ITS, Tef1a, and GAPDH), which subsequently were identified as E. rostratum. The investigation of symptom development on rice leaves after artificial infection with these isolates under greenhouse conditions (temperature range between 30–35 °C) and high humidity (> 80% RH) conditions revealed that the fungal isolates developed reddish-brown, irregular ellipsoidal spots, along with merging lesions resulting in chlorosis and necrotic tissue. Tiny black spots were observed under growth room conditions of 27 °C and > 80% RH. Experiments conducted on detached rice leaves infected under controlled conditions, with the only variable being temperature variation (33 °C and 27 °C) at constant RH, demonstrated that leaves treated at 33 °C exhibited greater symptom progression compared to those treated at 27 °C. This suggests that temperature influences the expression of symptoms related to rice leaf spot. Additionally, histopathological examination of rice leaf sheaths infected with E. rostratum confirmed the parasitic lifestyle of the fungus and could aid in tracking the pathogen development during exposure to different temperatures. Inhibition assays using flusilazole, azoxystrobin, tricyclazole, and mancozeb against E. rostratum mycelial growth showed that flusilazole had the strongest inhibition effect against all the isolates tested. Overall, our study contributes to a better understanding of the biological responses during E. rostratum infection in rice, particularly during distinct temperature exposure, as well as the response to fungicides of this pathogen, providing insights for future research and management practices in combating E. rostratum.