Johnson Spot

Johnson spot is a fungal disease in 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, ryegrasses, and St. Augustine grass.

Johnson spot lesions on rice (USDA ARS) Public Domain

The fungal pathogen

The fungus that causes Johnson spot is called Magnaporthe grisea (previously known as Pyriculria grisea). It 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. And thousands of new spores are generated each night. With numbers like that, it's no surprise that this disease can overwhelm a plant.

Johnson spot symptoms

Early signs of Johnson spot infection include white to grayish-green spots with dark borders. As they age, the lesions take on a more elliptical shape. Eventually, affected leaves die and fall off. These symptoms can be seen on many parts of the plant, including the leaf collar, stems (culms), and flowers (panicles). And seed production nearly stops, which is a problem since crops such as barley, millet, or rye are seeds.

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

How to prevent and control Johnson spot

Moisture is the key ingredient to this fungal growth. If leaves are wet and temperatures are between 77 and 82°F, Johnson spot can quickly take hold. Use these tips to prevent Johnson spot:

• Select resistant plant varieties.
• Space plants for good airflow.
• Avoid overhead watering.
• Water early in the day.
• Allow the soil to dry out between waterings (without causing water stress).
• Apply the minimum amount of nitrogen needed.
• Employ crop rotation.
• Dispose of infected plants in the trash, not the compost pile.

The Johnson spot fungus is resistant to chemical treatments, so these cultural practices are your only option.

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 annually.