Toward the end of April 2012, we received samples of rice seedlings with reddish brown spots (Fig.1a & b), one with tip burnings (Fig. 2), and the other dead seedlings among living spotted seedlings (Fig. 3) from producers’ farms in Phillips, Monroe, and Poinsett County, respectively.
We were not able to isolate any pathogenic fungi from the spotted living plants that should cause those kinds of symptoms. During our field visits, we checked fields with rice seedlings at three-leaf stage and beyond. All rice fields inspected had the leaf spots throughout the field. The seedlings at three leaf-stages had more spots than older seedlings. Older seedlings had started showing clean newer leaves. The symptom pattern in the field did not suggest chemical drift but was more indicative of a weather-related event. Past and present picture comparison of symptoms with Dr. Chuck Wilson suggested the possible cause for the spots to be a recent cold spell, most likely freezing dew at night. The young tissues of seedlings could be more sensitive to lower temperature conditions, resulting in the leaf spot reaction where dew collected on leaf tissue. We have observed various strange symptoms related to “frost” in the past when temperatures dropped to near 40 at weather stations, but actual temperatures in low-lying fields (seedlings) could have been several degrees lower. The good news is that new leaves were okay and suggested the symptoms were not progressive and thus not a disease we would have to worry about trying to control later. Obviously, we should continue to manage affected fields appropriately to boost growth and vigor of the seedlings.
Upon discovery of the seedlings with tip burns, our first impression was a salinity problem. However, the symptoms were also common in the barrow ditches which discounted this theory to a degree. In many areas, it appeared that affected plants had suffered early season drying stress from continual wind and lack of rainfall or flushing, so the tip drying may have simply been them drying out in response.
As to dead seedlings sent from Poinsett County on April 30, 2012, we observed active grape colaspis larvae using the microscope. Feeding injury and discoloration of the roots were also evident. With drying weather, grape colaspis feeding can cause visible symptoms including wilting, discoloration, and death. In severe situations, the injury may result in stand loss.
Early season weather, salinity, grape colaspis and pythium can all be tough on seedling rice. Figuring out the cause of the symptoms can sometimes be a challenge, but it is the first step to appropriate management and saving a stand of rice, so please contact us if you see unusual problems.