I got this question today. Blast spores need free moisture on the tissue (leaf wetness) to be able to produce infections. Rainy weather provides this free moisture and can make blast worse. However, spore production and germination also require warm temperatures. The rainy weather we had this last couple of days has been accompanied by a considerable cool down. This reduces spore production and germination.
The graph below shows daily hours of leaf wetness at three locations in the Valley: Willows, Maxwell, and Knights Landing. This year, I have seen blast in Willows and Maxwell. Hours of leaf wetness has been substantially higher at these locations than at Knights Landing. At Willows and Maxwell, hours of leaf wetness between mid-July and late August has hovered between 12 and 14 hours. These seem to be enough to produce some infections. The red line in the graph shows the theoretical hours of leaf wetness needed for infection based on nighttime temperatures, which was between 12 and 14 hours. However, once the weather started to cool down the hours of leaf wetness needed for infection shot up to 18 and 19 hours, which we are not getting. This rainy weather may extend the hours of leaf wetness, but we are also getting some wind that is drying down the canopy quickly. For example, I was in the field this morning in the Willows area and foliage was dry. Although we are seeing the hours of leaf wetness increase with the cool weather, they may not be enough to cause more infections because of the cooler temperatures.
