Johnson Spot

Johnson spot is a fungal disease of rice, wheat, barley, rye, and millet. It also attacks your lawn.


Other names for this disease include rice blast fungus, pitting disease, and ryegrass blast. As a threat to your lawn, Johnson spot can infect kikuyugrass, fescues, rye grasses, and St. Augustine grass.

Johnson spot lesions on rice (USDA ARS) Public Domain

The fungal pathogen

The fungi that causes Johnson spot is called Magnaporthe grisea (previously known as Pyriculria grisea). Magnaporthe grisea is a highly effective fungus. Spores attach themselves to plant surfaces. They can reproduce both sexually and asexually, and they are prolific. A single spore can complete its reproductive cycle in one week, though it can live for 20 days. Thousands of new spores are generated each night. I don’t know how to calculate the math on all that, but I am certain that those numbers would be overwhelming to a plant. As the fungi perform all that precreation, seed production is reduced and entire leaves are killed.


Johnson spot symptoms

Early signs of fungal infection include white to grayish green spots with dark borders. As they age, the lesions take on a more elliptical shape. These symptoms can be seen on many parts of the plant, including the leaf collar, stems (culms), and flowers (panicles).

Rice blast lesions on nodes (Donald Groth, USDA) Public Domain

How to prevent and control Johnson spot

Moisture is a key ingredient to this fungal growth. If leaves are wet and temperatures are between 77 and 82°F, Johnson spot can quickly take hold. To break this disease triangle, be sure to space plants in such a way that supports good air flow, avoid overhead watering, allow the soil to dry out between waterings (without causing water stress), and only apply the minimum amounts of nitrogen needed by the plants.


Of course, that advice is only partially useful when it comes to lawn care. To help your lawn avoid becoming infected with Johnson spot, water as early in the day as you can. This will allow plants to dry off before evening comes around.


This fungus has developed resistance to chemical treatments, so cultural practices are your only option. These practices include crop rotation, selecting resistant varieties, and disposing of infected plant material in the trash.


Johnson spot is the most significant disease of rice in the world. Experts estimate that this disease destroys enough rice to feed 60 million people every year.