Resistance of two cotton (Gossypium hirsutum L.) cultivars to Pythium ultimum Trow

Two commercial cotton (Gossypium hirsutum L.) cultivars were tested for their relative susceptibility to the fungal seedling blight pathogen, Pythium ultimum Trow. Cotton plants grew in sterilized sand for eight days, and then were inoculated with the pathogen. The seedlings were incubated at 18 C, and after a seven day period disease severity was rated. 'Dixie King 3' was consistently more resistant to P. ultimum than 'Delcot 277'.

Nutrient status of the fungus affected its pathogenicity.Disease severity on cotton seedlings was greater when the fungus grew on potato dextrose agar (PDA) than when it grew on dilutePDA or water agar.

Since the epidermal cell wall is the first physical barrier to invasion by a fungal pathogen, the two cultivars were compared for differences in cell wall thickness. There was no significant difference between the cultivars.

Pythium ultimum was cultured on various substrates to test linear growth responses. No differences in growth rates were found when the fungus was cultured on water agar mixed with ground hypocotyl tissue of each cultivar.

Gossypol, a terpenoid aldehyde, catechin, a polyphenol, and catechol, all previously implicated in plant resistance to other pathogens, were incorporated into culture media. Growth inhibition of P. ultimum occurred on such media containing catechol at 10 ppm and gossypol at 1,000 ppm. Catechin did not inhibit growth at1,000 ppm, the highest concentration tested. Cotton hypocotyl tissue was extracted and gossypol and catechin"equivalents" were measured with a spectrophotometer. Gossypol concentrations decreased in both cultivars after infection by P. ultimum .

Concentrations of catechin "equivalents" increased threefold in thePythium-tolerant cultivar 'Dixie King 3' following infection, but decreased in the susceptible cultivar 'Delcot 277'. This phytoalexin-like response in the resistant cultivar may be involved in cotton resistance to infection by Pythium ultimum.