Rain, cloudy skies and overcast conditions that continued to hold up the planting of rice, did not hold up the rice blast pathogen from reproducing and infecting rice at an early growth stage in Southwest Louisiana. Rice leaf blast has been reported in Southwest Louisiana on the medium grain variety Jupiter to a large extent and to a lesser extent on the long grain CL151, particularly on lighter sandy soils. Although the rice crop in Arkansas is behind that of Louisiana, blast can start infecting the plant as early as tillering stage. If conditions are favorable, it can appear early and prevail again at heading.
Field conditions that favor blast
- Fields with heavy tree lines, especially on the east side – which prolongs night-time dew period – are more likely to develop blast if planted with susceptible varieties.
- Soil types where a deep flood cannot be maintained are more prone for blast disease.
- High nitrogen in fields with blast history makes susceptible varieties prone to the disease.
- Fields with history likely get the disease if planted with susceptible varieties.
- Fields at river bottoms with longer dew periods are prone for the disease.
- Fields low in potash and low irrigation capacity favor disease development.
- Low cut fields when leveling get low in fertility particularly potash and are prone to blast disease.
Weather conditions that favor blast
- Frequent light rains that allow extended leaf wetness
- Extended cloudy days that allow slower leaf drying
- Slow wind that transport spores and slow leaf dryness
- Warmer days and cooler nights that allow long dew periods
- Combinations of rain, cloudy skies and overcast
Note that spores of the blast pathogen are windborne and can travel long distances so we have to be watchful and scout the fields. What happened in Louisiana may happen here, too, particularly if we go from extreme wet to extreme dry. Damage from leaf blast can be substantially suppressed if detected early, diagnosed correctly (Figure 1) and managed timely. Late detection, incorrect diagnosis and poor management could be costly with increasing the frequency of fungicide application and yield reduction. Maintaining a consistent > 4 inch flood at all times until the field is drained for harvest is essential to suppress further leaf blast disease development. However, in severe conditions (Figure 2) fungicide application may be required to suppress blast disease development. Once the leaf blast is under control, you still need to be prepared to apply preventative fungicides at late boot stage to 10% heading followed by a second application at 50-75% heading. Blast is a devastating disease of rice.
The blast fungus infects several parts of the rice plant:
- It infects leaves causing “Leaf Blast”
- It infects leaf collars causing “Collar Blast”
- It infects panicles causing “Panicle Blast”
- It infects panicle node and/ or neck causing “Neck blast (Neck rot)”
The blast pathogen is the most effective organism in promoting its survival. It is versatile to adapt to different environmental conditions and infect rice varieties. Get ahead of this notorious disease.
Scout fields for leaf blast symptoms on susceptible varieties grown in blast prone fields. Scouting fields historically prone to blast is always warranted. Varieties known to be resistant are not totally immune from blast infection. New races of the pathogen may change history. When scouting for leaf blast looking for the typical spindle-shaped lesions on older and lower leaves (Figure 1) is advisable. Early blast lesions may be confused with brown spot or spots formed due to herbicide damage such as Gramoxone (Paraquat).