Last year was a blast year. I first saw leaf lesions in some fields right about heading time. In other fields there were no leaf lesions, but neck blast developed later. It would be helpful to have a way to predict when weather conditions are favorable for blast so more information is available to make monitoring and treatment decisions. 

Weather plays a key role in the development of blast. One of the factors that allows blast development is leaf wetness. This is free moisture in the surface of the leave. Spores need this moisture to germinate and infect the plant tissue. If the period of leaf wetness is too short, the spores may germinate but they might not have enough time to produce an infection. When the period of leaf wetness is sufficiently long, infections can occur. 

This year I have deployed leaf wetness and temperature sensors in three rice fields in areas where blast is common (fig. 1). At all locations leaf wetness is very low during the day, increases to 100% at night, and then decreases again as the day warms up (fig2). 

Figure 1. Fields where leaf wetness and temperature is being monitored at the canopy level. The three sites are indicated by the blue circles.

Figure 2. Leaf wetness during July 11 and 12 at the Rd 55 field.

Using data from the three fields, I calculated the hours of leaf wetness available each day for blast spore germination and infection and compared that to the time needed by spores to germinate and infect (fig. 3). The data shows that during the first week of July the hours of leaf wetness were sufficient for spore germination and infection. After infection, lesions may form 4 to 14 days later. This means that, if blast spores were present during early July, we may start seeing infections soon.

Figure 3. Graph comparing the hours available and needed for blast spore germination and infection at the Rd 55 location. When the hours available (orange line) are higher than the hours required (blue line), infections may occur if blast spores are present.

I will continue to monitor field conditions and update the forecast next week.