Effects of cultivar, seed quality, pathogen virulence, inoculum density, and seed density, and seed depth on host resistance to the seed-seedling disease complex of cotton

Abstract

Cotton stand establishment and the seed-seedling disease complex are influenced by several factors. Studies were conducted at three phases to determine the effect of seed quality, isolate and inoculum density of Pythium ultimum and Rhizoctonia solani, and planting depth on host reaction of cotton cultivars and disease development. Methanol stress was evaluated for artificial deterioration of cottonseed. Tests were performed in germinators, controlled soil temperature tanks, and in the field. Under field conditions, differences were obtained between the 1984 and 1985 tests and among cultivars in disease incidence and stand. Under controlled soil temperature, P. ultimum isolate B6-1 and R. solani isolate F366 were more virulent than the Texas isolates causing more seed rot and damping-off, and stand reduction. Disease incidence was more severe as inoculum density was increased from 60 to 90 propagules, and planting depth from 1.3 to 3.8 cm. A differential response was obtained among cultivars for disease incidence and stand in the presence of both pathogens. The more virulent isolates did not allow differentiation among cultivars for resistance. The levels of pathogen virulence and inoculum density, planting depth, and seed quality are critical for differentiating germplasm for resistance to the seed-seedling disease complex. Laboratory and field tests in 1986 and 1987 indicated that disease incidence increased, and germination, emergence, and stand decreased as exposure time to methanol increased from 0 to 6 hr. The effects of methanol stress were more pronounced in the presence of P. ultimum than R. solani. Lankart 57 and TX-LEBO-3-80 were severely affected by methanol and pathogens, and Tamcot CAMD-E was affected the least. Planting date, temperature, seed quality, and methanol influenced cultivar response for germination, emergence, disease incidence, and stand. A new method, methanol stress, is suggested as an effective agent for artificially deteriorating cottonseed. Methanol stress mimicked the effects of field weathering relative to seed deterioration, germination, reaction to seed-seedling pathogens, and stand establishment. Immersing cottonseed in 20% methanol for 6 hr is a reliable technique to determine the quality, viability and vigor of cottonseed.