Effect of nutrients and inoculum quantity on Pythium ultimum infection of cotton hypocotyls

Seedling disease, caused by Pythium ultimum Trow., is one of the most destructive disease of cotton in Tennessee. The objective of this study was to evaluate the effect of the nutritional status of the pathogen on infection and disease development.

The nutritional status of Pythium ultimum mycelium used to inoculate cotton hypocotyls was varied by inoculating the pathogen on agar media containing various concentrations of nutrients. Sucrose, nitrate-nitrogen, ammonium-nitrogen and potassium in agar media were found to significantly affect vegetative growth and pathogenicity. Disease was more severe when inocula were grown on media with medium to high levels of these materials than when grown on media with low levels. Magnesium and phosphate at low or high levels in agar media did not affect pathogenicity . The pathogen grown on a nitrate-nitrogen medium produced significantly more severe disease than when grown on an ammonium-nitrogen medium.

Light transmission through blended agar cultures was used as a measure of inoculum quantity. There was a significant positive correlation among three factors: levels of sucrose, nitrogen, or potassium in the culture media, inoculum quantity, and disease severity.

In liquid media, Pythium ultimum became progressively more virulent as the concentration of nitrogen or sucrose was increased, but the weight of the hyphae increased sharply to a maxium at low to median levels of these nutrients. Nitrogen or carbohydrate deficient hyphae produced as much disease as did non-deficient hyphae, provided that the quantities of inocula were similar.

These results emphasize that sources of nutrients and the capacity of the pathogen to utilize these materials are important considerations in understanding disease development in nature.