Start Scouting for Leaf Blast

by Yeshi Wamishe, Extension Plant Pathologist - June 24, 2022

Rice blast is unpredictable. It may be lurking in fields with a history if planted with susceptible or moderately susceptible varieties. Infection may go fast when conditions are favorable for the spores to germinate.

What are favorable conditions for blast fungus spore germination?

• Susceptible or moderately susceptible varieties
• 9 hours + leaf wetness
• Dew, fog, shaded areas (trees), frequent light rains
• Cooler temperatures than needed for rice sheath blight
• Late planting
• Excessive nitrogen (N) rate
• Sandier soils that do not hold water
• Low grounds or river valleys
• Dry field edges, levees, and fields not adequately flooded

Where should you scout for rice leaf blast?

Look at tree lines, dry field edges, levees, and spots in the field with a greener canopy due to excessive N. 

What are the symptoms of rice blast?

Early Symptoms
Early symptoms may look like greyish-black spots, as seen in Figure 1. You may find the typical blast symptoms if you open the canopy and look at the lower leaves (Figure 2). 

Fig. 1. Early symptoms may look like greyish-black spots. 

Fig. 2. Often typical blast symptoms are clearly seen when you open the canopy and look at lower leaves in susceptible varieties planted in fields with a history.

Later Symptoms
Leaf lesions are spindle-shaped and elongated with brown borders and grayish centers. A brownish lesion on the internode at the base of the panicle causes “blasting” of heads followed by breaking over of the head, later producing the “rotten neck” symptoms after heading. 

Learn the Source and Spread

Source of Inoculum
The source of inoculum for early infection has not been satisfactorily determined. The fungus may overwinter on diseased straw and stubble or, in some cases, carried on infested or infected seeds. 

Means of Spread
The spores can easily be carried by wind and hence, blown from a long distance across the field and neighboring fields.

What are management practices for rice leaf blast?

For leaf stages of the disease, maintain proper flood level. Levels of infection tend to be less severe where maintained flood water is at adequate but not excessive depths. Avoid excessive nitrogen rates. (Nitrogen amounts vary with cropping history, soil type, varieties, etc.)

For later stages of the blast disease, using fungicides will be helpful. Fungicide timing is critical for effective control. Early scouting aids in protective fungicide decisions, particularly for neck and panicle blast. 

Note: When scouting for blast, you may need to scout for other diseases such as sheath blight. They can be found together as in Figure 3.

Fig. 3.  Blast and sheath blight on the same variety of rice.