Soil moisture and temperature effects on shrunken2 sweet corn decay and seedling blight caused by Penicillium oxalicum

Sweet corn (Zea mays L.) seed is commonly infected or infested with fungi that can impair stand establishment. Among these, Penicillium oxalicum Currie and Thom is known to cause preemergence damping-off or postemergence seedling blight. Supersweet, or shrunken2 (sh2), sweet corn cultivars are particularly affected by seedborne fungal pathogens, although the effects of seed infection on seedling emergence and stand are variable under field conditions. This study was conducted to examine factors that could influence the impact of P. oxalicum on seedling stand, including P. oxalicum inoculum density on seed and in soil, soil moisture, soil temperature, and control of seed decay caused by soilborne Pythium ultimum Trow. Seed surface disinfestation usually had no effect on seedling stand under conditions favoring infection by P. ultimum. Inoculation of sh2 sweet corn seeds or infestation of soil with conidia of P. oxalicum resulted in increasing severity of damping-off and seedling blight as inoculum density increased. In pasteurized soil in the greenhouse, an inoculum density of 10(2) P. oxalicum conidia per seed reduced emergence and induced seedling blight. In the field, where P. ultimum was also a factor, 10(6) conidia per seed were needed to reduce emergence and 10(5) conidia per seed to reduce healthy seedling stand. When pythium seed decay was controlled by metalaxyl seed treatment, seedling emergence and healthy seedling stand were both reduced at 1 X 10(6) P. oxalicum conidia per seed. When sh2 sweet corn seed was inoculated with conidia of P. oxalicum and incubated in soil at subgermination moisture contents (-4.2 to -7.8 MPa) for 2-4 weeks before planting and irrigating, P. oxalicum reduced seedling emergence at all soil moisture levels, but caused the greatest amount of injury after planting when seeds were incubated in soil above -5.1 MPa. As soil temperature increased from 9-25C, seedling emergence from seed inoculated with P. oxalicum was progressively reduced, with a decrease of nearly 50% at 25C. Penicillium oxalicum has the greatest potential to reduce seedling stand when infected sweet corn seeds are planted in warm, dry soil, but the effects of this and other seedborne fungal pathogens may be masked under conditions favoring infection by P. ultimum.