Downy mildew of Sweet Sorghum

(Peronosclerospora sorghi [Weston and Uppal (Shaw)

Symptoms

 

• Systemic infection occurs when the young meristematic tissues of the growing seedlings are infected and the symptoms appear as chlorotic foliage and stunted, often resulting in death of seedlings. The first infected leaf shows chlorosis on the lower part of the lamina, which further grows to cover a larger part of the leaf. The other leaves on a plant that get infected subsequently show more choruses. Under cool and humid weather conditions, the abaxial (below) surface of chlorotic leaves produce abundant spores (conidia) that appear as white, downy growth. As the plant grows, new emerging leaves exhibit parallel stripes of green and white tissue; the white interveinal tissue dies and leaf shredding occurs. These shredded tissues usually contain numerous oospores.
• The local lesions on foliage are the result of infection by conidia. These appear as stippled, necrotic lesions on leaf blades. The fungus produces white downy growth of conidiophores and conidia on these lesions. Conidia from these lesions get dispersed through wind currents to neighboring plants and cause disease. The local lesions become systemic when the conidia from these lesions infect meristematic tissues.
  
  

  

  Fig. 1: Symptoms on young seedlings and under surface of leaf
  

Control measures

Downy mildew is not a serious and widespread disease in India, and thus most commercial cultivars are resistant to the disease as of now. However, in some cases where disease appears, the following measures can be taken.

• Uproot and destroy the diseased seedlings to prevent disease spread.
• Treat  seed with Apron SD 35/Ridomil  (metalaxyl)  @ 2 g a.i kg-1  seed, followed by one spray with metalaxyl @1g a.i l-1 (750L ha-1) at 40 days after planting.