Seedling diseases are a major problem for rice production in the United States. Understanding the causal agents of seedling diseases is the key to successfully growing rice, a primary staple food crop feeding more than half of the world’s population.
Knowing the geographic distribution and importance of pathogens causing seedling diseases is vital to effectively managing these important diseases in rice. No surveys of major pathogens causing seedling diseases in rice in the United States had been conducted. Therefore, we completed systematic disease surveys in Arkansas, Louisiana, Mississippi, Missouri and Texas, the five major southern states producing rice in the United States. This revealed the prevalence of seedling diseases and identified new major pathogens currently causing seedling blight in the US.
Seedling diseases affect crop yield by reducing plant stands in the field. Seedling blight or “damping-off” is a seedling disease complex caused by several soilborne and seedborne pathogens in rice (particularly drill-seeded rice). This disease becomes more severe when the temperatures at planting are low. As a result, “stand loss” where large areas of rice crops die-off due to disease, frequently occurs every year.
We surveyed 70 commercial rice fields across the five southern states at seedling stage (2-3 weeks after emergence). We used traditional pathogen identification methods, including analysis of disease symptoms (symptomatology), pathogen appearance (morphology) and molecular diagnosis (DNA sequencing). Koch’s postulates were also applied to confirm the identity of each pathogen. (These are a set of criteria used to establish whether a particular microorganism is the cause of the disease symptoms found on the affected plant.)
Disease surveys identified seedling blight disease in all the rice fields surveyed (affecting 2 to 20% of the crop). The soilborne fungal pathogens Rhizoctonia solani AG-4, Rhizoctonia solani AG-11, Fusarium species, Sclerotium rolfsii, and Marasmius graminum were found to cause seedling blight in rice.
R. solani AG-11 was the most common disease agent found across all states, followed by Fusarium species. R. solani AG-4 occurred in Arkansas, Missouri, and Texas. S. rolfsii, and M. graminum were present only in Louisiana and Texas. None of the species of Pythium and Phytophthora that can cause seedling blight was present in the current disease surveys.
For the first time, R. solani AG-4 and M. graminum have been identified as new pathogens causing seedling blight in rice. Moreover, both R. solani AG-4 and M. graminum were found to be more aggressive than the other fungi tested.
Fungicide seed treatment can be an effective approach for managing soilborne pathogens and the diseases they cause. However, the geographical distribution and importance of fungal pathogens causing seedling diseases in rice in the southern United States remained largely unknown before this study. Identifying these major seedling disease pathogens from our disease surveys provides the foundation to improve the current seedling disease-management practices. We can design and develop fungicide seed treatments that are more effective and more economic by targeting the most prevalent pathogen populations for each region. In addition, the findings of this study can also help screen or breed rice cultivars and elite lines for resistance against these major pathogens.
Shankar P. Gaire, Xin-Gen Zhou, Yong Zhou, Jun Shi and Young-Ki Jo published this study in Plant Pathology:
TITLE IMAGE: A rice field with seedling disease. All images used with permission of the author.
