Factors affecting Pythium ultimum oospores : production, germination, and virulence to cotton seedlings in soil

In a field test, severity of cotton (Gossypium hirsutum) seedling disease was not affected by nitrogen fertilization. There were no differences in disease severity in seedlings grown in soils that had received 0, 90, or 180 kg/ha, either in the form of NH₄CL or NaNO₃.

Oospore production by Pythium ultimum (Trow) was determined on five liquid culture media. Higher numbers of oospores were produced on a basic mineral medium containing lecithin and on a com meal medium than on three other media evaluated. Numbers of oospores increased progressively during the 30-day incubation period on four of the media. Maximum oospore formation occurred in a medium containing 1.7 g/L of lecithin in a soybean oil formulation.

The addition of oospores of ultimum to soil significantly increased cotton seedling disease in previously sterilized and in nonsterilized soil which had been air-dried for 1 year, but not in oospore-amended fresh field soil. A significant amount of infection occurred in sterilized soil amended with 10 oospores/g, and very severe disease occurred in sterilized soil containing 100 oospores/g.

Conversion (change of thick walled spores to thin ones) and germination of oospores were studied with the use of an "agar-slide" technique, in which oospore-amended agar was solidified as a film on a glass slide. When agar-slides were placed in soil, oospores began the conversion process immediately, and some thick walls were converted to thin walls after 24 hours of incubation. Most of the oospores added to soil had thin walls after 16 days of incubation. Oospores did not germinate in soil during the three month incubation period.

P. ultimum oospores in soil immediately adjacent to the primary root of cotton germinated within 48 hours after cotton seeds were planted. Germination occurred at a greater distance (within 1.6 mm) from secondary root surfaces. All germ tubes observed grew toward root surfaces. Roots were mostly infected in the root hair zone and occasionally the root tips. Germ tube infection of the region of elongation was rare.

The "agar-slide" technique was very efficient for studying oospores of Pythium in soil, and could be adapted for determining the fate of propagules of other plant pathogens that cause root disease.