First Report of Pythium Root Rot on Grain Sorghum in Mississippi

Area of Science:

• Plant Pathology
• Mycology
• Agronomy

Background:

• Grain sorghum (Sorghum bicolor L.) experienced significant seedling death in Mississippi during May 2000.
• The preceding wet planting season, with elevated precipitation and humidity, created conducive conditions for disease development.
• Observed symptoms included leaf necrosis, collar rot, root rot, and vascular streaking, leading to plant mortality.

Purpose of the Study:

• To identify the causal agent of seedling death in grain sorghum in Mississippi.
• To confirm the pathogenicity of the isolated fungus on grain sorghum under controlled conditions.

Main Methods:

• Fungal isolation from symptomatic sorghum tissues using potato dextrose, V8, and cornmeal agars.
• Morphological characterization of isolated fungi, including oogonia, oospores, antheridia, and zoospores.
• Pathogenicity assays in a controlled growth chamber using inoculated soil and grain sorghum seeds.

Main Results:

• Pythium ultimum was consistently isolated from diseased sorghum tissues, accounting for 63% of isolations.
• Inoculation with P. ultimum successfully reproduced field symptoms, including collar rot, root rot, and leaf necrosis, in growth chamber experiments.
• Control plants remained asymptomatic, confirming the pathogenicity of the isolated P. ultimum.

Conclusions:

• Pythium ultimum is identified as the primary pathogen responsible for grain sorghum seedling death in Mississippi.
• This study represents the first report of P. ultimum as a pathogen of grain sorghum in Mississippi.
• The findings highlight the importance of P. ultimum in sorghum production and suggest potential management strategies.