Charcoal Stalk Rot of Sorghum: Effect of Environment on Host-Parasite Relations

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Abstract
The influence of high temperatures and low water potentials on the susceptibility of sorghum [S. bicolor] to Macrophomina phaseolina, infection phenomena, disease development and response of the pathogen in culture was investigated. Roots of fertilized CK60 B (self-fertile) and nonfertilized CK60 A (male-sterile) sorghum lines differed in susceptibility to M. phaseolina when plants were subjected to drought stress at the soft dough stage. Only stressed fertile plants developed charcoal stalk and root rot. Root systems of stressed male-sterile plants had a high percentage of roots with latent infections but did not develop symptoms. Most root infections of both fertile and male-sterile sorghums occurred only after the onset of stress conditions. Growth rates of M. phaseolina on potato-dextrose agar (PDA) declined as osmotic potential was decreased from -1.5 to -129 bars at 30.degree. and 35.degree. C. At 40.degree. C, the highest growth rates occurred between -13 and -40 bars, although growth rates at all corresponding osmotic potentials were lower than at 30.degree. and 35.degree. C. At all temperatures, germination of sclerotia on PDA was .gtoreq. 50% from -1.5 to -90 bars. Mycelial growth and sclerotial germination occurred at lower osmotic potentials on PDA than on osmotically adjusted water agar.

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